feat(solver): KCSolve solver addon with assembly integration (#289)

Adds the Kindred constraint solver as a pluggable Assembly workbench
backend, covering phases 3d through 5 of the solver roadmap.

Phase 3d: SolveContext packing
- Pack/unpack SolveContext into .kc archive on document save

Solver addon (mods/solver):
- Phase 1: Expression DAG, Newton-Raphson + BFGS, 3 basic constraints
- Phase 2: Full constraint vocabulary — all 24 BaseJointKind types
- Phase 3: Graph decomposition for cluster-by-cluster solving
- Phase 4: Per-entity DOF diagnostics, overconstrained detection,
  half-space preference tracking, minimum-movement weighting
- Phase 5: _build_system extraction, diagnose(), drag protocol,
  joint limits warning

Assembly workbench integration:
- Preference-driven solver selection (reads Mod/Assembly/Solver param)
- Solver backend combo box in Assembly preferences UI
- resetSolver() on AssemblyObject for live preference switching
- Integration tests (TestKindredSolverIntegration.py)
- In-client console test script (console_test_phase5.py)

.gitmodules

@@ -13,6 +13,12 @@
 [submodule "mods/ztools"]
 	path = mods/ztools
 	url = https://git.kindred-systems.com/forbes/ztools.git
+	branch = main
 [submodule "mods/silo"]
 	path = mods/silo
 	url = https://git.kindred-systems.com/kindred/silo-mod.git
+	branch = main
+[submodule "mods/solver"]
+	path = mods/solver
+	url = https://git.kindred-systems.com/kindred/solver.git
+	branch = main

docs/INTER_SOLVER.md

@@ -0,0 +1,568 @@
+# Pluggable Assembly Solver Architecture
+
+**Status:** Phase 2 complete  
+**Last Updated:** 2026-02-19
+
+---
+
+## 1. Problem
+
+Kindred Create currently vendors OndselSolver as a monolithic assembly constraint solver. Different engineering domains benefit from different solver strategies — Lagrangian methods work well for rigid body assemblies but poorly for over-constrained or soft-constraint systems. A pluggable architecture lets us ship multiple solvers (including experimental ones) without touching core assembly logic, and lets the server farm out solve jobs to headless worker processes.
+
+---
+
+## 2. Design Goals
+
+1. **Stable C++ API** — A solver-agnostic interface that the Assembly module calls. Solvers are shared libraries loaded at runtime.
+2. **Python binding layer** — Every C++ solver is exposed to Python via pybind11, enabling rapid prototyping, debugging, and server-side execution without a full GUI build.
+3. **Solver-defined joint types** — Each solver declares its own joint/mate vocabulary, mapped from a common base set (inspired by SOLIDWORKS mates: coincident, concentric, tangent, distance, angle, lock, etc.).
+4. **Semi-deterministic solving** — Consistent results given consistent input ordering, with configurable tolerance and iteration limits.
+5. **Server-compatible** — Solvers run as detached processes claimed by `silorunner` workers via the existing job queue.
+
+---
+
+## 3. Architecture Layers
+
+```
+┌──────────────────────────────────────────────────────┐
+│  Layer 4: Server / Worker                            │
+│  silorunner claims solve jobs, executes via Python   │
+│  Headless Create or standalone solver process        │
+├──────────────────────────────────────────────────────┤
+│  Layer 3: Python Debug & Scripting                   │
+│  pybind11 bindings for all solvers                   │
+│  Introspection, step-through, constraint viz         │
+│  import kcsolve; s = kcsolve.load("ondsel")          │
+├──────────────────────────────────────────────────────┤
+│  Layer 2: Solver Plugins (.so / .dll / .dylib)       │
+│  Each implements IKCSolver interface                  │
+│  Registers joint types via manifest                   │
+│  Loaded by SolverRegistry at runtime                  │
+├──────────────────────────────────────────────────────┤
+│  Layer 1: C++ Solver API (libkcsolve)                │
+│  IKCSolver, JointDef, SolveContext, SolveResult      │
+│  SolverRegistry (discovery, loading, selection)       │
+│  Ships as a shared library linked by Assembly module  │
+└──────────────────────────────────────────────────────┘
+```
+
+---
+
+## 4. Layer 1: C++ Solver API
+
+Located at `src/Mod/Assembly/Solver/` (or `src/Lib/KCSolve/` if we want it independent of Assembly).
+
+### 4.1 Core Types
+
+```cpp
+namespace KCSolve {
+
+// Unique identifier for a joint type within a solver
+struct JointTypeId {
+    std::string solver_id;   // e.g. "ondsel", "gnn", "relaxation"
+    std::string joint_name;  // e.g. "coincident", "distance"
+};
+
+// Base joint categories (SOLIDWORKS-inspired vocabulary)
+enum class BaseJointKind {
+    Coincident,
+    Concentric,
+    Tangent,
+    Distance,
+    Angle,
+    Lock,
+    Parallel,
+    Perpendicular,
+    PointOnLine,
+    SymmetricPlane,
+    Gear,
+    Rack,
+    Cam,
+    Slot,
+    Hinge,
+    Slider,
+    Cylindrical,
+    Planar,
+    Ball,
+    Screw,
+    Universal,
+    Custom   // solver-specific extension
+};
+
+// A joint definition registered by a solver plugin
+struct JointDef {
+    JointTypeId id;
+    BaseJointKind base_kind;          // which vanilla category it maps to
+    std::string display_name;
+    std::string description;
+    uint32_t dof_removed;             // degrees of freedom this joint removes
+    std::vector<std::string> params;  // parameter names (e.g. "distance", "angle")
+    bool supports_limits = false;
+    bool supports_friction = false;
+};
+
+// A constraint instance in a solve problem
+struct Constraint {
+    JointTypeId joint_type;
+    std::string part_a;    // part label or id
+    std::string part_b;
+    // Geometry references (face, edge, vertex indices)
+    std::vector<std::string> refs_a;
+    std::vector<std::string> refs_b;
+    std::map<std::string, double> params;   // param_name -> value
+    bool suppressed = false;
+};
+
+// Input to a solve operation
+struct SolveContext {
+    std::vector<Constraint> constraints;
+    // Part placements as 4x4 transforms (initial guess)
+    std::map<std::string, std::array<double, 16>> placements;
+    // Which parts are grounded (fixed)
+    std::set<std::string> grounded;
+    // Solver config
+    double tolerance = 1e-10;
+    uint32_t max_iterations = 500;
+    bool deterministic = true;    // force consistent ordering
+    // Optional: previous solution for warm-starting
+    std::map<std::string, std::array<double, 16>> warm_start;
+};
+
+enum class SolveStatus {
+    Converged,
+    MaxIterationsReached,
+    Overconstrained,
+    Underconstrained,
+    Redundant,
+    Failed
+};
+
+struct ConstraintDiagnostic {
+    std::string constraint_id;
+    double residual;
+    bool satisfied;
+    std::string message;
+};
+
+struct SolveResult {
+    SolveStatus status;
+    uint32_t iterations;
+    double final_residual;
+    double solve_time_ms;
+    std::map<std::string, std::array<double, 16>> placements;
+    std::vector<ConstraintDiagnostic> diagnostics;
+    // For semi-deterministic: hash of input ordering
+    uint64_t input_hash;
+};
+
+} // namespace KCSolve
+```
+
+### 4.2 Solver Interface
+
+```cpp
+namespace KCSolve {
+
+class IKCSolver {
+public:
+    virtual ~IKCSolver() = default;
+
+    // Identity
+    virtual std::string id() const = 0;
+    virtual std::string name() const = 0;
+    virtual std::string version() const = 0;
+
+    // Joint type registry — called once at load
+    virtual std::vector<JointDef> supported_joints() const = 0;
+
+    // Solve
+    virtual SolveResult solve(const SolveContext& ctx) = 0;
+
+    // Incremental: update a single constraint without full re-solve
+    // Default impl falls back to full solve
+    virtual SolveResult update(const SolveContext& ctx,
+                               const std::string& changed_constraint) {
+        return solve(ctx);
+    }
+
+    // Diagnostic: check if a constraint set is well-posed before solving
+    virtual SolveStatus diagnose(const SolveContext& ctx) {
+        return SolveStatus::Converged; // optimistic default
+    }
+
+    // Determinism: given identical input, produce identical output
+    virtual bool is_deterministic() const { return false; }
+};
+
+// Plugin entry point — each .so exports this symbol
+using CreateSolverFn = IKCSolver* (*)();
+
+} // namespace KCSolve
+```
+
+### 4.3 Solver Registry
+
+```cpp
+namespace KCSolve {
+
+class SolverRegistry {
+public:
+    // Scan a directory for solver plugins (*.so / *.dll / *.dylib)
+    void scan(const std::filesystem::path& plugin_dir);
+
+    // Manual registration (for built-in solvers like Ondsel)
+    void register_solver(std::unique_ptr<IKCSolver> solver);
+
+    // Lookup
+    IKCSolver* get(const std::string& solver_id) const;
+    std::vector<std::string> available() const;
+
+    // Joint type resolution: find which solvers support a given base kind
+    std::vector<JointTypeId> joints_for(BaseJointKind kind) const;
+
+    // Global default solver
+    void set_default(const std::string& solver_id);
+    IKCSolver* get_default() const;
+};
+
+} // namespace KCSolve
+```
+
+### 4.4 Plugin Loading
+
+Each solver plugin is a shared library exporting:
+
+```cpp
+extern "C" KCSolve::IKCSolver* kcsolve_create();
+extern "C" const char* kcsolve_api_version();  // "1.0"
+```
+
+The registry `dlopen`s each library, checks `kcsolve_api_version()` compatibility, and calls `kcsolve_create()`. Plugins are discovered from:
+
+1. `<install_prefix>/lib/kcsolve/` — system-installed solvers
+2. `~/.config/KindredCreate/solvers/` — user-installed solvers
+3. `KCSOLVE_PLUGIN_PATH` env var — development overrides
+
+---
+
+## 5. Layer 2: OndselSolver Adapter
+
+The first plugin wraps the existing OndselSolver, mapping its internal constraint types to the `IKCSolver` interface.
+
+```
+src/Mod/Assembly/Solver/
+├── IKCSolver.h            # Interface + types from §4
+├── SolverRegistry.cpp     # Plugin discovery and loading
+├── OndselAdapter.cpp      # Wraps OndselSolver as IKCSolver plugin
+└── CMakeLists.txt
+```
+
+`OndselAdapter` translates between `SolveContext` ↔ OndselSolver's Lagrangian formulation. This is the reference implementation and proves the API works before any new solvers are written.
+
+Joint mapping for OndselAdapter:
+
+| BaseJointKind | Ondsel Constraint | DOF Removed |
+|---------------|-------------------|-------------|
+| Coincident | PointOnPoint | 3 |
+| Concentric | CylindricalOnCylindrical | 4 |
+| Tangent | FaceOnFace (tangent mode) | 1 |
+| Distance | PointOnPoint + offset | 2 |
+| Angle | AxisAngle | 1 |
+| Lock | FullLock | 6 |
+| Hinge | RevoluteJoint | 5 |
+| Slider | PrismaticJoint | 5 |
+| Cylindrical | CylindricalJoint | 4 |
+| Ball | SphericalJoint | 3 |
+
+---
+
+## 6. Layer 3: Python Bindings
+
+### 6.1 pybind11 Module
+
+```
+src/Mod/Assembly/Solver/bindings/
+├── kcsolve_py.cpp         # pybind11 module definition
+└── CMakeLists.txt
+```
+
+```python
+import kcsolve
+
+# List available solvers
+print(kcsolve.available())  # ["ondsel", ...]
+
+# Load a solver
+solver = kcsolve.load("ondsel")
+print(solver.name, solver.version)
+print(solver.supported_joints())
+
+# Build a problem
+ctx = kcsolve.SolveContext()
+ctx.add_part("base", placement=..., grounded=True)
+ctx.add_part("arm", placement=...)
+ctx.add_constraint("coincident", "base", "arm",
+                    refs_a=["Face6"], refs_b=["Face1"])
+
+# Solve
+result = solver.solve(ctx)
+print(result.status)          # SolveStatus.Converged
+print(result.iterations)      # 12
+print(result.solve_time_ms)   # 3.4
+print(result.placements["arm"])
+
+# Diagnostics per constraint
+for d in result.diagnostics:
+    print(f"{d.constraint_id}: residual={d.residual:.2e} ok={d.satisfied}")
+```
+
+### 6.2 Debug / Introspection API
+
+The Python layer adds capabilities the C++ interface intentionally omits for performance:
+
+```python
+# Step-through solving (debug mode)
+debugger = kcsolve.Debugger(solver, ctx)
+for step in debugger.iterate():
+    print(f"iter {step.iteration}: residual={step.residual:.6e}")
+    print(f"  moved: {step.parts_moved}")
+    print(f"  worst constraint: {step.worst_constraint}")
+    if step.residual < 1e-8:
+        break
+
+# Constraint dependency graph
+graph = kcsolve.dependency_graph(ctx)
+# Returns dict: constraint_id -> [dependent_constraint_ids]
+
+# DOF analysis
+analysis = kcsolve.dof_analysis(ctx)
+print(f"Total DOF: {analysis.total_dof}")
+print(f"Removed: {analysis.constrained_dof}")
+print(f"Remaining: {analysis.free_dof}")
+for part, dofs in analysis.per_part.items():
+    print(f"  {part}: {dofs} free")
+```
+
+### 6.3 Pure-Python Solver Support
+
+The Python layer also supports solvers written entirely in Python (no C++ required). This is the fast path for prototyping new approaches (GNN, relaxation, etc.):
+
+```python
+class RelaxationSolver(kcsolve.PySolver):
+    """A pure-Python iterative relaxation solver for prototyping."""
+
+    id = "relaxation"
+    name = "Iterative Relaxation"
+    version = "0.1.0"
+
+    def supported_joints(self):
+        return [
+            kcsolve.JointDef("coincident", kcsolve.BaseJointKind.Coincident, dof_removed=3),
+            kcsolve.JointDef("distance", kcsolve.BaseJointKind.Distance, dof_removed=2),
+            # ...
+        ]
+
+    def solve(self, ctx: kcsolve.SolveContext) -> kcsolve.SolveResult:
+        placements = dict(ctx.placements)
+        for i in range(ctx.max_iterations):
+            max_residual = 0.0
+            for c in ctx.constraints:
+                residual = self._eval_constraint(c, placements)
+                correction = self._compute_correction(c, residual)
+                self._apply_correction(placements, c, correction)
+                max_residual = max(max_residual, abs(residual))
+            if max_residual < ctx.tolerance:
+                return kcsolve.SolveResult(
+                    status=kcsolve.SolveStatus.Converged,
+                    iterations=i + 1,
+                    final_residual=max_residual,
+                    placements=placements
+                )
+        return kcsolve.SolveResult(
+            status=kcsolve.SolveStatus.MaxIterationsReached,
+            iterations=ctx.max_iterations,
+            final_residual=max_residual,
+            placements=placements
+        )
+
+# Register at runtime
+kcsolve.register(RelaxationSolver())
+```
+
+Python solvers are discovered from:
+- `<user_macros>/solvers/*.py` — user-written solvers
+- `mods/*/solvers/*.py` — addon-provided solvers
+
+---
+
+## 7. Layer 4: Server Integration
+
+### 7.1 Solve Job Definition
+
+Extends the existing worker system (WORKERS.md) with a new job type:
+
+```yaml
+job:
+  name: assembly-solve
+  version: 1
+  description: "Solve assembly constraints using specified solver"
+  trigger:
+    type: revision_created
+    filter:
+      item_type: assembly
+  scope:
+    type: assembly
+  compute:
+    type: solve
+    command: create-solve
+    args:
+      solver: ondsel           # or "auto" for registry default
+      tolerance: 1e-10
+      max_iterations: 500
+      deterministic: true
+      output_placements: true  # write solved placements back to revision
+      output_diagnostics: true # store constraint diagnostics in job result
+  runner:
+    tags: [create, solver]
+  timeout: 300
+  max_retries: 1
+  priority: 75
+```
+
+### 7.2 Headless Solve via Runner
+
+The `create-solve` command in `silorunner`:
+
+1. Claims job from Silo server
+2. Downloads the assembly `.kc` file
+3. Launches Headless Create (or standalone Python if pure-Python solver)
+4. Loads the assembly, extracts constraint graph → `SolveContext`
+5. Calls `solver.solve(ctx)`
+6. Reports `SolveResult` back via `POST /api/runner/jobs/{id}/complete`
+7. Optionally writes updated placements as a new revision
+
+### 7.3 Standalone Solve Process (No GUI)
+
+For server-side batch solving without Headless Create overhead:
+
+```python
+#!/usr/bin/env python3
+"""Standalone solver worker — no FreeCAD dependency."""
+import kcsolve
+import json, sys
+
+problem = json.load(sys.stdin)
+ctx = kcsolve.SolveContext.from_dict(problem)
+
+solver = kcsolve.load(problem.get("solver", "ondsel"))
+result = solver.solve(ctx)
+
+json.dump(result.to_dict(), sys.stdout)
+```
+
+This enables lightweight solver containers that don't need the full Create installation — useful for CI validation, quick constraint checks, and scaling solver capacity independently of geometry workers.
+
+---
+
+## 8. Semi-Deterministic Behavior
+
+"Semi-deterministic" means: given the same constraint set and initial placements, the solver produces the same result. This is achieved by:
+
+1. **Canonical input ordering** — `SolveContext` sorts constraints and parts by a stable key (part label + constraint index) before passing to the solver. The ordering hash is stored in `SolveResult.input_hash`.
+
+2. **Solver contract** — `IKCSolver::is_deterministic()` reports whether the implementation guarantees this. OndselAdapter does (Lagrangian formulation with fixed pivot ordering). A GNN solver might not.
+
+3. **Tolerance-aware comparison** — Two `SolveResult`s are "equivalent" if all placement deltas are within tolerance, even if iteration counts differ. Used for regression testing.
+
+4. **Warm-start stability** — When `warm_start` placements are provided, the solver should converge to the same solution as a cold start (within tolerance), just faster. This is validated in the test suite.
+
+---
+
+## 9. Implementation Phases
+
+### Phase 1: API + OndselAdapter (foundation) -- COMPLETE
+
+- Defined `IKCSolver.h`, core types (`Types.h`), `SolverRegistry`
+- Implemented `OndselAdapter` wrapping existing solver
+- Assembly module calls through `SolverRegistry` instead of directly calling OndselSolver
+- 18 C++ tests, 6 Python integration tests
+- **PR:** #297 (merged)
+
+### Phase 2: pybind11 Bindings -- COMPLETE
+
+- Built `kcsolve` pybind11 module exposing all enums, structs, and classes
+- `PyIKCSolver` trampoline for pure-Python solver subclasses
+- `register_solver()` for runtime Python solver registration
+- `PySolverHolder` for GIL-safe forwarding of virtual calls
+- 16 Python tests covering types, registry, and Python solver round-trips
+- Debug/introspection API (Debugger, `dependency_graph()`, `dof_analysis()`) deferred to Phase 4+
+- Automatic Python solver discovery (`mods/*/solvers/`) deferred -- users call `register_solver()` explicitly
+- **PR:** #298
+- **Docs:** `docs/src/architecture/ondsel-solver.md`, `docs/src/reference/kcsolve-python.md`
+
+### Phase 3: Server Integration
+
+- `create-solve` command for `silorunner`
+- YAML job definition for solve jobs
+- Standalone solver process (no FreeCAD dependency)
+- `SolveContext` JSON serialization for inter-process communication
+- **Deliverable:** Solve jobs run async through the worker system
+
+### Phase 4: Second Solver (validation)
+
+- Implement a simple relaxation or gradient-descent solver as a Python plugin
+- Validates that the API actually supports different solving strategies
+- Benchmark against OndselAdapter for correctness and performance
+- **Deliverable:** Two interchangeable solvers, selectable per-assembly
+
+### Phase 5: GNN Solver (future)
+
+- Graph Neural Network approach from existing roadmap
+- Likely a Python solver wrapping a trained model
+- Focus on fast approximate solutions for interactive editing
+- Falls back to OndselAdapter for final precision solve
+- **Deliverable:** Hybrid solve pipeline (GNN fast-guess → Lagrangian refinement)
+
+---
+
+## 10. File Locations
+
+```
+src/Lib/KCSolve/              # or src/Mod/Assembly/Solver/
+├── include/
+│   └── KCSolve/
+│       ├── IKCSolver.h        # Interface + all types
+│       ├── SolverRegistry.h   # Plugin loading and lookup
+│       └── Types.h            # Enums, structs
+├── src/
+│   ├── SolverRegistry.cpp
+│   └── OndselAdapter.cpp
+├── bindings/
+│   └── kcsolve_py.cpp         # pybind11
+├── plugins/                   # Additional compiled solver plugins
+└── CMakeLists.txt
+```
+
+---
+
+## 11. Open Questions
+
+1. **Location**: `src/Lib/KCSolve/` (independent library, usable without Assembly module) vs `src/Mod/Assembly/Solver/` (tighter coupling, simpler build)? Leaning toward `src/Lib/` since server workers need it without the full Assembly module.
+
+2. **Geometry abstraction**: The C++ API uses string references for faces/edges/vertices. Should we pass actual OCC geometry (TopoDS_Shape) through the interface, or keep it abstract and let each solver adapter resolve references? Abstract is more portable but adds a translation step.
+
+3. **Constraint persistence**: Currently constraints live in the FCStd XML. Should the pluggable layer introduce its own serialization, or always read/write through FreeCAD's property system?
+
+4. **API versioning**: `kcsolve_api_version()` returns a string. Semver with major-only breaking changes? How strict on backward compat for the plugin ABI?
+
+5. **License implications**: OndselSolver is LGPL. New solver plugins could be any license since they're loaded at runtime via a stable C API boundary. Confirm this interpretation.
+
+---
+
+## 12. References
+
+- [ondsel-solver.md](ondsel-solver.md) — Current solver documentation
+- [WORKERS.md](WORKERS.md) — Worker/runner job system
+- [MULTI_USER_EDITS.md](MULTI_USER_EDITS.md) — Async validation pipeline
+- [DAG.md](DAG.md) — Dependency graph for incremental recompute
+- [ROADMAP.md](ROADMAP.md) — Tier 3 compute modules, GNN solver plans

docs/src/SUMMARY.md

@@ -19,7 +19,7 @@
 - [Python as Source of Truth](./architecture/python-source-of-truth.md)
 - [Silo Server](./architecture/silo-server.md)
 - [Signal Architecture](./architecture/signal-architecture.md)
-- [OndselSolver](./architecture/ondsel-solver.md)
+- [KCSolve: Pluggable Solver](./architecture/ondsel-solver.md)
 
 # Development
 
@@ -46,6 +46,7 @@
 - [Gap Analysis](./silo-server/GAP_ANALYSIS.md)
 - [Frontend Spec](./silo-server/frontend-spec.md)
 - [Installation](./silo-server/INSTALL.md)
+- [Solver Service](./silo-server/SOLVER.md)
 - [Roadmap](./silo-server/ROADMAP.md)
 
 # Reference
@@ -64,3 +65,4 @@
 - [OriginSelectorWidget](./reference/cpp-origin-selector-widget.md)
 - [FileOriginPython Bridge](./reference/cpp-file-origin-python.md)
 - [Creating a Custom Origin (C++)](./reference/cpp-custom-origin-guide.md)
+- [KCSolve Python API](./reference/kcsolve-python.md)

docs/src/architecture/ondsel-solver.md

@@ -1,27 +1,132 @@
-# OndselSolver
+# KCSolve: Pluggable Solver Architecture
 
-OndselSolver is the assembly constraint solver used by FreeCAD's Assembly workbench. Kindred Create vendors a fork of the solver as a git submodule.
+KCSolve is the pluggable assembly constraint solver framework for Kindred Create. It defines an abstract solver interface (`IKCSolver`) and a runtime registry (`SolverRegistry`) that lets the Assembly module work with any conforming solver backend. The default backend wraps OndselSolver via `OndselAdapter`.
 
-- **Path:** `src/3rdParty/OndselSolver/`
+- **Library:** `src/Mod/Assembly/Solver/` (builds `libKCSolve.so`)
-- **Source:** `git.kindred-systems.com/kindred/solver` (Kindred fork)
+- **Python module:** `src/Mod/Assembly/Solver/bindings/` (builds `kcsolve.so`)
+- **Tests:** `tests/src/Mod/Assembly/Solver/` (C++), `src/Mod/Assembly/AssemblyTests/TestKCSolvePy.py` (Python)
 
-## How it works
+## Architecture
 
-The solver uses a **Lagrangian constraint formulation** to resolve assembly constraints (mates, joints, fixed positions). Given a set of parts with geometric constraints between them, it computes positions and orientations that satisfy all constraints simultaneously.
+```
+┌──────────────────────────────────────────────────┐
+│  Assembly Module (AssemblyObject.cpp)             │
+│  Builds SolveContext from FreeCAD document,       │
+│  calls solver via SolverRegistry                  │
+├──────────────────────────────────────────────────┤
+│  SolverRegistry (singleton)                       │
+│  register_solver(), get(), available()            │
+│  Plugin discovery via scan() / scan_default_paths │
+├──────────────┬───────────────────────────────────┤
+│ OndselAdapter │  Python solvers  │  Future plugins │
+│ (C++ built-in)│  (via kcsolve)   │  (.so plugins)  │
+└──────────────┴───────────────────────────────────┘
+```
 
-The Assembly workbench (`src/Mod/Assembly/`) calls the solver whenever constraints are added or modified. Kindred Create has patches to `Assembly/` that extend `findPlacement()` for better datum and origin handling.
+The Assembly module never references OndselSolver directly. All solver access goes through `SolverRegistry::instance().get()`, which returns a `std::unique_ptr<IKCSolver>`.
 
-## Why a fork
+## IKCSolver interface
 
-The solver is forked from the upstream Ondsel project for:
+A solver backend implements `IKCSolver` (defined in `IKCSolver.h`). Only three methods are pure virtual; all others have sensible defaults:
-- **Pinned stability** — the submodule is pinned to a known-good commit
-- **Potential modifications** — the fork allows Kindred-specific patches if needed
-- **Availability** — hosted on Kindred's Gitea instance for reliable access
 
-## Future: GNN solver
+| Method | Required | Purpose |
+|--------|----------|---------|
+| `name()` | yes | Human-readable solver name |
+| `supported_joints()` | yes | List of `BaseJointKind` values the solver handles |
+| `solve(ctx)` | yes | Solve for static equilibrium |
+| `update(ctx)` | no | Incremental re-solve after parameter changes |
+| `pre_drag(ctx, parts)` | no | Begin interactive drag session |
+| `drag_step(placements)` | no | One mouse-move during drag |
+| `post_drag()` | no | End drag session |
+| `run_kinematic(ctx)` | no | Run kinematic simulation |
+| `num_frames()` | no | Frame count after simulation |
+| `update_for_frame(i)` | no | Retrieve frame placements |
+| `diagnose(ctx)` | no | Detect redundant/conflicting constraints |
+| `is_deterministic()` | no | Whether output is reproducible (default: true) |
+| `export_native(path)` | no | Write solver-native debug file (e.g. ASMT) |
+| `supports_bundle_fixed()` | no | Whether solver handles Fixed-joint bundling internally |
 
-There are plans to explore a Graph Neural Network (GNN) approach to constraint solving that could complement or supplement the Lagrangian solver for specific use cases. This is not yet implemented.
+## Core types
 
-## Related: GSL
+All types live in `Types.h` with no FreeCAD dependencies, making the header standalone for future server/worker use.
 
-The `src/3rdParty/GSL/` submodule is Microsoft's Guidelines Support Library (`github.com/microsoft/GSL`), providing C++ core guidelines utilities like `gsl::span` and `gsl::not_null`. It is a build dependency, not related to the constraint solver.
+**Transform** -- position `[x, y, z]` + unit quaternion `[w, x, y, z]`. Equivalent to `Base::Placement` but independent. Note the quaternion convention differs from `Base::Rotation` which uses `(x, y, z, w)` ordering; the adapter layer handles the swap.
+
+**BaseJointKind** -- 24 primitive constraint types decomposed from FreeCAD's `JointType` and `DistanceType` enums. Covers point constraints (Coincident, PointOnLine, PointInPlane), axis/surface constraints (Concentric, Tangent, Planar), kinematic joints (Fixed, Revolute, Cylindrical, Slider, Ball, Screw, Universal), mechanical elements (Gear, RackPinion), distance variants, and a `Custom` extension point.
+
+**SolveContext** -- complete solver input: parts (with placements, mass, grounded flag), constraints (with markers, parameters, limits), optional motion definitions and simulation parameters.
+
+**SolveResult** -- solver output: status code, updated part placements, DOF count, constraint diagnostics, and simulation frame count.
+
+## SolverRegistry
+
+Thread-safe singleton managing solver backends:
+
+```cpp
+auto& reg = SolverRegistry::instance();
+
+// Registration (at module init)
+reg.register_solver("ondsel", []() {
+    return std::make_unique<OndselAdapter>();
+});
+
+// Retrieval
+auto solver = reg.get();          // default solver
+auto solver = reg.get("ondsel");  // by name
+
+// Queries
+reg.available();          // ["ondsel", ...]
+reg.joints_for("ondsel"); // [Fixed, Revolute, ...]
+reg.set_default("ondsel");
+```
+
+Plugin discovery scans directories for shared libraries exporting `kcsolve_api_version()` and `kcsolve_create()`. Default paths: `KCSOLVE_PLUGIN_PATH` env var and `<prefix>/lib/kcsolve/`.
+
+## OndselAdapter
+
+The built-in solver backend wrapping OndselSolver's Lagrangian constraint formulation. Registered as `"ondsel"` at Assembly module initialization.
+
+Supports all 24 joint types. The adapter translates between `SolveContext`/`SolveResult` and OndselSolver's internal `ASMTAssembly` representation, including:
+
+- Part placement conversion (Transform <-> Base::Placement quaternion ordering)
+- Constraint parameter mapping (BaseJointKind -> OndselSolver joint classes)
+- Interactive drag protocol (pre_drag/drag_step/post_drag)
+- Kinematic simulation (run_kinematic/num_frames/update_for_frame)
+- Constraint diagnostics (redundancy detection via MbD system)
+
+## Python bindings (kcsolve module)
+
+The `kcsolve` pybind11 module exposes the full C++ API to Python. See [KCSolve Python API](../reference/kcsolve-python.md) for details.
+
+Key capabilities:
+- All enums, structs, and classes accessible from Python
+- Subclass `IKCSolver` in pure Python to create new solver backends
+- Register Python solvers at runtime via `kcsolve.register_solver()`
+- Query the registry from the FreeCAD console
+
+## File layout
+
+```
+src/Mod/Assembly/Solver/
+├── Types.h              # Enums and structs (no FreeCAD deps)
+├── IKCSolver.h          # Abstract solver interface
+├── SolverRegistry.h/cpp # Singleton registry + plugin loading
+├── OndselAdapter.h/cpp  # OndselSolver wrapper
+├── KCSolveGlobal.h      # DLL export macros
+├── CMakeLists.txt       # Builds libKCSolve.so
+└── bindings/
+    ├── PyIKCSolver.h    # pybind11 trampoline for Python subclasses
+    ├── kcsolve_py.cpp   # Module definition (enums, structs, classes)
+    └── CMakeLists.txt   # Builds kcsolve.so (pybind11 module)
+```
+
+## Testing
+
+- **18 C++ tests** (`KCSolve_tests_run`) covering SolverRegistry (8 tests) and OndselAdapter (10 tests including drag protocol and redundancy diagnosis)
+- **16 Python tests** (`TestKCSolvePy`) covering module import, type bindings, registry functions, Python solver subclassing, and full register/load/solve round-trips
+- **6 Python integration tests** (`TestSolverIntegration`) testing solver behavior through FreeCAD document objects
+
+## Related
+
+- [KCSolve Python API Reference](../reference/kcsolve-python.md)
+- [INTER_SOLVER.md](../../INTER_SOLVER.md) -- full architecture specification

docs/src/reference/kcsolve-python.md

@@ -0,0 +1,429 @@
+# KCSolve Python API Reference
+
+The `kcsolve` module provides Python access to the KCSolve pluggable solver framework. It is built with pybind11 and installed alongside the Assembly module.
+
+```python
+import kcsolve
+```
+
+## Module constants
+
+| Name | Value | Description |
+|------|-------|-------------|
+| `API_VERSION_MAJOR` | `1` | KCSolve API major version |
+
+## Enums
+
+### BaseJointKind
+
+Primitive constraint types. 24 values:
+
+`Coincident`, `PointOnLine`, `PointInPlane`, `Concentric`, `Tangent`, `Planar`, `LineInPlane`, `Parallel`, `Perpendicular`, `Angle`, `Fixed`, `Revolute`, `Cylindrical`, `Slider`, `Ball`, `Screw`, `Universal`, `Gear`, `RackPinion`, `Cam`, `Slot`, `DistancePointPoint`, `DistanceCylSph`, `Custom`
+
+### SolveStatus
+
+| Value | Meaning |
+|-------|---------|
+| `Success` | Solve converged |
+| `Failed` | Solve did not converge |
+| `InvalidFlip` | Orientation flipped past threshold |
+| `NoGroundedParts` | No grounded parts in assembly |
+
+### DiagnosticKind
+
+`Redundant`, `Conflicting`, `PartiallyRedundant`, `Malformed`
+
+### MotionKind
+
+`Rotational`, `Translational`, `General`
+
+### LimitKind
+
+`TranslationMin`, `TranslationMax`, `RotationMin`, `RotationMax`
+
+## Structs
+
+### Transform
+
+Rigid-body transform: position + unit quaternion.
+
+| Field | Type | Default | Description |
+|-------|------|---------|-------------|
+| `position` | `list[float]` (3) | `[0, 0, 0]` | Translation (x, y, z) |
+| `quaternion` | `list[float]` (4) | `[1, 0, 0, 0]` | Unit quaternion (w, x, y, z) |
+
+```python
+t = kcsolve.Transform()
+t = kcsolve.Transform.identity()  # same as default
+```
+
+Note: quaternion convention is `(w, x, y, z)`, which differs from FreeCAD's `Base.Rotation(x, y, z, w)`. The adapter layer handles conversion.
+
+### Part
+
+| Field | Type | Default |
+|-------|------|---------|
+| `id` | `str` | `""` |
+| `placement` | `Transform` | identity |
+| `mass` | `float` | `1.0` |
+| `grounded` | `bool` | `False` |
+
+### Constraint
+
+A constraint between two parts, built from a FreeCAD JointObject by the adapter layer.
+
+| Field | Type | Default | Description |
+|-------|------|---------|-------------|
+| `id` | `str` | `""` | FreeCAD document object name (e.g. `"Joint001"`) |
+| `part_i` | `str` | `""` | Solver-side part ID for first reference |
+| `marker_i` | `Transform` | identity | Coordinate system on `part_i` (attachment point/orientation) |
+| `part_j` | `str` | `""` | Solver-side part ID for second reference |
+| `marker_j` | `Transform` | identity | Coordinate system on `part_j` (attachment point/orientation) |
+| `type` | `BaseJointKind` | `Coincident` | Constraint type |
+| `params` | `list[float]` | `[]` | Scalar parameters (interpretation depends on `type`) |
+| `limits` | `list[Constraint.Limit]` | `[]` | Joint travel limits |
+| `activated` | `bool` | `True` | Whether this constraint is active |
+
+**`marker_i` / `marker_j`** -- Define the local coordinate frames on each part where the joint acts. For example, a Revolute joint's markers define the hinge axis direction and attachment points on each part.
+
+**`params`** -- Interpretation depends on `type`:
+
+| Type | params[0] | params[1] |
+|------|-----------|-----------|
+| `Angle` | angle (radians) | |
+| `RackPinion` | pitch radius | |
+| `Screw` | pitch | |
+| `Gear` | radius I | radius J (negative for belt) |
+| `DistancePointPoint` | distance | |
+| `DistanceCylSph` | distance | |
+| `Planar` | offset | |
+| `Concentric` | distance | |
+| `PointInPlane` | offset | |
+| `LineInPlane` | offset | |
+
+### Constraint.Limit
+
+Joint travel limits (translation or rotation bounds).
+
+| Field | Type | Default | Description |
+|-------|------|---------|-------------|
+| `kind` | `LimitKind` | `TranslationMin` | Which degree of freedom to limit |
+| `value` | `float` | `0.0` | Limit value (meters for translation, radians for rotation) |
+| `tolerance` | `float` | `1e-9` | Solver tolerance for limit enforcement |
+
+### MotionDef
+
+A motion driver for kinematic simulation. Defines time-dependent actuation of a constraint.
+
+| Field | Type | Default | Description |
+|-------|------|---------|-------------|
+| `kind` | `MotionKind` | `Rotational` | Type of motion: `Rotational`, `Translational`, or `General` (both) |
+| `joint_id` | `str` | `""` | ID of the constraint this motion drives |
+| `marker_i` | `str` | `""` | Reference marker on first part |
+| `marker_j` | `str` | `""` | Reference marker on second part |
+| `rotation_expr` | `str` | `""` | Rotation law as a function of time `t` (e.g. `"2*pi*t"`) |
+| `translation_expr` | `str` | `""` | Translation law as a function of time `t` (e.g. `"10*t"`) |
+
+For `Rotational` kind, only `rotation_expr` is used. For `Translational`, only `translation_expr`. For `General`, both are set.
+
+### SimulationParams
+
+Time-stepping parameters for kinematic simulation via `run_kinematic()`.
+
+| Field | Type | Default | Description |
+|-------|------|---------|-------------|
+| `t_start` | `float` | `0.0` | Simulation start time (seconds) |
+| `t_end` | `float` | `1.0` | Simulation end time (seconds) |
+| `h_out` | `float` | `0.01` | Output time step -- controls frame rate (e.g. `0.04` = 25 fps) |
+| `h_min` | `float` | `1e-9` | Minimum internal integration step |
+| `h_max` | `float` | `1.0` | Maximum internal integration step |
+| `error_tol` | `float` | `1e-6` | Error tolerance for adaptive time stepping |
+
+### SolveContext
+
+Complete input to a solve operation. Built by the adapter layer from FreeCAD document objects, or constructed manually for scripted solving.
+
+| Field | Type | Default | Description |
+|-------|------|---------|-------------|
+| `parts` | `list[Part]` | `[]` | All parts in the assembly |
+| `constraints` | `list[Constraint]` | `[]` | Constraints between parts |
+| `motions` | `list[MotionDef]` | `[]` | Motion drivers for kinematic simulation |
+| `simulation` | `SimulationParams` or `None` | `None` | Time-stepping parameters for `run_kinematic()` |
+| `bundle_fixed` | `bool` | `False` | Hint to merge Fixed-joint-connected parts into rigid bodies |
+
+**`motions`** -- Motion drivers define time-dependent joint actuation for kinematic simulation. Each `MotionDef` references a constraint by `joint_id` and provides expressions (functions of time `t`) for rotation and/or translation. Only used when calling `run_kinematic()`.
+
+**`simulation`** -- When set, provides time-stepping parameters (`t_start`, `t_end`, step sizes, error tolerance) for kinematic simulation via `run_kinematic()`. When `None`, kinematic simulation is not requested.
+
+**`bundle_fixed`** -- When `True`, parts connected by `Fixed` joints should be merged into single rigid bodies before solving, reducing the problem size. If the solver reports `supports_bundle_fixed() == True`, it handles this internally. Otherwise, the caller (adapter layer) pre-bundles before building the context.
+
+**Important:** pybind11 returns copies of `list` fields, not references. Use whole-list assignment:
+
+```python
+ctx = kcsolve.SolveContext()
+p = kcsolve.Part()
+p.id = "box1"
+ctx.parts = [p]          # correct
+# ctx.parts.append(p)    # does NOT modify ctx
+```
+
+### ConstraintDiagnostic
+
+| Field | Type | Default |
+|-------|------|---------|
+| `constraint_id` | `str` | `""` |
+| `kind` | `DiagnosticKind` | `Redundant` |
+| `detail` | `str` | `""` |
+
+### SolveResult
+
+| Field | Type | Default |
+|-------|------|---------|
+| `status` | `SolveStatus` | `Success` |
+| `placements` | `list[SolveResult.PartResult]` | `[]` |
+| `dof` | `int` | `-1` |
+| `diagnostics` | `list[ConstraintDiagnostic]` | `[]` |
+| `num_frames` | `int` | `0` |
+
+### SolveResult.PartResult
+
+| Field | Type | Default |
+|-------|------|---------|
+| `id` | `str` | `""` |
+| `placement` | `Transform` | identity |
+
+## Classes
+
+### IKCSolver
+
+Abstract base class for solver backends. Subclass in Python to create custom solvers.
+
+Three methods must be implemented:
+
+```python
+class MySolver(kcsolve.IKCSolver):
+    def name(self):
+        return "My Solver"
+
+    def supported_joints(self):
+        return [kcsolve.BaseJointKind.Fixed, kcsolve.BaseJointKind.Revolute]
+
+    def solve(self, ctx):
+        result = kcsolve.SolveResult()
+        result.status = kcsolve.SolveStatus.Success
+        return result
+```
+
+All other methods are optional and have default implementations. Override them to add capabilities beyond basic solving.
+
+#### update(ctx) -> SolveResult
+
+Incrementally re-solve after parameter changes (e.g. joint angle adjusted during creation). Solvers can optimize this path since only parameters changed, not topology. Default: delegates to `solve()`.
+
+```python
+def update(self, ctx):
+    # Only re-evaluate changed constraints, reuse cached factorization
+    return self._incremental_solve(ctx)
+```
+
+#### Interactive drag protocol
+
+Three-phase protocol for interactive part dragging in the viewport. Solvers can maintain internal state across the drag session for better performance.
+
+**pre_drag(ctx, drag_parts) -> SolveResult** -- Prepare for a drag session. `drag_parts` is a `list[str]` of part IDs being dragged. Solve the initial state and cache internal data. Default: delegates to `solve()`.
+
+**drag_step(drag_placements) -> SolveResult** -- Called on each mouse move. `drag_placements` is a `list[SolveResult.PartResult]` with the current positions of dragged parts. Returns updated placements for all affected parts. Default: returns Success with no placements.
+
+**post_drag()** -- End the drag session and release internal state. Default: no-op.
+
+```python
+def pre_drag(self, ctx, drag_parts):
+    self._cached_system = self._build_system(ctx)
+    return self.solve(ctx)
+
+def drag_step(self, drag_placements):
+    # Use cached system for fast incremental solve
+    for dp in drag_placements:
+        self._cached_system.set_placement(dp.id, dp.placement)
+    return self._cached_system.solve_incremental()
+
+def post_drag(self):
+    self._cached_system = None
+```
+
+#### Kinematic simulation
+
+**run_kinematic(ctx) -> SolveResult** -- Run a kinematic simulation over the time range in `ctx.simulation`. After this call, `num_frames()` returns the frame count and `update_for_frame(i)` retrieves individual frames. Requires `ctx.simulation` to be set and `ctx.motions` to contain at least one motion driver. Default: returns Failed.
+
+**num_frames() -> int** -- Number of simulation frames available after `run_kinematic()`. Default: returns 0.
+
+**update_for_frame(index) -> SolveResult** -- Retrieve part placements for simulation frame at `index` (0-based, must be < `num_frames()`). Default: returns Failed.
+
+```python
+# Run a kinematic simulation
+ctx.simulation = kcsolve.SimulationParams()
+ctx.simulation.t_start = 0.0
+ctx.simulation.t_end = 2.0
+ctx.simulation.h_out = 0.04  # 25 fps
+
+motion = kcsolve.MotionDef()
+motion.kind = kcsolve.MotionKind.Rotational
+motion.joint_id = "Joint001"
+motion.rotation_expr = "2*pi*t"  # one revolution per second
+ctx.motions = [motion]
+
+solver = kcsolve.load("ondsel")
+result = solver.run_kinematic(ctx)
+
+for i in range(solver.num_frames()):
+    frame = solver.update_for_frame(i)
+    for pr in frame.placements:
+        print(f"frame {i}: {pr.id} at {list(pr.placement.position)}")
+```
+
+#### diagnose(ctx) -> list[ConstraintDiagnostic]
+
+Analyze the assembly for redundant, conflicting, or malformed constraints. May require a prior `solve()` call for some solvers. Returns a list of `ConstraintDiagnostic` objects. Default: returns empty list.
+
+```python
+diags = solver.diagnose(ctx)
+for d in diags:
+    if d.kind == kcsolve.DiagnosticKind.Redundant:
+        print(f"Redundant: {d.constraint_id} - {d.detail}")
+    elif d.kind == kcsolve.DiagnosticKind.Conflicting:
+        print(f"Conflict: {d.constraint_id} - {d.detail}")
+```
+
+#### is_deterministic() -> bool
+
+Whether this solver produces identical results given identical input. Used for regression testing and result caching. Default: returns `True`.
+
+#### export_native(path)
+
+Write a solver-native debug/diagnostic file (e.g. ASMT format for OndselSolver). Requires a prior `solve()` or `run_kinematic()` call. Default: no-op.
+
+```python
+solver.solve(ctx)
+solver.export_native("/tmp/debug.asmt")
+```
+
+#### supports_bundle_fixed() -> bool
+
+Whether this solver handles Fixed-joint part bundling internally. When `False`, the caller merges Fixed-joint-connected parts into single rigid bodies before building the `SolveContext`, reducing problem size. When `True`, the solver receives unbundled parts and optimizes internally. Default: returns `False`.
+
+### OndselAdapter
+
+Built-in solver wrapping OndselSolver's Lagrangian constraint formulation. Inherits `IKCSolver`.
+
+```python
+solver = kcsolve.OndselAdapter()
+solver.name()  # "OndselSolver (Lagrangian)"
+```
+
+In practice, use `kcsolve.load("ondsel")` rather than constructing directly, as this goes through the registry.
+
+## Module functions
+
+### available()
+
+Return names of all registered solvers.
+
+```python
+kcsolve.available()  # ["ondsel"]
+```
+
+### load(name="")
+
+Create an instance of the named solver. If `name` is empty, uses the default. Returns `None` if the solver is not found.
+
+```python
+solver = kcsolve.load("ondsel")
+solver = kcsolve.load()  # default solver
+```
+
+### joints_for(name)
+
+Query supported joint types for a registered solver.
+
+```python
+joints = kcsolve.joints_for("ondsel")
+# [BaseJointKind.Coincident, BaseJointKind.Fixed, ...]
+```
+
+### set_default(name)
+
+Set the default solver name. Returns `True` if the name is registered.
+
+```python
+kcsolve.set_default("ondsel")  # True
+kcsolve.set_default("unknown")  # False
+```
+
+### get_default()
+
+Get the current default solver name.
+
+```python
+kcsolve.get_default()  # "ondsel"
+```
+
+### register_solver(name, solver_class)
+
+Register a Python solver class with the SolverRegistry. `solver_class` must be a callable that returns an `IKCSolver` subclass instance.
+
+```python
+class MySolver(kcsolve.IKCSolver):
+    def name(self): return "MySolver"
+    def supported_joints(self): return [kcsolve.BaseJointKind.Fixed]
+    def solve(self, ctx):
+        r = kcsolve.SolveResult()
+        r.status = kcsolve.SolveStatus.Success
+        return r
+
+kcsolve.register_solver("my_solver", MySolver)
+solver = kcsolve.load("my_solver")
+```
+
+## Complete example
+
+```python
+import kcsolve
+
+# Build a two-part assembly with a Fixed joint
+ctx = kcsolve.SolveContext()
+
+base = kcsolve.Part()
+base.id = "base"
+base.grounded = True
+
+arm = kcsolve.Part()
+arm.id = "arm"
+arm.placement.position = [100.0, 0.0, 0.0]
+
+joint = kcsolve.Constraint()
+joint.id = "Joint001"
+joint.part_i = "base"
+joint.part_j = "arm"
+joint.type = kcsolve.BaseJointKind.Fixed
+
+ctx.parts = [base, arm]
+ctx.constraints = [joint]
+
+# Solve
+solver = kcsolve.load("ondsel")
+result = solver.solve(ctx)
+
+print(result.status)  # SolveStatus.Success
+for pr in result.placements:
+    print(f"{pr.id}: pos={list(pr.placement.position)}")
+
+# Diagnostics
+diags = solver.diagnose(ctx)
+for d in diags:
+    print(f"{d.constraint_id}: {d.kind} - {d.detail}")
+```
+
+## Related
+
+- [KCSolve Architecture](../architecture/ondsel-solver.md)
+- [INTER_SOLVER.md](../../INTER_SOLVER.md) -- full architecture specification

docs/src/silo-server/MODULES.md

@@ -23,7 +23,7 @@ These cannot be disabled. They define what Silo *is*.
 |-----------|------|-------------|
 | `core` | Core PDM | Items, revisions, files, BOM, search, import/export, part number generation |
 | `schemas` | Schemas | Part numbering schema parsing, segment management, form descriptors |
-| `storage` | Storage | MinIO/S3 file storage, presigned uploads, versioning |
+| `storage` | Storage | Filesystem storage |
 
 ### 2.2 Optional Modules
 
@@ -470,12 +470,10 @@ Returns full config grouped by module with secrets redacted:
     "default": "kindred-rd"
   },
   "storage": {
-    "endpoint": "minio:9000",
+    "backend": "filesystem",
-    "bucket": "silo-files",
+    "filesystem": {
-    "access_key": "****",
+      "root_dir": "/var/lib/silo/data"
-    "secret_key": "****",
+    },
-    "use_ssl": false,
-    "region": "us-east-1",
     "status": "connected"
   },
   "database": {
@@ -566,7 +564,7 @@ Available for modules with external connections:
 
 | Module | Test Action |
 |--------|------------|
-| `storage` | Ping MinIO, verify bucket exists |
+| `storage` | Verify filesystem storage directory is accessible |
 | `auth` (ldap) | Attempt LDAP bind with configured credentials |
 | `auth` (oidc) | Fetch OIDC discovery document from issuer URL |
 | `odoo` | Attempt XML-RPC connection to Odoo |
@@ -602,11 +600,9 @@ database:
   sslmode: disable
 
 storage:
-  endpoint: minio:9000
+  backend: filesystem
-  bucket: silo-files
+  filesystem:
-  access_key: silominio
+    root_dir: /var/lib/silo/data
-  secret_key: silominiosecret
-  use_ssl: false
 
 schemas:
   directory: /etc/silo/schemas

docs/src/silo-server/SOLVER.md

@@ -0,0 +1,899 @@
+# Solver Service Specification
+
+**Status:** Draft
+**Last Updated:** 2026-02-19
+**Depends on:** KCSolve Phase 1 (PR #297), Phase 2 (PR #298)
+
+---
+
+## 1. Overview
+
+The solver service extends Silo's job queue system with assembly constraint solving capabilities. It enables server-side solving of assemblies stored in Silo, with results streamed back to clients in real time via SSE.
+
+This specification describes how the existing KCSolve client-side API (C++ library + pybind11 `kcsolve` module) integrates with Silo's worker infrastructure to provide headless, asynchronous constraint solving.
+
+### 1.1 Goals
+
+1. **Offload solving** -- Move heavy solve operations off the user's machine to server workers.
+2. **Batch validation** -- Automatically validate assemblies on commit (e.g. check for over-constrained systems).
+3. **Solver selection** -- Allow the server to run different solvers than the client (e.g. a more thorough solver for validation, a fast one for interactive editing).
+4. **Standalone execution** -- Solver workers can run without a full FreeCAD installation, using just the `kcsolve` Python module and the `.kc` file.
+
+### 1.2 Non-Goals
+
+- **Interactive drag** -- Real-time drag solving stays client-side (latency-sensitive).
+- **Geometry processing** -- Workers don't compute geometry; they receive pre-extracted constraint graphs.
+- **Solver development** -- Writing new solver backends is out of scope; this spec covers the transport and execution layer.
+
+---
+
+## 2. Architecture
+
+```
+                          ┌─────────────────────┐
+                          │   Kindred Create     │
+                          │   (FreeCAD client)   │
+                          └───────┬──────────────┘
+                                  │ 1. POST /api/solver/jobs
+                                  │    (SolveContext JSON)
+                                  │
+                                  │ 4. GET /api/events (SSE)
+                                  │    solver.progress, solver.completed
+                                  ▼
+                          ┌─────────────────────┐
+                          │   Silo Server        │
+                          │   (silod)            │
+                          │                     │
+                          │   solver module      │
+                          │   REST + SSE + queue  │
+                          └───────┬──────────────┘
+                                  │ 2. POST /api/runner/claim
+                                  │ 3. POST /api/runner/jobs/{id}/complete
+                                  ▼
+                          ┌─────────────────────┐
+                          │   Solver Runner      │
+                          │   (silorunner)       │
+                          │                     │
+                          │   kcsolve module     │
+                          │   OndselAdapter      │
+                          │   Python solvers     │
+                          └─────────────────────┘
+```
+
+### 2.1 Components
+
+| Component | Role | Deployment |
+|-----------|------|------------|
+| **Silo server** | Job queue management, REST API, SSE broadcast, result storage | Existing `silod` binary |
+| **Solver runner** | Claims solver jobs, executes `kcsolve`, reports results | New runner tag `solver` on existing `silorunner` |
+| **kcsolve module** | Python/C++ solver library (Phase 1+2) | Installed on runner nodes |
+| **Create client** | Submits jobs, receives results via SSE | Existing FreeCAD client |
+
+### 2.2 Module Registration
+
+The solver service is a Silo module with ID `solver`, gated behind the existing module system:
+
+```yaml
+# config.yaml
+modules:
+  solver:
+    enabled: true
+```
+
+It depends on the `jobs` module being enabled. All solver endpoints return `404` with `{"error": "module not enabled"}` when disabled.
+
+---
+
+## 3. Data Model
+
+### 3.1 SolveContext JSON Schema
+
+The `SolveContext` is the input to a solve operation. Currently it exists only as a C++ struct and pybind11 binding with no serialization. Phase 3 adds JSON serialization to enable server transport.
+
+```json
+{
+  "api_version": 1,
+  "parts": [
+    {
+      "id": "Part001",
+      "placement": {
+        "position": [0.0, 0.0, 0.0],
+        "quaternion": [1.0, 0.0, 0.0, 0.0]
+      },
+      "mass": 1.0,
+      "grounded": true
+    },
+    {
+      "id": "Part002",
+      "placement": {
+        "position": [100.0, 0.0, 0.0],
+        "quaternion": [1.0, 0.0, 0.0, 0.0]
+      },
+      "mass": 1.0,
+      "grounded": false
+    }
+  ],
+  "constraints": [
+    {
+      "id": "Joint001",
+      "part_i": "Part001",
+      "marker_i": {
+        "position": [50.0, 0.0, 0.0],
+        "quaternion": [1.0, 0.0, 0.0, 0.0]
+      },
+      "part_j": "Part002",
+      "marker_j": {
+        "position": [0.0, 0.0, 0.0],
+        "quaternion": [1.0, 0.0, 0.0, 0.0]
+      },
+      "type": "Revolute",
+      "params": [],
+      "limits": [],
+      "activated": true
+    }
+  ],
+  "motions": [],
+  "simulation": null,
+  "bundle_fixed": false
+}
+```
+
+**Field reference:** See [KCSolve Python API](../reference/kcsolve-python.md) for full field documentation. The JSON schema maps 1:1 to the Python/C++ types.
+
+**Enum serialization:** Enums serialize as strings matching their Python names (e.g. `"Revolute"`, `"Success"`, `"Redundant"`).
+
+**Transform shorthand:** The `placement` and `marker_*` fields use the `Transform` struct: `position` is `[x, y, z]`, `quaternion` is `[w, x, y, z]`.
+
+**Constraint.Limit:**
+```json
+{
+  "kind": "RotationMin",
+  "value": -1.5708,
+  "tolerance": 1e-9
+}
+```
+
+**MotionDef:**
+```json
+{
+  "kind": "Rotational",
+  "joint_id": "Joint001",
+  "marker_i": "",
+  "marker_j": "",
+  "rotation_expr": "2*pi*t",
+  "translation_expr": ""
+}
+```
+
+**SimulationParams:**
+```json
+{
+  "t_start": 0.0,
+  "t_end": 2.0,
+  "h_out": 0.04,
+  "h_min": 1e-9,
+  "h_max": 1.0,
+  "error_tol": 1e-6
+}
+```
+
+### 3.2 SolveResult JSON Schema
+
+```json
+{
+  "status": "Success",
+  "placements": [
+    {
+      "id": "Part002",
+      "placement": {
+        "position": [50.0, 0.0, 0.0],
+        "quaternion": [0.707, 0.0, 0.707, 0.0]
+      }
+    }
+  ],
+  "dof": 1,
+  "diagnostics": [
+    {
+      "constraint_id": "Joint003",
+      "kind": "Redundant",
+      "detail": "6 DOF removed by Joint003 are already constrained"
+    }
+  ],
+  "num_frames": 0
+}
+```
+
+### 3.3 Solver Job Record
+
+Solver jobs are stored in the existing `jobs` table. The solver-specific data is in the `args` and `result` JSONB columns.
+
+**Job args (input):**
+```json
+{
+  "solver": "ondsel",
+  "operation": "solve",
+  "context": { /* SolveContext JSON */ },
+  "item_part_number": "ASM-001",
+  "revision_number": 3
+}
+```
+
+**Operation types:**
+| Operation | Description | Requires simulation? |
+|-----------|-------------|---------------------|
+| `solve` | Static equilibrium solve | No |
+| `diagnose` | Constraint analysis only (no placement update) | No |
+| `kinematic` | Time-domain kinematic simulation | Yes |
+
+**Job result (output):**
+```json
+{
+  "result": { /* SolveResult JSON */ },
+  "solver_name": "OndselSolver (Lagrangian)",
+  "solver_version": "1.0",
+  "solve_time_ms": 127.4
+}
+```
+
+---
+
+## 4. REST API
+
+All endpoints are prefixed with `/api/solver/` and gated behind `RequireModule("solver")`.
+
+### 4.1 Submit Solve Job
+
+```
+POST /api/solver/jobs
+Authorization: Bearer silo_...
+Content-Type: application/json
+
+{
+  "solver": "ondsel",
+  "operation": "solve",
+  "context": { /* SolveContext */ },
+  "priority": 50
+}
+```
+
+**Optional fields:**
+| Field | Type | Default | Description |
+|-------|------|---------|-------------|
+| `solver` | string | `""` (default solver) | Solver name from registry |
+| `operation` | string | `"solve"` | `solve`, `diagnose`, or `kinematic` |
+| `context` | object | required | SolveContext JSON |
+| `priority` | int | `50` | Lower = higher priority |
+| `item_part_number` | string | `null` | Silo item reference (for result association) |
+| `revision_number` | int | `null` | Revision that generated this context |
+| `callback_url` | string | `null` | Webhook URL for completion notification |
+
+**Response `201 Created`:**
+```json
+{
+  "job_id": "550e8400-e29b-41d4-a716-446655440000",
+  "status": "pending",
+  "created_at": "2026-02-19T18:30:00Z"
+}
+```
+
+**Error responses:**
+| Code | Condition |
+|------|-----------|
+| `400` | Invalid SolveContext (missing required fields, unknown enum values) |
+| `401` | Not authenticated |
+| `404` | Module not enabled |
+| `422` | Unknown solver name, invalid operation |
+
+### 4.2 Get Job Status
+
+```
+GET /api/solver/jobs/{jobID}
+```
+
+**Response `200 OK`:**
+```json
+{
+  "job_id": "550e8400-...",
+  "status": "completed",
+  "operation": "solve",
+  "solver": "ondsel",
+  "priority": 50,
+  "item_part_number": "ASM-001",
+  "revision_number": 3,
+  "runner_id": "runner-01",
+  "runner_name": "solver-worker-01",
+  "created_at": "2026-02-19T18:30:00Z",
+  "claimed_at": "2026-02-19T18:30:01Z",
+  "completed_at": "2026-02-19T18:30:02Z",
+  "result": {
+    "result": { /* SolveResult */ },
+    "solver_name": "OndselSolver (Lagrangian)",
+    "solve_time_ms": 127.4
+  }
+}
+```
+
+### 4.3 List Solver Jobs
+
+```
+GET /api/solver/jobs?status=completed&item=ASM-001&limit=20&offset=0
+```
+
+**Query parameters:**
+| Param | Type | Description |
+|-------|------|-------------|
+| `status` | string | Filter by status: `pending`, `claimed`, `running`, `completed`, `failed` |
+| `item` | string | Filter by item part number |
+| `operation` | string | Filter by operation type |
+| `solver` | string | Filter by solver name |
+| `limit` | int | Page size (default 20, max 100) |
+| `offset` | int | Pagination offset |
+
+**Response `200 OK`:**
+```json
+{
+  "jobs": [ /* array of job objects */ ],
+  "total": 42,
+  "limit": 20,
+  "offset": 0
+}
+```
+
+### 4.4 Cancel Job
+
+```
+POST /api/solver/jobs/{jobID}/cancel
+```
+
+Only `pending` and `claimed` jobs can be cancelled. Running jobs must complete or time out.
+
+**Response `200 OK`:**
+```json
+{
+  "job_id": "550e8400-...",
+  "status": "cancelled"
+}
+```
+
+### 4.5 Get Solver Registry
+
+```
+GET /api/solver/solvers
+```
+
+Returns available solvers on registered runners. Runners report their solver capabilities during heartbeat.
+
+**Response `200 OK`:**
+```json
+{
+  "solvers": [
+    {
+      "name": "ondsel",
+      "display_name": "OndselSolver (Lagrangian)",
+      "deterministic": true,
+      "supported_joints": [
+        "Coincident", "Fixed", "Revolute", "Cylindrical",
+        "Slider", "Ball", "Screw", "Gear", "RackPinion",
+        "Parallel", "Perpendicular", "Angle", "Planar",
+        "Concentric", "PointOnLine", "PointInPlane",
+        "LineInPlane", "Tangent", "DistancePointPoint",
+        "DistanceCylSph", "Universal"
+      ],
+      "runner_count": 2
+    }
+  ],
+  "default_solver": "ondsel"
+}
+```
+
+---
+
+## 5. Server-Sent Events
+
+Solver jobs emit events on the existing `/api/events` SSE stream.
+
+### 5.1 Event Types
+
+| Event | Payload | When |
+|-------|---------|------|
+| `solver.created` | `{job_id, operation, solver, item_part_number}` | Job submitted |
+| `solver.claimed` | `{job_id, runner_id, runner_name}` | Runner starts work |
+| `solver.progress` | `{job_id, progress, message}` | Progress update (0-100) |
+| `solver.completed` | `{job_id, status, dof, diagnostics_count, solve_time_ms}` | Job succeeded |
+| `solver.failed` | `{job_id, error_message}` | Job failed |
+
+### 5.2 Example Stream
+
+```
+event: solver.created
+data: {"job_id":"abc-123","operation":"solve","solver":"ondsel","item_part_number":"ASM-001"}
+
+event: solver.claimed
+data: {"job_id":"abc-123","runner_id":"r1","runner_name":"solver-worker-01"}
+
+event: solver.progress
+data: {"job_id":"abc-123","progress":50,"message":"Building constraint system..."}
+
+event: solver.completed
+data: {"job_id":"abc-123","status":"Success","dof":3,"diagnostics_count":1,"solve_time_ms":127.4}
+```
+
+### 5.3 Client Integration
+
+The Create client subscribes to the SSE stream and updates the Assembly workbench UI:
+
+1. **Silo viewport widget** shows job status indicator (pending/running/done/failed)
+2. On `solver.completed`, the client can fetch the full result via `GET /api/solver/jobs/{id}` and apply placements
+3. On `solver.failed`, the client shows the error in the report panel
+4. Diagnostic results (redundant/conflicting constraints) surface in the constraint tree
+
+---
+
+## 6. Runner Integration
+
+### 6.1 Runner Requirements
+
+Solver runners are standard `silorunner` instances with the `solver` tag. They require:
+
+- Python 3.11+ with `kcsolve` module installed
+- `libKCSolve.so` and solver backend libraries (e.g. `libOndselSolver.so`)
+- Network access to the Silo server
+
+No FreeCAD installation is required. The runner operates on pre-extracted `SolveContext` JSON.
+
+### 6.2 Runner Registration
+
+```bash
+# Register a solver runner (admin)
+curl -X POST https://silo.example.com/api/runners \
+  -H "Authorization: Bearer admin_token" \
+  -d '{"name":"solver-01","tags":["solver"]}'
+
+# Response includes one-time token
+{"id":"uuid","token":"silo_runner_xyz..."}
+```
+
+### 6.3 Runner Heartbeat
+
+Runners report solver capabilities during heartbeat:
+
+```json
+POST /api/runner/heartbeat
+{
+  "capabilities": {
+    "solvers": ["ondsel"],
+    "api_version": 1,
+    "python_version": "3.11.11"
+  }
+}
+```
+
+### 6.4 Runner Execution Flow
+
+```python
+#!/usr/bin/env python3
+"""Solver runner entry point."""
+
+import json
+import kcsolve
+
+
+def execute_solve_job(args: dict) -> dict:
+    """Execute a solver job from parsed args."""
+    solver_name = args.get("solver", "")
+    operation = args.get("operation", "solve")
+    ctx_dict = args["context"]
+
+    # Deserialize SolveContext from JSON
+    ctx = kcsolve.SolveContext.from_dict(ctx_dict)
+
+    # Load solver
+    solver = kcsolve.load(solver_name)
+    if solver is None:
+        raise ValueError(f"Unknown solver: {solver_name!r}")
+
+    # Execute operation
+    if operation == "solve":
+        result = solver.solve(ctx)
+    elif operation == "diagnose":
+        diags = solver.diagnose(ctx)
+        result = kcsolve.SolveResult()
+        result.diagnostics = diags
+    elif operation == "kinematic":
+        result = solver.run_kinematic(ctx)
+    else:
+        raise ValueError(f"Unknown operation: {operation!r}")
+
+    # Serialize result
+    return {
+        "result": result.to_dict(),
+        "solver_name": solver.name(),
+        "solver_version": "1.0",
+    }
+```
+
+### 6.5 Standalone Process Mode
+
+For minimal deployments, the runner can invoke a standalone solver process:
+
+```bash
+echo '{"solver":"ondsel","operation":"solve","context":{...}}' | \
+  python3 -m kcsolve.runner
+```
+
+The `kcsolve.runner` module reads JSON from stdin, executes the solve, and writes the result JSON to stdout. Exit code 0 = success, non-zero = failure with error JSON on stderr.
+
+---
+
+## 7. Job Definitions
+
+### 7.1 Manual Solve Job
+
+Triggered by the client when the user requests a server-side solve:
+
+```yaml
+job:
+  name: assembly-solve
+  version: 1
+  description: "Solve assembly constraints on server"
+
+  trigger:
+    type: manual
+
+  scope:
+    type: assembly
+
+  compute:
+    type: solver
+    command: solver-run
+
+  runner:
+    tags: [solver]
+
+  timeout: 300
+  max_retries: 1
+  priority: 50
+```
+
+### 7.2 Commit-Time Validation
+
+Automatically validates assembly constraints when a new revision is committed:
+
+```yaml
+job:
+  name: assembly-validate
+  version: 1
+  description: "Validate assembly constraints on commit"
+
+  trigger:
+    type: revision_created
+    filter:
+      item_type: assembly
+
+  scope:
+    type: assembly
+
+  compute:
+    type: solver
+    command: solver-diagnose
+    args:
+      operation: diagnose
+
+  runner:
+    tags: [solver]
+
+  timeout: 120
+  max_retries: 2
+  priority: 75
+```
+
+### 7.3 Kinematic Simulation
+
+Server-side kinematic simulation for assemblies with motion definitions:
+
+```yaml
+job:
+  name: assembly-kinematic
+  version: 1
+  description: "Run kinematic simulation"
+
+  trigger:
+    type: manual
+
+  scope:
+    type: assembly
+
+  compute:
+    type: solver
+    command: solver-kinematic
+    args:
+      operation: kinematic
+
+  runner:
+    tags: [solver]
+
+  timeout: 1800
+  max_retries: 0
+  priority: 100
+```
+
+---
+
+## 8. SolveContext Extraction
+
+When a solver job is triggered by a revision commit (rather than a direct context submission), the server or runner must extract a `SolveContext` from the `.kc` file.
+
+### 8.1 Extraction via Headless Create
+
+For full-fidelity extraction that handles geometry classification:
+
+```bash
+create --console -e "
+import kcsolve_extract
+kcsolve_extract.extract_and_solve('input.kc', 'output.json', solver='ondsel')
+"
+```
+
+This requires a full Create installation on the runner and uses the Assembly module's existing adapter layer to build `SolveContext` from document objects.
+
+### 8.2 Extraction from .kc Silo Directory
+
+For lightweight extraction without FreeCAD, the constraint graph can be stored in the `.kc` archive's `silo/` directory during commit:
+
+```
+silo/solver/context.json    # Pre-extracted SolveContext
+silo/solver/result.json     # Last solve result (if any)
+```
+
+The client extracts the `SolveContext` locally before committing the `.kc` file. The server reads it from the archive, avoiding the need for geometry processing on the runner.
+
+**Commit-time packing** (client side):
+```python
+# In the Assembly workbench commit hook:
+ctx = assembly_object.build_solve_context()
+kc_archive.write("silo/solver/context.json", ctx.to_json())
+```
+
+**Runner-side extraction:**
+```python
+import zipfile, json
+
+with zipfile.ZipFile("assembly.kc") as zf:
+    ctx_json = json.loads(zf.read("silo/solver/context.json"))
+```
+
+---
+
+## 9. Database Schema
+
+### 9.1 Migration
+
+The solver module uses the existing `jobs` table. One new table is added for result caching:
+
+```sql
+-- Migration: 020_solver_results.sql
+
+CREATE TABLE solver_results (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    item_id UUID NOT NULL REFERENCES items(id) ON DELETE CASCADE,
+    revision_number INTEGER NOT NULL,
+    job_id UUID REFERENCES jobs(id) ON DELETE SET NULL,
+    operation TEXT NOT NULL,         -- 'solve', 'diagnose', 'kinematic'
+    solver_name TEXT NOT NULL,
+    status TEXT NOT NULL,            -- SolveStatus string
+    dof INTEGER,
+    diagnostics JSONB DEFAULT '[]',
+    placements JSONB DEFAULT '[]',
+    num_frames INTEGER DEFAULT 0,
+    solve_time_ms DOUBLE PRECISION,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT now(),
+    UNIQUE(item_id, revision_number, operation)
+);
+
+CREATE INDEX idx_solver_results_item ON solver_results(item_id);
+CREATE INDEX idx_solver_results_status ON solver_results(status);
+```
+
+The `UNIQUE(item_id, revision_number, operation)` constraint means each revision has at most one result per operation type. Re-running overwrites the previous result.
+
+### 9.2 Result Association
+
+When a solver job completes, the server:
+1. Stores the full result in the `jobs.result` JSONB column (standard job result)
+2. Upserts a row in `solver_results` for quick lookup by item/revision
+3. Broadcasts `solver.completed` SSE event
+
+---
+
+## 10. Configuration
+
+### 10.1 Server Config
+
+```yaml
+# config.yaml
+modules:
+  solver:
+    enabled: true
+    default_solver: "ondsel"
+    max_context_size_mb: 10       # Reject oversized SolveContext payloads
+    default_timeout: 300          # Default job timeout (seconds)
+    auto_diagnose_on_commit: true # Auto-submit diagnose job on revision commit
+```
+
+### 10.2 Environment Variables
+
+| Variable | Description |
+|----------|-------------|
+| `SILO_SOLVER_ENABLED` | Override module enabled state |
+| `SILO_SOLVER_DEFAULT` | Default solver name |
+
+### 10.3 Runner Config
+
+```yaml
+# runner.yaml
+server_url: https://silo.example.com
+token: silo_runner_xyz...
+tags: [solver]
+
+solver:
+  kcsolve_path: /opt/create/lib  # LD_LIBRARY_PATH for kcsolve.so
+  python: /opt/create/bin/python3
+  max_concurrent: 2              # Parallel job slots per runner
+```
+
+---
+
+## 11. Security
+
+### 11.1 Authentication
+
+All solver endpoints use the existing Silo authentication:
+- **User endpoints** (`/api/solver/jobs`): Session or API token, requires `viewer` role to read, `editor` role to submit
+- **Runner endpoints** (`/api/runner/...`): Runner token authentication (existing)
+
+### 11.2 Input Validation
+
+The server validates SolveContext JSON before queuing:
+- Maximum payload size (configurable, default 10 MB)
+- Required fields present (`parts`, `constraints`)
+- Enum values are valid strings
+- Transform arrays have correct length (position: 3, quaternion: 4)
+- No duplicate part or constraint IDs
+
+### 11.3 Runner Isolation
+
+Solver runners execute untrusted constraint data. Mitigations:
+- Runners should run in containers or sandboxed environments
+- Python solver registration (`kcsolve.register_solver()`) is disabled in runner mode
+- Solver execution has a configurable timeout (killed on expiry)
+- Result size is bounded (large kinematic simulations are truncated)
+
+---
+
+## 12. Client SDK
+
+### 12.1 Python Client
+
+The existing `silo-client` package is extended with solver methods:
+
+```python
+from silo_client import SiloClient
+
+client = SiloClient("https://silo.example.com", token="silo_...")
+
+# Submit a solve job
+import kcsolve
+ctx = kcsolve.SolveContext()
+# ... build context ...
+
+job = client.solver.submit(ctx.to_dict(), solver="ondsel")
+print(job.id, job.status)  # "pending"
+
+# Poll for completion
+result = client.solver.wait(job.id, timeout=60)
+print(result.status)  # "Success"
+
+# Or use SSE for real-time updates
+for event in client.solver.stream(job.id):
+    print(event.type, event.data)
+
+# Query results for an item
+results = client.solver.results("ASM-001")
+```
+
+### 12.2 Create Workbench Integration
+
+The Assembly workbench adds a "Solve on Server" command:
+
+```python
+# CommandSolveOnServer.py (sketch)
+def activated(self):
+    assembly = get_active_assembly()
+    ctx = assembly.build_solve_context()
+    
+    # Submit to Silo
+    from silo_client import get_client
+    client = get_client()
+    job = client.solver.submit(ctx.to_dict())
+    
+    # Subscribe to SSE for updates
+    self.watch_job(job.id)
+
+def on_solver_completed(self, job_id, result):
+    # Apply placements back to assembly
+    assembly = get_active_assembly()
+    for pr in result["placements"]:
+        assembly.set_part_placement(pr["id"], pr["placement"])
+    assembly.recompute()
+```
+
+---
+
+## 13. Implementation Plan
+
+### Phase 3a: JSON Serialization
+
+Add `to_dict()` / `from_dict()` methods to all KCSolve types in the pybind11 module.
+
+**Files to modify:**
+- `src/Mod/Assembly/Solver/bindings/kcsolve_py.cpp` -- add dict conversion methods
+
+**Verification:** `ctx.to_dict()` round-trips through `SolveContext.from_dict()`.
+
+### Phase 3b: Server Endpoints
+
+Add the solver module to the Silo server.
+
+**Files to create (in silo repository):**
+- `internal/modules/solver/solver.go` -- Module registration and config
+- `internal/modules/solver/handlers.go` -- REST endpoint handlers
+- `internal/modules/solver/events.go` -- SSE event definitions
+- `migrations/020_solver_results.sql` -- Database migration
+
+### Phase 3c: Runner Support
+
+Add solver job execution to `silorunner`.
+
+**Files to create:**
+- `src/Mod/Assembly/Solver/bindings/runner.py` -- `kcsolve.runner` entry point
+- Runner capability reporting during heartbeat
+
+### Phase 3d: .kc Context Packing
+
+Pack `SolveContext` into `.kc` archives on commit.
+
+**Files to modify:**
+- `mods/silo/freecad/silo_origin.py` -- Hook into commit to pack solver context
+
+### Phase 3e: Client Integration
+
+Add "Solve on Server" command to the Assembly workbench.
+
+**Files to modify:**
+- `mods/silo/freecad/` -- Solver client methods
+- `src/Mod/Assembly/` -- Server solve command
+
+---
+
+## 14. Open Questions
+
+1. **Context size limits** -- Large assemblies may produce multi-MB SolveContext JSON. Should we compress (gzip) or use a binary format (msgpack)?
+
+2. **Result persistence** -- How long should solver results be retained? Per-revision (overwritten on next commit) or historical (keep all)?
+
+3. **Kinematic frame storage** -- Kinematic simulations can produce thousands of frames. Store all frames in JSONB, or write to a separate file and reference it?
+
+4. **Multi-solver comparison** -- Should the API support running the same context through multiple solvers and comparing results? Useful for Phase 4 (second solver validation).
+
+5. **Webhook notifications** -- The `callback_url` field allows external integrations (e.g. CI). What authentication should the webhook use?
+
+---
+
+## 15. References
+
+- [KCSolve Architecture](../architecture/ondsel-solver.md)
+- [KCSolve Python API Reference](../reference/kcsolve-python.md)
+- [INTER_SOLVER.md](../../INTER_SOLVER.md) -- Full pluggable solver spec
+- [WORKERS.md](WORKERS.md) -- Worker/runner job system
+- [SPECIFICATION.md](SPECIFICATION.md) -- Silo server specification
+- [MODULES.md](MODULES.md) -- Module system

docs/src/silo-server/frontend-spec.md

@@ -337,7 +337,7 @@ Supporting files:
 | File | Purpose |
 |------|---------|
 | `web/src/components/items/CategoryPicker.tsx` | Multi-stage domain/subcategory selector |
-| `web/src/components/items/FileDropZone.tsx` | Drag-and-drop file upload with MinIO presigned URLs |
+| `web/src/components/items/FileDropZone.tsx` | Drag-and-drop file upload |
 | `web/src/components/items/TagInput.tsx` | Multi-select tag input for projects |
 | `web/src/hooks/useFormDescriptor.ts` | Fetches and caches form descriptor from `/api/schemas/{name}/form` |
 | `web/src/hooks/useFileUpload.ts` | Manages presigned URL upload flow |
@@ -421,7 +421,7 @@ Below the picker, the selected category is shown as a breadcrumb: `Fasteners ›
 
 ### FileDropZone
 
-Handles drag-and-drop and click-to-browse file uploads with MinIO presigned URL flow.
+Handles drag-and-drop and click-to-browse file uploads.
 
 **Props**:
 
@@ -435,7 +435,7 @@ interface FileDropZoneProps {
 
 interface PendingAttachment {
   file: File;
-  objectKey: string;          // MinIO key after upload
+  objectKey: string;          // storage key after upload
   uploadProgress: number;     // 0-100
   uploadStatus: 'pending' | 'uploading' | 'complete' | 'error';
   error?: string;
@@ -462,7 +462,7 @@ Clicking the zone opens a hidden `<input type="file" multiple>`.
 
 1. On file selection/drop, immediately request a presigned upload URL: `POST /api/uploads/presign` with `{ filename, content_type, size }`.
 2. Backend returns `{ object_key, upload_url, expires_at }`.
-3. `PUT` the file directly to the presigned MinIO URL using `XMLHttpRequest` (for progress tracking).
+3. `PUT` the file directly to the presigned URL using `XMLHttpRequest` (for progress tracking).
 4. On completion, update `PendingAttachment.uploadStatus` to `'complete'` and store the `object_key`.
 5. The `object_key` is later sent to the item creation endpoint to associate the file.
 
@@ -589,10 +589,10 @@ Items 1-5 below are implemented. Item 4 (hierarchical categories) is resolved by
 ```
 POST /api/uploads/presign
 Request:  { "filename": "bracket.FCStd", "content_type": "application/octet-stream", "size": 2400000 }
-Response: { "object_key": "uploads/tmp/{uuid}/{filename}", "upload_url": "https://minio.../...", "expires_at": "2026-02-06T..." }
+Response: { "object_key": "uploads/tmp/{uuid}/{filename}", "upload_url": "https://...", "expires_at": "2026-02-06T..." }
 ```
 
-The Go handler generates a presigned PUT URL via the MinIO SDK. Objects are uploaded to a temporary prefix. On item creation, they're moved/linked to the item's permanent prefix.
+The Go handler generates a presigned PUT URL for direct upload. Objects are uploaded to a temporary prefix. On item creation, they're moved/linked to the item's permanent prefix.
 
 ### 2. File Association -- IMPLEMENTED
 
@@ -612,7 +612,7 @@ Request:  { "object_key": "uploads/tmp/{uuid}/thumb.png" }
 Response: 204
 ```
 
-Stores the thumbnail at `items/{item_id}/thumbnail.png` in MinIO. Updates `item.thumbnail_key` column.
+Stores the thumbnail at `items/{item_id}/thumbnail.png` in storage. Updates `item.thumbnail_key` column.
 
 ### 4. Hierarchical Categories -- IMPLEMENTED (via Form Descriptor)
 

docs/src/silo-server/overview.md

@@ -34,7 +34,7 @@ silo/
 │   ├── ods/               # ODS spreadsheet library
 │   ├── partnum/           # Part number generation
 │   ├── schema/            # YAML schema parsing
-│   ├── storage/           # MinIO file storage
+│   ├── storage/           # Filesystem storage
 │   └── testutil/          # Test helpers
 ├── web/                   # React SPA (Vite + TypeScript)
 │   └── src/
@@ -55,7 +55,7 @@ silo/
 
 See the **[Installation Guide](docs/INSTALL.md)** for complete setup instructions.
 
-**Docker Compose (quickest — includes PostgreSQL, MinIO, OpenLDAP, and Silo):**
+**Docker Compose (quickest — includes PostgreSQL, OpenLDAP, and Silo):**
 
 ```bash
 ./scripts/setup-docker.sh
@@ -65,7 +65,7 @@ docker compose -f deployments/docker-compose.allinone.yaml up -d
 **Development (local Go + Docker services):**
 
 ```bash
-make docker-up    # Start PostgreSQL + MinIO in Docker
+make docker-up    # Start PostgreSQL in Docker
 make run          # Run silo locally with Go
 ```
 

src/Gui/EditingContext.cpp

@@ -267,7 +267,8 @@ void EditingContextResolver::registerBuiltinContexts()
         {QStringLiteral("Part Design Helper Features"),
          QStringLiteral("Part Design Modeling Features"),
          QStringLiteral("Part Design Dress-Up Features"),
-         QStringLiteral("Part Design Transformation Features")},
+         QStringLiteral("Part Design Transformation Features"),
+         QStringLiteral("Sketcher")},
         /*.priority =*/40,
         /*.match =*/
         []() {
@@ -292,7 +293,11 @@ void EditingContextResolver::registerBuiltinContexts()
         /*.labelTemplate =*/QStringLiteral("Body: {name}"),
         /*.color =*/QLatin1String(CatppuccinMocha::Mauve),
         /*.toolbars =*/
-        {QStringLiteral("Part Design Helper Features"), QStringLiteral("Sketcher")},
+        {QStringLiteral("Part Design Helper Features"),
+         QStringLiteral("Part Design Modeling Features"),
+         QStringLiteral("Part Design Dress-Up Features"),
+         QStringLiteral("Part Design Transformation Features"),
+         QStringLiteral("Sketcher")},
         /*.priority =*/30,
         /*.match =*/
         []() {
@@ -307,7 +312,9 @@ void EditingContextResolver::registerBuiltinContexts()
         /*.labelTemplate =*/QStringLiteral("Assembly: {name}"),
         /*.color =*/QLatin1String(CatppuccinMocha::Blue),
         /*.toolbars =*/
-        {QStringLiteral("Assembly")},
+        {QStringLiteral("Assembly"),
+         QStringLiteral("Assembly Joints"),
+         QStringLiteral("Assembly Management")},
         /*.priority =*/30,
         /*.match =*/
         []() {
@@ -340,7 +347,11 @@ void EditingContextResolver::registerBuiltinContexts()
         /*.labelTemplate =*/QStringLiteral("Part Design"),
         /*.color =*/QLatin1String(CatppuccinMocha::Mauve),
         /*.toolbars =*/
-        {QStringLiteral("Part Design Helper Features"), QStringLiteral("Sketcher")},
+        {QStringLiteral("Part Design Helper Features"),
+         QStringLiteral("Part Design Modeling Features"),
+         QStringLiteral("Part Design Dress-Up Features"),
+         QStringLiteral("Part Design Transformation Features"),
+         QStringLiteral("Sketcher")},
         /*.priority =*/20,
         /*.match =*/
         []() {
@@ -353,7 +364,12 @@ void EditingContextResolver::registerBuiltinContexts()
         /*.labelTemplate =*/QStringLiteral("Sketcher"),
         /*.color =*/QLatin1String(CatppuccinMocha::Green),
         /*.toolbars =*/
-        {QStringLiteral("Sketcher"), QStringLiteral("Sketcher Tools")},
+        {QStringLiteral("Sketcher"),
+         QStringLiteral("Sketcher Tools"),
+         QStringLiteral("Geometries"),
+         QStringLiteral("Constraints"),
+         QStringLiteral("B-Spline Tools"),
+         QStringLiteral("Visual Helpers")},
         /*.priority =*/20,
         /*.match =*/
         []() {
@@ -366,7 +382,9 @@ void EditingContextResolver::registerBuiltinContexts()
         /*.labelTemplate =*/QStringLiteral("Assembly"),
         /*.color =*/QLatin1String(CatppuccinMocha::Blue),
         /*.toolbars =*/
-        {QStringLiteral("Assembly")},
+        {QStringLiteral("Assembly"),
+         QStringLiteral("Assembly Joints"),
+         QStringLiteral("Assembly Management")},
         /*.priority =*/20,
         /*.match =*/
         []() {

src/Mod/Assembly/App/AppAssembly.cpp

@@ -26,6 +26,8 @@
 #include <Base/Interpreter.h>
 #include <Base/PyObjectBase.h>
 
+#include <Mod/Assembly/Solver/OndselAdapter.h>
+
 #include "AssemblyObject.h"
 #include "AssemblyLink.h"
 #include "BomObject.h"
@@ -54,6 +56,10 @@ PyMOD_INIT_FUNC(AssemblyApp)
     }
 
     PyObject* mod = Assembly::initModule();
+
+    // Register the built-in OndselSolver adapter with the solver registry.
+    KCSolve::OndselAdapter::register_solver();
+
     Base::Console().log("Loading Assembly module... done\n");
 
 

src/Mod/Assembly/App/AssemblyObject.h

@@ -25,24 +25,21 @@
 #ifndef ASSEMBLY_AssemblyObject_H
 #define ASSEMBLY_AssemblyObject_H
 
+#include <memory>
+
 #include <boost/signals2.hpp>
 
 #include <Mod/Assembly/AssemblyGlobal.h>
+#include <Mod/Assembly/Solver/Types.h>
 
 #include <App/FeaturePython.h>
 #include <App/Part.h>
 #include <App/PropertyLinks.h>
 
-#include <OndselSolver/enum.h>
+namespace KCSolve
-
-namespace MbD
 {
-class ASMTPart;
+class IKCSolver;
-class ASMTAssembly;
+}  // namespace KCSolve
-class ASMTJoint;
-class ASMTMarker;
-class ASMTPart;
-}  // namespace MbD
 
 namespace App
 {
@@ -101,11 +98,15 @@ public:
     void postDrag();
     void savePlacementsForUndo();
     void undoSolve();
+    void resetSolver();
     void clearUndo();
 
     void exportAsASMT(std::string fileName);
 
-    Base::Placement getMbdPlacement(std::shared_ptr<MbD::ASMTPart> mbdPart);
+    /// Build the assembly constraint graph without solving.
+    /// Returns an empty SolveContext if no parts are grounded.
+    KCSolve::SolveContext getSolveContext();
+
     bool validateNewPlacements();
     void setNewPlacements();
     static void redrawJointPlacements(std::vector<App::DocumentObject*> joints);
@@ -114,42 +115,8 @@ public:
     // This makes sure that LinkGroups or sub-assemblies have identity placements.
     void ensureIdentityPlacements();
 
-    // Ondsel Solver interface
-    std::shared_ptr<MbD::ASMTAssembly> makeMbdAssembly();
-    void create_mbdSimulationParameters(App::DocumentObject* sim);
-    std::shared_ptr<MbD::ASMTPart> makeMbdPart(
-        std::string& name,
-        Base::Placement plc = Base::Placement(),
-        double mass = 1.0
-    );
-    std::shared_ptr<MbD::ASMTPart> getMbDPart(App::DocumentObject* obj);
-    // To help the solver, during dragging, we are bundling parts connected by a fixed joint.
-    // So several assembly components are bundled in a single ASMTPart.
-    // So we need to store the plc of each bundled object relative to the bundle origin (first obj
-    // of objectPartMap).
-    struct MbDPartData
-    {
-        std::shared_ptr<MbD::ASMTPart> part;
-        Base::Placement offsetPlc;  // This is the offset within the bundled parts
-    };
-    MbDPartData getMbDData(App::DocumentObject* part);
-    std::shared_ptr<MbD::ASMTMarker> makeMbdMarker(std::string& name, Base::Placement& plc);
-    std::vector<std::shared_ptr<MbD::ASMTJoint>> makeMbdJoint(App::DocumentObject* joint);
-    std::shared_ptr<MbD::ASMTJoint> makeMbdJointOfType(App::DocumentObject* joint, JointType jointType);
-    std::shared_ptr<MbD::ASMTJoint> makeMbdJointDistance(App::DocumentObject* joint);
-    std::string handleOneSideOfJoint(
-        App::DocumentObject* joint,
-        const char* propRefName,
-        const char* propPlcName
-    );
-    void getRackPinionMarkers(
-        App::DocumentObject* joint,
-        std::string& markerNameI,
-        std::string& markerNameJ
-    );
     int slidingPartIndex(App::DocumentObject* joint);
 
-    void jointParts(std::vector<App::DocumentObject*> joints);
     JointGroup* getJointGroup() const;
     ViewGroup* getExplodedViewGroup() const;
     template<typename T>
@@ -169,8 +136,6 @@ public:
         const std::vector<App::DocumentObject*>& excludeJoints = {}
     );
     std::unordered_set<App::DocumentObject*> getGroundedParts();
-    std::unordered_set<App::DocumentObject*> fixGroundedParts();
-    void fixGroundedPart(App::DocumentObject* obj, Base::Placement& plc, std::string& jointName);
 
     bool isJointConnectingPartToGround(App::DocumentObject* joint, const char* partPropName);
     bool isJointTypeConnecting(App::DocumentObject* joint);
@@ -210,7 +175,7 @@ public:
 
     std::vector<App::DocumentObject*> getMotionsFromSimulation(App::DocumentObject* sim);
 
-    bool isMbDJointValid(App::DocumentObject* joint);
+    bool isJointValid(App::DocumentObject* joint);
 
     bool isEmpty() const;
     int numberOfComponents() const;
@@ -259,12 +224,56 @@ public:
     fastsignals::signal<void()> signalSolverUpdate;
 
 private:
-    std::shared_ptr<MbD::ASMTAssembly> mbdAssembly;
+    // ── Solver integration ─────────────────────────────────────────
+
+    KCSolve::IKCSolver* getOrCreateSolver();
+
+    KCSolve::SolveContext buildSolveContext(
+        const std::vector<App::DocumentObject*>& joints,
+        bool forSimulation = false,
+        App::DocumentObject* sim = nullptr
+    );
+
+    KCSolve::Transform computeMarkerTransform(
+        App::DocumentObject* joint,
+        const char* propRefName,
+        const char* propPlcName
+    );
+
+    struct RackPinionResult
+    {
+        std::string partIdI;
+        KCSolve::Transform markerI;
+        std::string partIdJ;
+        KCSolve::Transform markerJ;
+    };
+    RackPinionResult computeRackPinionMarkers(App::DocumentObject* joint);
+
+    // ── Part ↔ solver ID mapping ───────────────────────────────────
+
+    // Maps a solver part ID to the FreeCAD objects it represents.
+    // Multiple objects map to one ID when parts are bundled by Fixed joints.
+    struct PartMapping
+    {
+        App::DocumentObject* obj;
+        Base::Placement offset;  // identity for primary, non-identity for bundled
+    };
+    std::unordered_map<std::string, std::vector<PartMapping>> partIdToObjs_;
+    std::unordered_map<App::DocumentObject*, std::string> objToPartId_;
+
+    // Register a part (and recursively its fixed-joint bundle when bundleFixed is set).
+    // Returns the solver part ID.
+    std::string registerPart(App::DocumentObject* obj);
+
+    // ── Solver state ───────────────────────────────────────────────
+
+    std::unique_ptr<KCSolve::IKCSolver> solver_;
+    KCSolve::SolveResult lastResult_;
+
+    // ── Existing state (unchanged) ─────────────────────────────────
 
-    std::unordered_map<App::DocumentObject*, MbDPartData> objectPartMap;
     std::vector<std::pair<App::DocumentObject*, double>> objMasses;
     std::vector<App::DocumentObject*> draggedParts;
-    std::vector<App::DocumentObject*> motions;
 
     std::vector<std::pair<App::DocumentObject*, Base::Placement>> previousPositions;
 

src/Mod/Assembly/App/AssemblyObject.pyi

@@ -4,10 +4,9 @@ from __future__ import annotations
 
 from typing import Any, Final
 
-from Base.Metadata import constmethod, export
-
-from App.Part import Part
 from App.DocumentObject import DocumentObject
+from App.Part import Part
+from Base.Metadata import constmethod, export
 
 @export(Include="Mod/Assembly/App/AssemblyObject.h", Namespace="Assembly")
 class AssemblyObject(Part):
@@ -119,7 +118,9 @@ class AssemblyObject(Part):
         ...
 
     @constmethod
-    def isJointConnectingPartToGround(self, joint: DocumentObject, prop_name: str, /) -> Any:
+    def isJointConnectingPartToGround(
+        self, joint: DocumentObject, prop_name: str, /
+    ) -> Any:
         """
         Check if a joint is connecting a part to the ground.
 
@@ -153,6 +154,16 @@ class AssemblyObject(Part):
         """
         ...
 
+    @constmethod
+    def getSolveContext(self) -> dict:
+        """Build the assembly constraint graph as a serializable dict.
+
+        Returns a dict matching kcsolve.SolveContext.to_dict() format,
+        or an empty dict if the assembly has no grounded parts.
+        Does NOT trigger a solve.
+        """
+        ...
+
     @constmethod
     def getDownstreamParts(
         self, start_part: DocumentObject, joint_to_ignore: DocumentObject, /

src/Mod/Assembly/App/AssemblyObjectPyImp.cpp

@@ -21,13 +21,161 @@
  *                                                                          *
  ***************************************************************************/
 
-
+ 
-// inclusion of the generated files (generated out of AssemblyObject.xml)
+ // inclusion of the generated files (generated out of AssemblyObject.xml)
 #include "AssemblyObjectPy.h"
 #include "AssemblyObjectPy.cpp"
 
+#include <Mod/Assembly/Solver/SolverRegistry.h>
+
 using namespace Assembly;
 
+namespace
+{
+
+// ── Enum-to-string tables for dict serialization ───────────────────
+// String values must match kcsolve_py.cpp py::enum_ .value() names exactly.
+
+const char* baseJointKindStr(KCSolve::BaseJointKind k)
+{
+    switch (k) {
+        case KCSolve::BaseJointKind::Coincident:         return "Coincident";
+        case KCSolve::BaseJointKind::PointOnLine:        return "PointOnLine";
+        case KCSolve::BaseJointKind::PointInPlane:       return "PointInPlane";
+        case KCSolve::BaseJointKind::Concentric:         return "Concentric";
+        case KCSolve::BaseJointKind::Tangent:            return "Tangent";
+        case KCSolve::BaseJointKind::Planar:             return "Planar";
+        case KCSolve::BaseJointKind::LineInPlane:        return "LineInPlane";
+        case KCSolve::BaseJointKind::Parallel:           return "Parallel";
+        case KCSolve::BaseJointKind::Perpendicular:      return "Perpendicular";
+        case KCSolve::BaseJointKind::Angle:              return "Angle";
+        case KCSolve::BaseJointKind::Fixed:              return "Fixed";
+        case KCSolve::BaseJointKind::Revolute:           return "Revolute";
+        case KCSolve::BaseJointKind::Cylindrical:        return "Cylindrical";
+        case KCSolve::BaseJointKind::Slider:             return "Slider";
+        case KCSolve::BaseJointKind::Ball:               return "Ball";
+        case KCSolve::BaseJointKind::Screw:              return "Screw";
+        case KCSolve::BaseJointKind::Universal:          return "Universal";
+        case KCSolve::BaseJointKind::Gear:               return "Gear";
+        case KCSolve::BaseJointKind::RackPinion:         return "RackPinion";
+        case KCSolve::BaseJointKind::Cam:                return "Cam";
+        case KCSolve::BaseJointKind::Slot:               return "Slot";
+        case KCSolve::BaseJointKind::DistancePointPoint: return "DistancePointPoint";
+        case KCSolve::BaseJointKind::DistanceCylSph:     return "DistanceCylSph";
+        case KCSolve::BaseJointKind::Custom:             return "Custom";
+    }
+    return "Custom";
+}
+
+const char* limitKindStr(KCSolve::Constraint::Limit::Kind k)
+{
+    switch (k) {
+        case KCSolve::Constraint::Limit::Kind::TranslationMin: return "TranslationMin";
+        case KCSolve::Constraint::Limit::Kind::TranslationMax: return "TranslationMax";
+        case KCSolve::Constraint::Limit::Kind::RotationMin:    return "RotationMin";
+        case KCSolve::Constraint::Limit::Kind::RotationMax:    return "RotationMax";
+    }
+    return "TranslationMin";
+}
+
+const char* motionKindStr(KCSolve::MotionDef::Kind k)
+{
+    switch (k) {
+        case KCSolve::MotionDef::Kind::Rotational:    return "Rotational";
+        case KCSolve::MotionDef::Kind::Translational: return "Translational";
+        case KCSolve::MotionDef::Kind::General:       return "General";
+    }
+    return "Rotational";
+}
+
+// ── Python dict builders ───────────────────────────────────────────
+// Layout matches solve_context_to_dict() in kcsolve_py.cpp exactly.
+
+Py::Dict transformToDict(const KCSolve::Transform& t)
+{
+    Py::Dict d;
+    d.setItem("position", Py::TupleN(
+        Py::Float(t.position[0]),
+        Py::Float(t.position[1]),
+        Py::Float(t.position[2])));
+    d.setItem("quaternion", Py::TupleN(
+        Py::Float(t.quaternion[0]),
+        Py::Float(t.quaternion[1]),
+        Py::Float(t.quaternion[2]),
+        Py::Float(t.quaternion[3])));
+    return d;
+}
+
+Py::Dict partToDict(const KCSolve::Part& p)
+{
+    Py::Dict d;
+    d.setItem("id", Py::String(p.id));
+    d.setItem("placement", transformToDict(p.placement));
+    d.setItem("mass", Py::Float(p.mass));
+    d.setItem("grounded", Py::Boolean(p.grounded));
+    return d;
+}
+
+Py::Dict limitToDict(const KCSolve::Constraint::Limit& lim)
+{
+    Py::Dict d;
+    d.setItem("kind", Py::String(limitKindStr(lim.kind)));
+    d.setItem("value", Py::Float(lim.value));
+    d.setItem("tolerance", Py::Float(lim.tolerance));
+    return d;
+}
+
+Py::Dict constraintToDict(const KCSolve::Constraint& c)
+{
+    Py::Dict d;
+    d.setItem("id", Py::String(c.id));
+    d.setItem("part_i", Py::String(c.part_i));
+    d.setItem("marker_i", transformToDict(c.marker_i));
+    d.setItem("part_j", Py::String(c.part_j));
+    d.setItem("marker_j", transformToDict(c.marker_j));
+    d.setItem("type", Py::String(baseJointKindStr(c.type)));
+
+    Py::List params;
+    for (double v : c.params) {
+        params.append(Py::Float(v));
+    }
+    d.setItem("params", params);
+
+    Py::List lims;
+    for (const auto& l : c.limits) {
+        lims.append(limitToDict(l));
+    }
+    d.setItem("limits", lims);
+    d.setItem("activated", Py::Boolean(c.activated));
+    return d;
+}
+
+Py::Dict motionToDict(const KCSolve::MotionDef& m)
+{
+    Py::Dict d;
+    d.setItem("kind", Py::String(motionKindStr(m.kind)));
+    d.setItem("joint_id", Py::String(m.joint_id));
+    d.setItem("marker_i", Py::String(m.marker_i));
+    d.setItem("marker_j", Py::String(m.marker_j));
+    d.setItem("rotation_expr", Py::String(m.rotation_expr));
+    d.setItem("translation_expr", Py::String(m.translation_expr));
+    return d;
+}
+
+Py::Dict simToDict(const KCSolve::SimulationParams& s)
+{
+    Py::Dict d;
+    d.setItem("t_start", Py::Float(s.t_start));
+    d.setItem("t_end", Py::Float(s.t_end));
+    d.setItem("h_out", Py::Float(s.h_out));
+    d.setItem("h_min", Py::Float(s.h_min));
+    d.setItem("h_max", Py::Float(s.h_max));
+    d.setItem("error_tol", Py::Float(s.error_tol));
+    return d;
+}
+
+}  // anonymous namespace
+
 // returns a string which represents the object e.g. when printed in python
 std::string AssemblyObjectPy::representation() const
 {
@@ -243,3 +391,52 @@ PyObject* AssemblyObjectPy::getDownstreamParts(PyObject* args) const
 
     return Py::new_reference_to(ret);
 }
+
+PyObject* AssemblyObjectPy::getSolveContext(PyObject* args) const
+{
+    if (!PyArg_ParseTuple(args, "")) {
+        return nullptr;
+    }
+    PY_TRY
+    {
+        KCSolve::SolveContext ctx = getAssemblyObjectPtr()->getSolveContext();
+
+        // Empty context (no grounded parts) → return empty dict
+        if (ctx.parts.empty()) {
+            return Py::new_reference_to(Py::Dict());
+        }
+
+        Py::Dict d;
+        d.setItem("api_version", Py::Long(KCSolve::API_VERSION_MAJOR));
+
+        Py::List parts;
+        for (const auto& p : ctx.parts) {
+            parts.append(partToDict(p));
+        }
+        d.setItem("parts", parts);
+
+        Py::List constraints;
+        for (const auto& c : ctx.constraints) {
+            constraints.append(constraintToDict(c));
+        }
+        d.setItem("constraints", constraints);
+
+        Py::List motions;
+        for (const auto& m : ctx.motions) {
+            motions.append(motionToDict(m));
+        }
+        d.setItem("motions", motions);
+
+        if (ctx.simulation.has_value()) {
+            d.setItem("simulation", simToDict(*ctx.simulation));
+        }
+        else {
+            d.setItem("simulation", Py::None());
+        }
+
+        d.setItem("bundle_fixed", Py::Boolean(ctx.bundle_fixed));
+
+        return Py::new_reference_to(d);
+    }
+    PY_CATCH;
+}

src/Mod/Assembly/App/CMakeLists.txt

@@ -5,7 +5,7 @@ set(Assembly_LIBS
     PartDesign
     Spreadsheet
     FreeCADApp
-    OndselSolver
+    KCSolve
 )
 
 generate_from_py(AssemblyObject)

src/Mod/Assembly/AssemblyTests/TestKCSolvePy.py

@@ -0,0 +1,520 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+# ***************************************************************************
+# *                                                                         *
+# *   Copyright (c) 2025 Kindred Systems <development@kindred-systems.com>  *
+# *                                                                         *
+# *   This file is part of FreeCAD.                                         *
+# *                                                                         *
+# *   FreeCAD is free software: you can redistribute it and/or modify it    *
+# *   under the terms of the GNU Lesser General Public License as           *
+# *   published by the Free Software Foundation, either version 2.1 of the  *
+# *   License, or (at your option) any later version.                       *
+# *                                                                         *
+# *   FreeCAD is distributed in the hope that it will be useful, but        *
+# *   WITHOUT ANY WARRANTY; without even the implied warranty of            *
+# *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU      *
+# *   Lesser General Public License for more details.                       *
+# *                                                                         *
+# *   You should have received a copy of the GNU Lesser General Public      *
+# *   License along with FreeCAD. If not, see                               *
+# *   <https://www.gnu.org/licenses/>.                                      *
+# *                                                                         *
+# ***************************************************************************
+
+"""Unit tests for the kcsolve pybind11 module."""
+
+import unittest
+
+
+class TestKCSolveImport(unittest.TestCase):
+    """Verify that the kcsolve module loads and exposes expected symbols."""
+
+    def test_import(self):
+        import kcsolve
+
+        for sym in (
+            "IKCSolver",
+            "OndselAdapter",
+            "Transform",
+            "Part",
+            "Constraint",
+            "SolveContext",
+            "SolveResult",
+            "BaseJointKind",
+            "SolveStatus",
+            "available",
+            "load",
+            "register_solver",
+        ):
+            self.assertTrue(hasattr(kcsolve, sym), f"missing symbol: {sym}")
+
+    def test_api_version(self):
+        import kcsolve
+
+        self.assertEqual(kcsolve.API_VERSION_MAJOR, 1)
+
+
+class TestKCSolveTypes(unittest.TestCase):
+    """Verify struct/enum bindings behave correctly."""
+
+    def test_transform_identity(self):
+        import kcsolve
+
+        t = kcsolve.Transform.identity()
+        self.assertEqual(list(t.position), [0.0, 0.0, 0.0])
+        self.assertEqual(list(t.quaternion), [1.0, 0.0, 0.0, 0.0])  # w,x,y,z
+
+    def test_part_defaults(self):
+        import kcsolve
+
+        p = kcsolve.Part()
+        self.assertEqual(p.id, "")
+        self.assertAlmostEqual(p.mass, 1.0)
+        self.assertFalse(p.grounded)
+
+    def test_solve_context_construction(self):
+        import kcsolve
+
+        ctx = kcsolve.SolveContext()
+        self.assertEqual(len(ctx.parts), 0)
+        self.assertEqual(len(ctx.constraints), 0)
+
+        p = kcsolve.Part()
+        p.id = "part1"
+        # pybind11 def_readwrite on std::vector returns a copy,
+        # so we must assign the whole list back.
+        ctx.parts = [p]
+        self.assertEqual(len(ctx.parts), 1)
+        self.assertEqual(ctx.parts[0].id, "part1")
+
+    def test_enum_values(self):
+        import kcsolve
+
+        self.assertEqual(int(kcsolve.SolveStatus.Success), 0)
+        # BaseJointKind.Fixed should exist
+        self.assertIsNotNone(kcsolve.BaseJointKind.Fixed)
+        # DiagnosticKind should exist
+        self.assertIsNotNone(kcsolve.DiagnosticKind.Redundant)
+
+    def test_constraint_fields(self):
+        import kcsolve
+
+        c = kcsolve.Constraint()
+        c.id = "Joint001"
+        c.part_i = "part1"
+        c.part_j = "part2"
+        c.type = kcsolve.BaseJointKind.Fixed
+        self.assertEqual(c.id, "Joint001")
+        self.assertEqual(c.type, kcsolve.BaseJointKind.Fixed)
+
+    def test_solve_result_fields(self):
+        import kcsolve
+
+        r = kcsolve.SolveResult()
+        self.assertEqual(r.status, kcsolve.SolveStatus.Success)
+        self.assertEqual(r.dof, -1)
+        self.assertEqual(len(r.placements), 0)
+
+
+class TestKCSolveRegistry(unittest.TestCase):
+    """Verify SolverRegistry wrapper functions."""
+
+    def test_available_returns_list(self):
+        import kcsolve
+
+        result = kcsolve.available()
+        self.assertIsInstance(result, list)
+
+    def test_load_ondsel(self):
+        import kcsolve
+
+        solver = kcsolve.load("ondsel")
+        # Ondsel should be registered by FreeCAD init
+        if solver is not None:
+            self.assertIn("Ondsel", solver.name())
+
+    def test_load_unknown_returns_none(self):
+        import kcsolve
+
+        solver = kcsolve.load("nonexistent_solver_xyz")
+        self.assertIsNone(solver)
+
+    def test_get_set_default(self):
+        import kcsolve
+
+        original = kcsolve.get_default()
+        # Setting unknown solver should return False
+        self.assertFalse(kcsolve.set_default("nonexistent_solver_xyz"))
+        # Default should be unchanged
+        self.assertEqual(kcsolve.get_default(), original)
+
+
+class TestKCSolveSerialization(unittest.TestCase):
+    """Verify to_dict() / from_dict() round-trip on all KCSolve types."""
+
+    def test_transform_round_trip(self):
+        import kcsolve
+
+        t = kcsolve.Transform()
+        t.position = [1.0, 2.0, 3.0]
+        t.quaternion = [0.5, 0.5, 0.5, 0.5]
+        d = t.to_dict()
+        self.assertEqual(list(d["position"]), [1.0, 2.0, 3.0])
+        self.assertEqual(list(d["quaternion"]), [0.5, 0.5, 0.5, 0.5])
+        t2 = kcsolve.Transform.from_dict(d)
+        self.assertEqual(list(t2.position), [1.0, 2.0, 3.0])
+        self.assertEqual(list(t2.quaternion), [0.5, 0.5, 0.5, 0.5])
+
+    def test_transform_identity_round_trip(self):
+        import kcsolve
+
+        t = kcsolve.Transform.identity()
+        t2 = kcsolve.Transform.from_dict(t.to_dict())
+        self.assertEqual(list(t2.position), [0.0, 0.0, 0.0])
+        self.assertEqual(list(t2.quaternion), [1.0, 0.0, 0.0, 0.0])
+
+    def test_part_round_trip(self):
+        import kcsolve
+
+        p = kcsolve.Part()
+        p.id = "box"
+        p.mass = 2.5
+        p.grounded = True
+        p.placement = kcsolve.Transform.identity()
+        d = p.to_dict()
+        self.assertEqual(d["id"], "box")
+        self.assertAlmostEqual(d["mass"], 2.5)
+        self.assertTrue(d["grounded"])
+        p2 = kcsolve.Part.from_dict(d)
+        self.assertEqual(p2.id, "box")
+        self.assertAlmostEqual(p2.mass, 2.5)
+        self.assertTrue(p2.grounded)
+
+    def test_constraint_with_limits_round_trip(self):
+        import kcsolve
+
+        c = kcsolve.Constraint()
+        c.id = "Joint001"
+        c.part_i = "part1"
+        c.part_j = "part2"
+        c.type = kcsolve.BaseJointKind.Revolute
+        c.params = [1.5, 2.5]
+        lim = kcsolve.Constraint.Limit()
+        lim.kind = kcsolve.LimitKind.RotationMin
+        lim.value = -3.14
+        lim.tolerance = 0.01
+        c.limits = [lim]
+        d = c.to_dict()
+        self.assertEqual(d["type"], "Revolute")
+        self.assertEqual(len(d["limits"]), 1)
+        self.assertEqual(d["limits"][0]["kind"], "RotationMin")
+        c2 = kcsolve.Constraint.from_dict(d)
+        self.assertEqual(c2.type, kcsolve.BaseJointKind.Revolute)
+        self.assertEqual(len(c2.limits), 1)
+        self.assertEqual(c2.limits[0].kind, kcsolve.LimitKind.RotationMin)
+        self.assertAlmostEqual(c2.limits[0].value, -3.14)
+
+    def test_solve_context_full_round_trip(self):
+        import kcsolve
+
+        ctx = kcsolve.SolveContext()
+        p = kcsolve.Part()
+        p.id = "box"
+        p.grounded = True
+        ctx.parts = [p]
+
+        c = kcsolve.Constraint()
+        c.id = "J1"
+        c.part_i = "box"
+        c.part_j = "cyl"
+        c.type = kcsolve.BaseJointKind.Fixed
+        ctx.constraints = [c]
+        ctx.bundle_fixed = True
+
+        d = ctx.to_dict()
+        self.assertEqual(d["api_version"], kcsolve.API_VERSION_MAJOR)
+        self.assertEqual(len(d["parts"]), 1)
+        self.assertEqual(len(d["constraints"]), 1)
+        self.assertTrue(d["bundle_fixed"])
+
+        ctx2 = kcsolve.SolveContext.from_dict(d)
+        self.assertEqual(ctx2.parts[0].id, "box")
+        self.assertTrue(ctx2.parts[0].grounded)
+        self.assertEqual(ctx2.constraints[0].type, kcsolve.BaseJointKind.Fixed)
+        self.assertTrue(ctx2.bundle_fixed)
+
+    def test_solve_context_with_simulation(self):
+        import kcsolve
+
+        ctx = kcsolve.SolveContext()
+        ctx.parts = []
+        ctx.constraints = []
+        sim = kcsolve.SimulationParams()
+        sim.t_start = 0.0
+        sim.t_end = 10.0
+        sim.h_out = 0.01
+        ctx.simulation = sim
+        d = ctx.to_dict()
+        self.assertIsNotNone(d["simulation"])
+        self.assertAlmostEqual(d["simulation"]["t_end"], 10.0)
+        ctx2 = kcsolve.SolveContext.from_dict(d)
+        self.assertIsNotNone(ctx2.simulation)
+        self.assertAlmostEqual(ctx2.simulation.t_end, 10.0)
+
+    def test_solve_context_simulation_null(self):
+        import kcsolve
+
+        ctx = kcsolve.SolveContext()
+        ctx.parts = []
+        ctx.constraints = []
+        ctx.simulation = None
+        d = ctx.to_dict()
+        self.assertIsNone(d["simulation"])
+        ctx2 = kcsolve.SolveContext.from_dict(d)
+        self.assertIsNone(ctx2.simulation)
+
+    def test_solve_result_round_trip(self):
+        import kcsolve
+
+        r = kcsolve.SolveResult()
+        r.status = kcsolve.SolveStatus.Success
+        r.dof = 6
+        pr = kcsolve.SolveResult.PartResult()
+        pr.id = "box"
+        pr.placement = kcsolve.Transform.identity()
+        r.placements = [pr]
+        diag = kcsolve.ConstraintDiagnostic()
+        diag.constraint_id = "J1"
+        diag.kind = kcsolve.DiagnosticKind.Redundant
+        diag.detail = "over-constrained"
+        r.diagnostics = [diag]
+        r.num_frames = 100
+
+        d = r.to_dict()
+        self.assertEqual(d["status"], "Success")
+        self.assertEqual(d["dof"], 6)
+        self.assertEqual(d["num_frames"], 100)
+        self.assertEqual(len(d["placements"]), 1)
+        self.assertEqual(len(d["diagnostics"]), 1)
+
+        r2 = kcsolve.SolveResult.from_dict(d)
+        self.assertEqual(r2.status, kcsolve.SolveStatus.Success)
+        self.assertEqual(r2.dof, 6)
+        self.assertEqual(r2.num_frames, 100)
+        self.assertEqual(r2.placements[0].id, "box")
+        self.assertEqual(r2.diagnostics[0].kind, kcsolve.DiagnosticKind.Redundant)
+
+    def test_motion_def_round_trip(self):
+        import kcsolve
+
+        m = kcsolve.MotionDef()
+        m.kind = kcsolve.MotionKind.Rotational
+        m.joint_id = "J1"
+        m.marker_i = "part1"
+        m.marker_j = "part2"
+        m.rotation_expr = "2*pi*time"
+        m.translation_expr = ""
+        d = m.to_dict()
+        self.assertEqual(d["kind"], "Rotational")
+        self.assertEqual(d["joint_id"], "J1")
+        m2 = kcsolve.MotionDef.from_dict(d)
+        self.assertEqual(m2.kind, kcsolve.MotionKind.Rotational)
+        self.assertEqual(m2.rotation_expr, "2*pi*time")
+
+    def test_all_base_joint_kinds_round_trip(self):
+        import kcsolve
+
+        all_kinds = [
+            "Coincident",
+            "PointOnLine",
+            "PointInPlane",
+            "Concentric",
+            "Tangent",
+            "Planar",
+            "LineInPlane",
+            "Parallel",
+            "Perpendicular",
+            "Angle",
+            "Fixed",
+            "Revolute",
+            "Cylindrical",
+            "Slider",
+            "Ball",
+            "Screw",
+            "Universal",
+            "Gear",
+            "RackPinion",
+            "Cam",
+            "Slot",
+            "DistancePointPoint",
+            "DistanceCylSph",
+            "Custom",
+        ]
+        for name in all_kinds:
+            c = kcsolve.Constraint()
+            c.id = "test"
+            c.part_i = "a"
+            c.part_j = "b"
+            c.type = getattr(kcsolve.BaseJointKind, name)
+            d = c.to_dict()
+            self.assertEqual(d["type"], name)
+            c2 = kcsolve.Constraint.from_dict(d)
+            self.assertEqual(c2.type, getattr(kcsolve.BaseJointKind, name))
+
+    def test_all_solve_statuses_round_trip(self):
+        import kcsolve
+
+        for name in ("Success", "Failed", "InvalidFlip", "NoGroundedParts"):
+            r = kcsolve.SolveResult()
+            r.status = getattr(kcsolve.SolveStatus, name)
+            d = r.to_dict()
+            self.assertEqual(d["status"], name)
+            r2 = kcsolve.SolveResult.from_dict(d)
+            self.assertEqual(r2.status, getattr(kcsolve.SolveStatus, name))
+
+    def test_json_stdlib_round_trip(self):
+        import json
+
+        import kcsolve
+
+        ctx = kcsolve.SolveContext()
+        p = kcsolve.Part()
+        p.id = "box"
+        p.grounded = True
+        ctx.parts = [p]
+        ctx.constraints = []
+        d = ctx.to_dict()
+        json_str = json.dumps(d)
+        d2 = json.loads(json_str)
+        ctx2 = kcsolve.SolveContext.from_dict(d2)
+        self.assertEqual(ctx2.parts[0].id, "box")
+
+    def test_from_dict_missing_required_key(self):
+        import kcsolve
+
+        with self.assertRaises(KeyError):
+            kcsolve.Part.from_dict({"mass": 1.0, "grounded": False})
+
+    def test_from_dict_invalid_enum_string(self):
+        import kcsolve
+
+        d = {
+            "id": "J1",
+            "part_i": "a",
+            "part_j": "b",
+            "type": "Bogus",
+            "marker_i": {"position": [0, 0, 0], "quaternion": [1, 0, 0, 0]},
+            "marker_j": {"position": [0, 0, 0], "quaternion": [1, 0, 0, 0]},
+        }
+        with self.assertRaises(ValueError):
+            kcsolve.Constraint.from_dict(d)
+
+    def test_from_dict_bad_position_length(self):
+        import kcsolve
+
+        with self.assertRaises(ValueError):
+            kcsolve.Transform.from_dict(
+                {
+                    "position": [1.0, 2.0],
+                    "quaternion": [1, 0, 0, 0],
+                }
+            )
+
+    def test_from_dict_bad_api_version(self):
+        import kcsolve
+
+        d = {
+            "api_version": 99,
+            "parts": [],
+            "constraints": [],
+        }
+        with self.assertRaises(ValueError):
+            kcsolve.SolveContext.from_dict(d)
+
+
+class TestPySolver(unittest.TestCase):
+    """Verify Python IKCSolver subclassing and registration."""
+
+    def _make_solver_class(self):
+        import kcsolve
+
+        class _DummySolver(kcsolve.IKCSolver):
+            def name(self):
+                return "DummyPySolver"
+
+            def supported_joints(self):
+                return [
+                    kcsolve.BaseJointKind.Fixed,
+                    kcsolve.BaseJointKind.Revolute,
+                ]
+
+            def solve(self, ctx):
+                r = kcsolve.SolveResult()
+                r.status = kcsolve.SolveStatus.Success
+                parts = ctx.parts  # copy from C++ vector
+                r.dof = len(parts) * 6
+                placements = []
+                for p in parts:
+                    pr = kcsolve.SolveResult.PartResult()
+                    pr.id = p.id
+                    pr.placement = p.placement
+                    placements.append(pr)
+                r.placements = placements
+                return r
+
+        return _DummySolver
+
+    def test_instantiate_python_solver(self):
+        cls = self._make_solver_class()
+        solver = cls()
+        self.assertEqual(solver.name(), "DummyPySolver")
+        self.assertEqual(len(solver.supported_joints()), 2)
+
+    def test_python_solver_solve(self):
+        import kcsolve
+
+        cls = self._make_solver_class()
+        solver = cls()
+
+        ctx = kcsolve.SolveContext()
+        p = kcsolve.Part()
+        p.id = "box1"
+        p.grounded = True
+        ctx.parts = [p]
+
+        result = solver.solve(ctx)
+        self.assertEqual(result.status, kcsolve.SolveStatus.Success)
+        self.assertEqual(result.dof, 6)
+        self.assertEqual(len(result.placements), 1)
+        self.assertEqual(result.placements[0].id, "box1")
+
+    def test_register_and_roundtrip(self):
+        import kcsolve
+
+        cls = self._make_solver_class()
+        # Use a unique name to avoid collision across test runs
+        name = "test_dummy_roundtrip"
+        kcsolve.register_solver(name, cls)
+
+        self.assertIn(name, kcsolve.available())
+
+        loaded = kcsolve.load(name)
+        self.assertIsNotNone(loaded)
+        self.assertEqual(loaded.name(), "DummyPySolver")
+
+        ctx = kcsolve.SolveContext()
+        result = loaded.solve(ctx)
+        self.assertEqual(result.status, kcsolve.SolveStatus.Success)
+
+    def test_default_virtuals(self):
+        """Default implementations of optional virtuals should not crash."""
+        import kcsolve
+
+        cls = self._make_solver_class()
+        solver = cls()
+        self.assertTrue(solver.is_deterministic())
+        self.assertFalse(solver.supports_bundle_fixed())
+
+        ctx = kcsolve.SolveContext()
+        diags = solver.diagnose(ctx)
+        self.assertEqual(len(diags), 0)

src/Mod/Assembly/AssemblyTests/TestKindredSolverIntegration.py

@@ -0,0 +1,180 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+# /****************************************************************************
+#                                                                           *
+#    Copyright (c) 2025 Kindred Systems <development@kindred-systems.com>   *
+#                                                                           *
+#    This file is part of FreeCAD.                                          *
+#                                                                           *
+#    FreeCAD is free software: you can redistribute it and/or modify it     *
+#    under the terms of the GNU Lesser General Public License as            *
+#    published by the Free Software Foundation, either version 2.1 of the   *
+#    License, or (at your option) any later version.                        *
+#                                                                           *
+#    FreeCAD is distributed in the hope that it will be useful, but         *
+#    WITHOUT ANY WARRANTY; without even the implied warranty of             *
+#    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU       *
+#    Lesser General Public License for more details.                        *
+#                                                                           *
+#    You should have received a copy of the GNU Lesser General Public       *
+#    License along with FreeCAD. If not, see                                *
+#    <https://www.gnu.org/licenses/>.                                       *
+#                                                                           *
+# ***************************************************************************/
+
+"""
+Integration tests for the Kindred solver backend.
+
+These tests mirror TestSolverIntegration but force the solver preference
+to "kindred" so the full pipeline (AssemblyObject → IKCSolver →
+KindredSolver) is exercised.
+"""
+
+import unittest
+
+import FreeCAD as App
+import JointObject
+
+
+def _pref():
+    return App.ParamGet("User parameter:BaseApp/Preferences/Mod/Assembly")
+
+
+class TestKindredSolverIntegration(unittest.TestCase):
+    """Full-stack solver tests using the Kindred (Newton-Raphson) backend."""
+
+    def setUp(self):
+        # Force the kindred solver backend
+        self._prev_solver = _pref().GetString("Solver", "")
+        _pref().SetString("Solver", "kindred")
+
+        doc_name = self.__class__.__name__
+        if App.ActiveDocument:
+            if App.ActiveDocument.Name != doc_name:
+                App.newDocument(doc_name)
+        else:
+            App.newDocument(doc_name)
+        App.setActiveDocument(doc_name)
+        self.doc = App.ActiveDocument
+
+        self.assembly = self.doc.addObject("Assembly::AssemblyObject", "Assembly")
+        # Reset the solver so it picks up the new preference
+        self.assembly.resetSolver()
+        self.jointgroup = self.assembly.newObject("Assembly::JointGroup", "Joints")
+
+    def tearDown(self):
+        App.closeDocument(self.doc.Name)
+        _pref().SetString("Solver", self._prev_solver)
+
+    # ── Helpers ─────────────────────────────────────────────────────
+
+    def _make_box(self, x=0, y=0, z=0, size=10):
+        box = self.assembly.newObject("Part::Box", "Box")
+        box.Length = size
+        box.Width = size
+        box.Height = size
+        box.Placement = App.Placement(App.Vector(x, y, z), App.Rotation())
+        return box
+
+    def _ground(self, obj):
+        gnd = self.jointgroup.newObject("App::FeaturePython", "GroundedJoint")
+        JointObject.GroundedJoint(gnd, obj)
+        return gnd
+
+    def _make_joint(self, joint_type, ref1, ref2):
+        joint = self.jointgroup.newObject("App::FeaturePython", "Joint")
+        JointObject.Joint(joint, joint_type)
+        refs = [
+            [ref1[0], ref1[1]],
+            [ref2[0], ref2[1]],
+        ]
+        joint.Proxy.setJointConnectors(joint, refs)
+        return joint
+
+    # ── Tests ───────────────────────────────────────────────────────
+
+    def test_solve_fixed_joint(self):
+        """Two boxes + grounded + fixed joint -> placements match."""
+        box1 = self._make_box(10, 20, 30)
+        box2 = self._make_box(40, 50, 60)
+        self._ground(box2)
+
+        self._make_joint(
+            0,
+            [box2, ["Face6", "Vertex7"]],
+            [box1, ["Face6", "Vertex7"]],
+        )
+
+        self.assertTrue(
+            box1.Placement.isSame(box2.Placement, 1e-6),
+            "Fixed joint: box1 should match box2 placement",
+        )
+
+    def test_solve_revolute_joint(self):
+        """Two boxes + grounded + revolute joint -> solve succeeds."""
+        box1 = self._make_box(0, 0, 0)
+        box2 = self._make_box(100, 0, 0)
+        self._ground(box1)
+
+        self._make_joint(
+            1,
+            [box1, ["Face6", "Vertex7"]],
+            [box2, ["Face6", "Vertex7"]],
+        )
+
+        result = self.assembly.solve()
+        self.assertEqual(result, 0, "Revolute joint solve should succeed")
+
+    def test_solve_returns_code_for_no_ground(self):
+        """Assembly with no grounded parts -> solve returns -6."""
+        box1 = self._make_box(0, 0, 0)
+        box2 = self._make_box(50, 0, 0)
+
+        joint = self.jointgroup.newObject("App::FeaturePython", "Joint")
+        JointObject.Joint(joint, 0)
+        refs = [
+            [box1, ["Face6", "Vertex7"]],
+            [box2, ["Face6", "Vertex7"]],
+        ]
+        joint.Proxy.setJointConnectors(joint, refs)
+
+        result = self.assembly.solve()
+        self.assertEqual(result, -6, "No grounded parts should return -6")
+
+    def test_solve_dof_reporting(self):
+        """Revolute joint -> DOF = 1."""
+        box1 = self._make_box(0, 0, 0)
+        box2 = self._make_box(100, 0, 0)
+        self._ground(box1)
+
+        self._make_joint(
+            1,
+            [box1, ["Face6", "Vertex7"]],
+            [box2, ["Face6", "Vertex7"]],
+        )
+
+        self.assembly.solve()
+        dof = self.assembly.getLastDoF()
+        self.assertEqual(dof, 1, "Revolute joint should leave 1 DOF")
+
+    def test_solve_stability(self):
+        """Solving twice produces identical placements."""
+        box1 = self._make_box(10, 20, 30)
+        box2 = self._make_box(40, 50, 60)
+        self._ground(box2)
+
+        self._make_joint(
+            0,
+            [box2, ["Face6", "Vertex7"]],
+            [box1, ["Face6", "Vertex7"]],
+        )
+
+        self.assembly.solve()
+        plc_first = App.Placement(box1.Placement)
+
+        self.assembly.solve()
+        plc_second = box1.Placement
+
+        self.assertTrue(
+            plc_first.isSame(plc_second, 1e-6),
+            "Deterministic solver should produce identical results",
+        )

src/Mod/Assembly/AssemblyTests/TestSolverIntegration.py

@@ -0,0 +1,216 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+# /****************************************************************************
+#                                                                           *
+#    Copyright (c) 2025 Kindred Systems <development@kindred-systems.com>   *
+#                                                                           *
+#    This file is part of FreeCAD.                                          *
+#                                                                           *
+#    FreeCAD is free software: you can redistribute it and/or modify it     *
+#    under the terms of the GNU Lesser General Public License as            *
+#    published by the Free Software Foundation, either version 2.1 of the   *
+#    License, or (at your option) any later version.                        *
+#                                                                           *
+#    FreeCAD is distributed in the hope that it will be useful, but         *
+#    WITHOUT ANY WARRANTY; without even the implied warranty of             *
+#    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU       *
+#    Lesser General Public License for more details.                        *
+#                                                                           *
+#    You should have received a copy of the GNU Lesser General Public       *
+#    License along with FreeCAD. If not, see                                *
+#    <https://www.gnu.org/licenses/>.                                       *
+#                                                                           *
+# ***************************************************************************/
+
+"""
+Solver integration tests for Phase 1e (KCSolve refactor).
+
+These tests verify that the AssemblyObject → IKCSolver → OndselAdapter pipeline
+produces correct results via the full FreeCAD stack. They complement the C++
+unit tests in tests/src/Mod/Assembly/Solver/.
+"""
+
+import os
+import tempfile
+import unittest
+
+import FreeCAD as App
+import JointObject
+
+
+class TestSolverIntegration(unittest.TestCase):
+    """Full-stack solver regression tests exercising AssemblyObject.solve()."""
+
+    def setUp(self):
+        doc_name = self.__class__.__name__
+        if App.ActiveDocument:
+            if App.ActiveDocument.Name != doc_name:
+                App.newDocument(doc_name)
+        else:
+            App.newDocument(doc_name)
+        App.setActiveDocument(doc_name)
+        self.doc = App.ActiveDocument
+
+        self.assembly = self.doc.addObject("Assembly::AssemblyObject", "Assembly")
+        self.jointgroup = self.assembly.newObject("Assembly::JointGroup", "Joints")
+
+    def tearDown(self):
+        App.closeDocument(self.doc.Name)
+
+    # ── Helpers ─────────────────────────────────────────────────────
+
+    def _make_box(self, x=0, y=0, z=0, size=10):
+        """Create a Part::Box inside the assembly with a given offset."""
+        box = self.assembly.newObject("Part::Box", "Box")
+        box.Length = size
+        box.Width = size
+        box.Height = size
+        box.Placement = App.Placement(App.Vector(x, y, z), App.Rotation())
+        return box
+
+    def _ground(self, obj):
+        """Create a grounded joint for the given object."""
+        gnd = self.jointgroup.newObject("App::FeaturePython", "GroundedJoint")
+        JointObject.GroundedJoint(gnd, obj)
+        return gnd
+
+    def _make_joint(self, joint_type, ref1, ref2):
+        """Create a joint of the given type connecting two (obj, subelements) pairs.
+
+        joint_type: integer JointType enum value (0=Fixed, 1=Revolute, etc.)
+        ref1, ref2: tuples of (obj, [sub_element, sub_element])
+        """
+        joint = self.jointgroup.newObject("App::FeaturePython", "Joint")
+        JointObject.Joint(joint, joint_type)
+
+        refs = [
+            [ref1[0], ref1[1]],
+            [ref2[0], ref2[1]],
+        ]
+        joint.Proxy.setJointConnectors(joint, refs)
+        return joint
+
+    # ── Tests ───────────────────────────────────────────────────────
+
+    def test_solve_fixed_joint(self):
+        """Two boxes + grounded + fixed joint → placements match."""
+        box1 = self._make_box(10, 20, 30)
+        box2 = self._make_box(40, 50, 60)
+        self._ground(box2)
+
+        # Fixed joint (type 0) connecting Face6+Vertex7 on each box.
+        self._make_joint(
+            0,
+            [box2, ["Face6", "Vertex7"]],
+            [box1, ["Face6", "Vertex7"]],
+        )
+
+        # After setJointConnectors, solve() was already called internally.
+        # Verify that box1 moved to match box2.
+        self.assertTrue(
+            box1.Placement.isSame(box2.Placement, 1e-6),
+            "Fixed joint: box1 should match box2 placement",
+        )
+
+    def test_solve_revolute_joint(self):
+        """Two boxes + grounded + revolute joint → solve succeeds (return 0)."""
+        box1 = self._make_box(0, 0, 0)
+        box2 = self._make_box(100, 0, 0)
+        self._ground(box1)
+
+        # Revolute joint (type 1) connecting Face6+Vertex7.
+        self._make_joint(
+            1,
+            [box1, ["Face6", "Vertex7"]],
+            [box2, ["Face6", "Vertex7"]],
+        )
+
+        result = self.assembly.solve()
+        self.assertEqual(result, 0, "Revolute joint solve should succeed")
+
+    def test_solve_returns_code_for_no_ground(self):
+        """Assembly with no grounded parts → solve returns -6."""
+        box1 = self._make_box(0, 0, 0)
+        box2 = self._make_box(50, 0, 0)
+
+        # Fixed joint but no ground.
+        joint = self.jointgroup.newObject("App::FeaturePython", "Joint")
+        JointObject.Joint(joint, 0)
+        refs = [
+            [box1, ["Face6", "Vertex7"]],
+            [box2, ["Face6", "Vertex7"]],
+        ]
+        joint.Proxy.setJointConnectors(joint, refs)
+
+        result = self.assembly.solve()
+        self.assertEqual(result, -6, "No grounded parts should return -6")
+
+    def test_solve_returns_redundancy(self):
+        """Over-constrained assembly → solve returns -2 (redundant)."""
+        box1 = self._make_box(0, 0, 0)
+        box2 = self._make_box(50, 0, 0)
+        self._ground(box1)
+
+        # Two fixed joints between the same faces → redundant.
+        self._make_joint(
+            0,
+            [box1, ["Face6", "Vertex7"]],
+            [box2, ["Face6", "Vertex7"]],
+        )
+        self._make_joint(
+            0,
+            [box1, ["Face5", "Vertex5"]],
+            [box2, ["Face5", "Vertex5"]],
+        )
+
+        result = self.assembly.solve()
+        self.assertEqual(result, -2, "Redundant constraints should return -2")
+
+    def test_export_asmt(self):
+        """exportAsASMT() produces a non-empty file."""
+        box1 = self._make_box(0, 0, 0)
+        box2 = self._make_box(50, 0, 0)
+        self._ground(box1)
+
+        self._make_joint(
+            0,
+            [box1, ["Face6", "Vertex7"]],
+            [box2, ["Face6", "Vertex7"]],
+        )
+
+        self.assembly.solve()
+
+        with tempfile.NamedTemporaryFile(suffix=".asmt", delete=False) as f:
+            path = f.name
+
+        try:
+            self.assembly.exportAsASMT(path)
+            self.assertTrue(os.path.exists(path), "ASMT file should exist")
+            self.assertGreater(
+                os.path.getsize(path), 0, "ASMT file should be non-empty"
+            )
+        finally:
+            if os.path.exists(path):
+                os.unlink(path)
+
+    def test_solve_multiple_times_stable(self):
+        """Solving the same assembly twice produces identical placements."""
+        box1 = self._make_box(10, 20, 30)
+        box2 = self._make_box(40, 50, 60)
+        self._ground(box2)
+
+        self._make_joint(
+            0,
+            [box2, ["Face6", "Vertex7"]],
+            [box1, ["Face6", "Vertex7"]],
+        )
+
+        self.assembly.solve()
+        plc_first = App.Placement(box1.Placement)
+
+        self.assembly.solve()
+        plc_second = box1.Placement
+
+        self.assertTrue(
+            plc_first.isSame(plc_second, 1e-6),
+            "Deterministic solver should produce identical results",
+        )

src/Mod/Assembly/CMakeLists.txt

@@ -11,6 +11,7 @@ else ()
     endif ()
 endif ()
 
+add_subdirectory(Solver)
 add_subdirectory(App)
 
 if(BUILD_GUI)
@@ -56,6 +57,9 @@ SET(AssemblyTests_SRCS
     AssemblyTests/__init__.py
     AssemblyTests/TestCore.py
     AssemblyTests/TestCommandInsertLink.py
+    AssemblyTests/TestSolverIntegration.py
+    AssemblyTests/TestKindredSolverIntegration.py
+    AssemblyTests/TestKCSolvePy.py
     AssemblyTests/mocks/__init__.py
     AssemblyTests/mocks/MockGui.py
 )

src/Mod/Assembly/Gui/Resources/preferences/Assembly.ui

@@ -84,6 +84,20 @@ The files are named "runPreDrag.asmt" and "dragging.log" and are located in the
     </spacer>
    </item>
    <item row="3" column="0">
+    <widget class="QLabel" name="solverBackendLabel">
+     <property name="text">
+      <string>Solver backend</string>
+     </property>
+    </widget>
+   </item>
+   <item row="3" column="1">
+    <widget class="QComboBox" name="solverBackend">
+     <property name="toolTip">
+      <string>Select the constraint solver used for assembly solving</string>
+     </property>
+    </widget>
+   </item>
+   <item row="4" column="0">
     <spacer name="verticalSpacer">
      <property name="orientation">
       <enum>Qt::Vertical</enum>

src/Mod/Assembly/Preferences.py

@@ -40,13 +40,34 @@ class PreferencesPage:
         pref.SetBool("LeaveEditWithEscape", self.form.checkBoxEnableEscape.isChecked())
         pref.SetBool("LogSolverDebug", self.form.checkBoxSolverDebug.isChecked())
         pref.SetInt("GroundFirstPart", self.form.groundFirstPart.currentIndex())
+        idx = self.form.solverBackend.currentIndex()
+        solver_name = self.form.solverBackend.itemData(idx) or ""
+        pref.SetString("Solver", solver_name)
 
     def loadSettings(self):
         pref = preferences()
-        self.form.checkBoxEnableEscape.setChecked(pref.GetBool("LeaveEditWithEscape", True))
+        self.form.checkBoxEnableEscape.setChecked(
+            pref.GetBool("LeaveEditWithEscape", True)
+        )
         self.form.checkBoxSolverDebug.setChecked(pref.GetBool("LogSolverDebug", False))
         self.form.groundFirstPart.clear()
         self.form.groundFirstPart.addItem(translate("Assembly", "Ask"))
         self.form.groundFirstPart.addItem(translate("Assembly", "Always"))
         self.form.groundFirstPart.addItem(translate("Assembly", "Never"))
         self.form.groundFirstPart.setCurrentIndex(pref.GetInt("GroundFirstPart", 0))
+
+        self.form.solverBackend.clear()
+        self.form.solverBackend.addItem(translate("Assembly", "Default"), "")
+        try:
+            import kcsolve
+
+            for name in kcsolve.available():
+                solver = kcsolve.load(name)
+                self.form.solverBackend.addItem(solver.name(), name)
+        except ImportError:
+            pass
+        current = pref.GetString("Solver", "")
+        for i in range(self.form.solverBackend.count()):
+            if self.form.solverBackend.itemData(i) == current:
+                self.form.solverBackend.setCurrentIndex(i)
+                break

src/Mod/Assembly/Solver/CMakeLists.txt

@@ -0,0 +1,46 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+
+set(KCSolve_SRCS
+    KCSolveGlobal.h
+    Types.h
+    IKCSolver.h
+    SolverRegistry.h
+    SolverRegistry.cpp
+    OndselAdapter.h
+    OndselAdapter.cpp
+)
+
+add_library(KCSolve SHARED ${KCSolve_SRCS})
+
+target_include_directories(KCSolve
+    PUBLIC
+        ${CMAKE_SOURCE_DIR}/src
+        ${CMAKE_BINARY_DIR}/src
+)
+
+target_compile_definitions(KCSolve
+    PRIVATE
+        CMAKE_INSTALL_PREFIX="${CMAKE_INSTALL_PREFIX}"
+)
+
+target_link_libraries(KCSolve
+    PRIVATE
+        FreeCADBase
+        OndselSolver
+)
+
+# Platform-specific dynamic loading library
+if(NOT WIN32)
+    target_link_libraries(KCSolve PRIVATE ${CMAKE_DL_LIBS})
+endif()
+
+if(FREECAD_WARN_ERROR)
+    target_compile_warn_error(KCSolve)
+endif()
+
+SET_BIN_DIR(KCSolve KCSolve /Mod/Assembly)
+INSTALL(TARGETS KCSolve DESTINATION ${CMAKE_INSTALL_LIBDIR})
+
+if(FREECAD_USE_PYBIND11)
+    add_subdirectory(bindings)
+endif()

src/Mod/Assembly/Solver/IKCSolver.h

@@ -0,0 +1,189 @@
+// SPDX-License-Identifier: LGPL-2.1-or-later
+/****************************************************************************
+ *                                                                          *
+ *   Copyright (c) 2025 Kindred Systems <development@kindred-systems.com>   *
+ *                                                                          *
+ *   This file is part of FreeCAD.                                          *
+ *                                                                          *
+ *   FreeCAD is free software: you can redistribute it and/or modify it     *
+ *   under the terms of the GNU Lesser General Public License as            *
+ *   published by the Free Software Foundation, either version 2.1 of the   *
+ *   License, or (at your option) any later version.                        *
+ *                                                                          *
+ *   FreeCAD is distributed in the hope that it will be useful, but         *
+ *   WITHOUT ANY WARRANTY; without even the implied warranty of             *
+ *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU       *
+ *   Lesser General Public License for more details.                        *
+ *                                                                          *
+ *   You should have received a copy of the GNU Lesser General Public       *
+ *   License along with FreeCAD. If not, see                                *
+ *   <https://www.gnu.org/licenses/>.                                       *
+ *                                                                          *
+ ***************************************************************************/
+
+#ifndef KCSOLVE_IKCSOLVER_H
+#define KCSOLVE_IKCSOLVER_H
+
+#include <cstddef>
+#include <string>
+#include <vector>
+
+#include "Types.h"
+
+namespace KCSolve
+{
+
+/// Abstract interface for a pluggable assembly constraint solver.
+///
+/// Solver backends implement this interface. The Assembly module calls
+/// through it via the SolverRegistry. A minimal solver only needs to
+/// implement solve(), name(), and supported_joints() — all other methods
+/// have default implementations that either delegate to solve() or
+/// return sensible defaults.
+///
+/// Method mapping to current AssemblyObject operations:
+///
+///   solve()            <->  AssemblyObject::solve()
+///   pre_drag()         <->  AssemblyObject::preDrag()
+///   drag_step()        <->  AssemblyObject::doDragStep()
+///   post_drag()        <->  AssemblyObject::postDrag()
+///   run_kinematic()    <->  AssemblyObject::generateSimulation()
+///   num_frames()       <->  AssemblyObject::numberOfFrames()
+///   update_for_frame() <->  AssemblyObject::updateForFrame()
+///   diagnose()         <->  AssemblyObject::updateSolveStatus()
+
+class IKCSolver
+{
+public:
+    virtual ~IKCSolver() = default;
+
+    /// Human-readable solver name (e.g. "OndselSolver (Lagrangian)").
+    virtual std::string name() const = 0;
+
+    /// Return the set of BaseJointKind values this solver supports.
+    /// The registry uses this for capability-based solver selection.
+    virtual std::vector<BaseJointKind> supported_joints() const = 0;
+
+    // ── Static solve ───────────────────────────────────────────────
+
+    /// Solve the assembly for static equilibrium.
+    /// @param ctx  Complete description of parts, constraints, and options.
+    /// @return     Result with updated placements and diagnostics.
+    virtual SolveResult solve(const SolveContext& ctx) = 0;
+
+    /// Incrementally update an already-solved assembly after parameter
+    /// changes (e.g. joint angle/distance changed during joint creation).
+    /// Default: delegates to solve().
+    virtual SolveResult update(const SolveContext& ctx)
+    {
+        return solve(ctx);
+    }
+
+    // ── Interactive drag ───────────────────────────────────────────
+    //
+    // Three-phase protocol for interactive part dragging:
+    //   1. pre_drag()   — solve initial state, prepare for dragging
+    //   2. drag_step()  — called on each mouse move with updated positions
+    //   3. post_drag()  — finalize and release internal solver state
+    //
+    // Solvers can maintain internal state across the drag session for
+    // better interactive performance. This addresses a known weakness
+    // in the current direct-OndselSolver integration.
+
+    /// Prepare for an interactive drag session.
+    /// @param ctx         Assembly state before dragging begins.
+    /// @param drag_parts  IDs of parts being dragged.
+    /// @return            Initial solve result.
+    virtual SolveResult pre_drag(const SolveContext& ctx,
+                                 const std::vector<std::string>& /*drag_parts*/)
+    {
+        return solve(ctx);
+    }
+
+    /// Perform one incremental drag step.
+    /// @param drag_placements  Current placements of the dragged parts
+    ///                         (part ID + new transform).
+    /// @return                 Updated placements for all affected parts.
+    virtual SolveResult drag_step(
+        const std::vector<SolveResult::PartResult>& /*drag_placements*/)
+    {
+        return SolveResult {SolveStatus::Success, {}, -1, {}, 0};
+    }
+
+    /// End an interactive drag session and finalize state.
+    virtual void post_drag()
+    {
+    }
+
+    // ── Kinematic simulation ───────────────────────────────────────
+
+    /// Run a kinematic simulation over the time range in ctx.simulation.
+    /// After this call, num_frames() returns the frame count and
+    /// update_for_frame(i) retrieves individual frame placements.
+    /// Default: delegates to solve() (ignoring simulation params).
+    virtual SolveResult run_kinematic(const SolveContext& /*ctx*/)
+    {
+        return SolveResult {SolveStatus::Failed, {}, -1, {}, 0};
+    }
+
+    /// Number of simulation frames available after run_kinematic().
+    virtual std::size_t num_frames() const
+    {
+        return 0;
+    }
+
+    /// Retrieve part placements for simulation frame at index.
+    /// @pre index < num_frames()
+    virtual SolveResult update_for_frame(std::size_t /*index*/)
+    {
+        return SolveResult {SolveStatus::Failed, {}, -1, {}, 0};
+    }
+
+    // ── Diagnostics ────────────────────────────────────────────────
+
+    /// Analyze the assembly for redundant, conflicting, or malformed
+    /// constraints. May require a prior solve() call for some solvers.
+    virtual std::vector<ConstraintDiagnostic> diagnose(const SolveContext& /*ctx*/)
+    {
+        return {};
+    }
+
+    // ── Capability queries ─────────────────────────────────────────
+
+    /// Whether this solver produces deterministic results given
+    /// identical input.
+    virtual bool is_deterministic() const
+    {
+        return true;
+    }
+
+    /// Export solver-native debug/diagnostic file (e.g. ASMT for OndselSolver).
+    /// Default: no-op.  Requires a prior solve() or run_kinematic() call.
+    virtual void export_native(const std::string& /*path*/)
+    {
+    }
+
+    /// Whether this solver handles fixed-joint part bundling internally.
+    /// When false, the caller bundles parts connected by Fixed joints
+    /// before building the SolveContext. When true, the solver receives
+    /// unbundled parts and optimizes internally.
+    virtual bool supports_bundle_fixed() const
+    {
+        return false;
+    }
+
+    // Public default constructor for pybind11 trampoline support.
+    // The class remains abstract (3 pure virtuals prevent direct instantiation).
+    IKCSolver() = default;
+
+private:
+    // Non-copyable, non-movable (polymorphic base class)
+    IKCSolver(const IKCSolver&) = delete;
+    IKCSolver& operator=(const IKCSolver&) = delete;
+    IKCSolver(IKCSolver&&) = delete;
+    IKCSolver& operator=(IKCSolver&&) = delete;
+};
+
+}  // namespace KCSolve
+
+#endif  // KCSOLVE_IKCSOLVER_H

src/Mod/Assembly/Solver/KCSolveGlobal.h

@@ -0,0 +1,37 @@
+// SPDX-License-Identifier: LGPL-2.1-or-later
+/****************************************************************************
+ *                                                                          *
+ *   Copyright (c) 2025 Kindred Systems <development@kindred-systems.com>   *
+ *                                                                          *
+ *   This file is part of FreeCAD.                                          *
+ *                                                                          *
+ *   FreeCAD is free software: you can redistribute it and/or modify it     *
+ *   under the terms of the GNU Lesser General Public License as            *
+ *   published by the Free Software Foundation, either version 2.1 of the   *
+ *   License, or (at your option) any later version.                        *
+ *                                                                          *
+ *   FreeCAD is distributed in the hope that it will be useful, but         *
+ *   WITHOUT ANY WARRANTY; without even the implied warranty of             *
+ *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU       *
+ *   Lesser General Public License for more details.                        *
+ *                                                                          *
+ *   You should have received a copy of the GNU Lesser General Public       *
+ *   License along with FreeCAD. If not, see                                *
+ *   <https://www.gnu.org/licenses/>.                                       *
+ *                                                                          *
+ ***************************************************************************/
+
+#include <FCGlobal.h>
+
+#ifndef KCSOLVE_GLOBAL_H
+#define KCSOLVE_GLOBAL_H
+
+#ifndef KCSolveExport
+# ifdef KCSolve_EXPORTS
+#  define KCSolveExport FREECAD_DECL_EXPORT
+# else
+#  define KCSolveExport FREECAD_DECL_IMPORT
+# endif
+#endif
+
+#endif  // KCSOLVE_GLOBAL_H

src/Mod/Assembly/Solver/OndselAdapter.cpp

@@ -0,0 +1,796 @@
+// SPDX-License-Identifier: LGPL-2.1-or-later
+/****************************************************************************
+ *                                                                          *
+ *   Copyright (c) 2025 Kindred Systems <development@kindred-systems.com>   *
+ *                                                                          *
+ *   This file is part of FreeCAD.                                          *
+ *                                                                          *
+ *   FreeCAD is free software: you can redistribute it and/or modify it     *
+ *   under the terms of the GNU Lesser General Public License as            *
+ *   published by the Free Software Foundation, either version 2.1 of the   *
+ *   License, or (at your option) any later version.                        *
+ *                                                                          *
+ *   FreeCAD is distributed in the hope that it will be useful, but         *
+ *   WITHOUT ANY WARRANTY; without even the implied warranty of             *
+ *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU       *
+ *   Lesser General Public License for more details.                        *
+ *                                                                          *
+ *   You should have received a copy of the GNU Lesser General Public       *
+ *   License along with FreeCAD. If not, see                                *
+ *   <https://www.gnu.org/licenses/>.                                       *
+ *                                                                          *
+ ***************************************************************************/
+
+#include "OndselAdapter.h"
+#include "SolverRegistry.h"
+
+#include <Base/Console.h>
+
+#include <OndselSolver/CREATE.h>
+#include <OndselSolver/ASMTAssembly.h>
+#include <OndselSolver/ASMTAngleJoint.h>
+#include <OndselSolver/ASMTConstantGravity.h>
+#include <OndselSolver/ASMTCylSphJoint.h>
+#include <OndselSolver/ASMTCylindricalJoint.h>
+#include <OndselSolver/ASMTFixedJoint.h>
+#include <OndselSolver/ASMTGearJoint.h>
+#include <OndselSolver/ASMTGeneralMotion.h>
+#include <OndselSolver/ASMTLineInPlaneJoint.h>
+#include <OndselSolver/ASMTMarker.h>
+#include <OndselSolver/ASMTParallelAxesJoint.h>
+#include <OndselSolver/ASMTPart.h>
+#include <OndselSolver/ASMTPerpendicularJoint.h>
+#include <OndselSolver/ASMTPlanarJoint.h>
+#include <OndselSolver/ASMTPointInPlaneJoint.h>
+#include <OndselSolver/ASMTRackPinionJoint.h>
+#include <OndselSolver/ASMTRevCylJoint.h>
+#include <OndselSolver/ASMTRevoluteJoint.h>
+#include <OndselSolver/ASMTRotationLimit.h>
+#include <OndselSolver/ASMTRotationalMotion.h>
+#include <OndselSolver/ASMTScrewJoint.h>
+#include <OndselSolver/ASMTSimulationParameters.h>
+#include <OndselSolver/ASMTSphSphJoint.h>
+#include <OndselSolver/ASMTSphericalJoint.h>
+#include <OndselSolver/ASMTTranslationLimit.h>
+#include <OndselSolver/ASMTTranslationalJoint.h>
+#include <OndselSolver/ASMTTranslationalMotion.h>
+#include <OndselSolver/ExternalSystem.h>
+
+// For System::jointsMotionsDo and diagnostic iteration
+#include <OndselSolver/Constraint.h>
+#include <OndselSolver/Joint.h>
+#include <OndselSolver/System.h>
+
+using namespace MbD;
+
+namespace KCSolve
+{
+
+// ── Static registration ────────────────────────────────────────────
+
+void OndselAdapter::register_solver()
+{
+    SolverRegistry::instance().register_solver(
+        "ondsel",
+        []() { return std::make_unique<OndselAdapter>(); });
+}
+
+
+// ── IKCSolver identity ─────────────────────────────────────────────
+
+std::string OndselAdapter::name() const
+{
+    return "OndselSolver (Lagrangian)";
+}
+
+bool OndselAdapter::is_deterministic() const
+{
+    return true;
+}
+
+bool OndselAdapter::supports_bundle_fixed() const
+{
+    return false;
+}
+
+std::vector<BaseJointKind> OndselAdapter::supported_joints() const
+{
+    return {
+        BaseJointKind::Coincident,
+        BaseJointKind::PointOnLine,
+        BaseJointKind::PointInPlane,
+        BaseJointKind::Concentric,
+        BaseJointKind::Tangent,
+        BaseJointKind::Planar,
+        BaseJointKind::LineInPlane,
+        BaseJointKind::Parallel,
+        BaseJointKind::Perpendicular,
+        BaseJointKind::Angle,
+        BaseJointKind::Fixed,
+        BaseJointKind::Revolute,
+        BaseJointKind::Cylindrical,
+        BaseJointKind::Slider,
+        BaseJointKind::Ball,
+        BaseJointKind::Screw,
+        BaseJointKind::Gear,
+        BaseJointKind::RackPinion,
+        BaseJointKind::DistancePointPoint,
+        BaseJointKind::DistanceCylSph,
+    };
+}
+
+
+// ── Quaternion → rotation matrix ───────────────────────────────────
+
+void OndselAdapter::quat_to_matrix(const std::array<double, 4>& q,
+                                    double (&mat)[3][3])
+{
+    double w = q[0], x = q[1], y = q[2], z = q[3];
+    double xx = x * x, yy = y * y, zz = z * z;
+    double xy = x * y, xz = x * z, yz = y * z;
+    double wx = w * x, wy = w * y, wz = w * z;
+
+    mat[0][0] = 1.0 - 2.0 * (yy + zz);
+    mat[0][1] = 2.0 * (xy - wz);
+    mat[0][2] = 2.0 * (xz + wy);
+    mat[1][0] = 2.0 * (xy + wz);
+    mat[1][1] = 1.0 - 2.0 * (xx + zz);
+    mat[1][2] = 2.0 * (yz - wx);
+    mat[2][0] = 2.0 * (xz - wy);
+    mat[2][1] = 2.0 * (yz + wx);
+    mat[2][2] = 1.0 - 2.0 * (xx + yy);
+}
+
+
+// ── Assembly building ──────────────────────────────────────────────
+
+std::shared_ptr<ASMTPart> OndselAdapter::make_part(const Part& part)
+{
+    auto mbdPart = CREATE<ASMTPart>::With();
+    mbdPart->setName(part.id);
+
+    auto massMarker = CREATE<ASMTPrincipalMassMarker>::With();
+    massMarker->setMass(part.mass);
+    massMarker->setDensity(1.0);
+    massMarker->setMomentOfInertias(1.0, 1.0, 1.0);
+    mbdPart->setPrincipalMassMarker(massMarker);
+
+    const auto& pos = part.placement.position;
+    mbdPart->setPosition3D(pos[0], pos[1], pos[2]);
+
+    double mat[3][3];
+    quat_to_matrix(part.placement.quaternion, mat);
+    mbdPart->setRotationMatrix(
+        mat[0][0], mat[0][1], mat[0][2],
+        mat[1][0], mat[1][1], mat[1][2],
+        mat[2][0], mat[2][1], mat[2][2]);
+
+    return mbdPart;
+}
+
+std::shared_ptr<ASMTMarker> OndselAdapter::make_marker(const std::string& markerName,
+                                                        const Transform& tf)
+{
+    auto mbdMarker = CREATE<ASMTMarker>::With();
+    mbdMarker->setName(markerName);
+
+    const auto& pos = tf.position;
+    mbdMarker->setPosition3D(pos[0], pos[1], pos[2]);
+
+    double mat[3][3];
+    quat_to_matrix(tf.quaternion, mat);
+    mbdMarker->setRotationMatrix(
+        mat[0][0], mat[0][1], mat[0][2],
+        mat[1][0], mat[1][1], mat[1][2],
+        mat[2][0], mat[2][1], mat[2][2]);
+
+    return mbdMarker;
+}
+
+std::shared_ptr<ASMTJoint> OndselAdapter::create_joint(const Constraint& c)
+{
+    auto param = [&](std::size_t i, double fallback = 0.0) -> double {
+        return i < c.params.size() ? c.params[i] : fallback;
+    };
+
+    switch (c.type) {
+        case BaseJointKind::Coincident:
+            return CREATE<ASMTSphericalJoint>::With();
+
+        case BaseJointKind::PointOnLine: {
+            auto j = CREATE<ASMTCylSphJoint>::With();
+            j->distanceIJ = param(0);
+            return j;
+        }
+
+        case BaseJointKind::PointInPlane: {
+            auto j = CREATE<ASMTPointInPlaneJoint>::With();
+            j->offset = param(0);
+            return j;
+        }
+
+        case BaseJointKind::Concentric: {
+            auto j = CREATE<ASMTRevCylJoint>::With();
+            j->distanceIJ = param(0);
+            return j;
+        }
+
+        case BaseJointKind::Tangent: {
+            auto j = CREATE<ASMTPlanarJoint>::With();
+            j->offset = param(0);
+            return j;
+        }
+
+        case BaseJointKind::Planar: {
+            auto j = CREATE<ASMTPlanarJoint>::With();
+            j->offset = param(0);
+            return j;
+        }
+
+        case BaseJointKind::LineInPlane: {
+            auto j = CREATE<ASMTLineInPlaneJoint>::With();
+            j->offset = param(0);
+            return j;
+        }
+
+        case BaseJointKind::Parallel:
+            return CREATE<ASMTParallelAxesJoint>::With();
+
+        case BaseJointKind::Perpendicular:
+            return CREATE<ASMTPerpendicularJoint>::With();
+
+        case BaseJointKind::Angle: {
+            auto j = CREATE<ASMTAngleJoint>::With();
+            j->theIzJz = param(0);
+            return j;
+        }
+
+        case BaseJointKind::Fixed:
+            return CREATE<ASMTFixedJoint>::With();
+
+        case BaseJointKind::Revolute:
+            return CREATE<ASMTRevoluteJoint>::With();
+
+        case BaseJointKind::Cylindrical:
+            return CREATE<ASMTCylindricalJoint>::With();
+
+        case BaseJointKind::Slider:
+            return CREATE<ASMTTranslationalJoint>::With();
+
+        case BaseJointKind::Ball:
+            return CREATE<ASMTSphericalJoint>::With();
+
+        case BaseJointKind::Screw: {
+            auto j = CREATE<ASMTScrewJoint>::With();
+            j->pitch = param(0);
+            return j;
+        }
+
+        case BaseJointKind::Gear: {
+            auto j = CREATE<ASMTGearJoint>::With();
+            j->radiusI = param(0);
+            j->radiusJ = param(1);
+            return j;
+        }
+
+        case BaseJointKind::RackPinion: {
+            auto j = CREATE<ASMTRackPinionJoint>::With();
+            j->pitchRadius = param(0);
+            return j;
+        }
+
+        case BaseJointKind::DistancePointPoint: {
+            auto j = CREATE<ASMTSphSphJoint>::With();
+            j->distanceIJ = param(0);
+            return j;
+        }
+
+        case BaseJointKind::DistanceCylSph: {
+            auto j = CREATE<ASMTCylSphJoint>::With();
+            j->distanceIJ = param(0);
+            return j;
+        }
+
+        // Unsupported types
+        case BaseJointKind::Universal:
+        case BaseJointKind::Cam:
+        case BaseJointKind::Slot:
+        case BaseJointKind::Custom:
+            Base::Console().warning(
+                "KCSolve: OndselAdapter does not support joint kind %d for constraint '%s'\n",
+                static_cast<int>(c.type), c.id.c_str());
+            return nullptr;
+    }
+
+    return nullptr;  // unreachable, but silences compiler warnings
+}
+
+void OndselAdapter::add_limits(const Constraint& c,
+                                const std::string& marker_i,
+                                const std::string& marker_j)
+{
+    for (const auto& lim : c.limits) {
+        switch (lim.kind) {
+            case Constraint::Limit::Kind::TranslationMin: {
+                auto limit = CREATE<ASMTTranslationLimit>::With();
+                limit->setName(c.id + "-LimitLenMin");
+                limit->setMarkerI(marker_i);
+                limit->setMarkerJ(marker_j);
+                limit->settype("=>");
+                limit->setlimit(std::to_string(lim.value));
+                limit->settol(std::to_string(lim.tolerance));
+                assembly_->addLimit(limit);
+                break;
+            }
+            case Constraint::Limit::Kind::TranslationMax: {
+                auto limit = CREATE<ASMTTranslationLimit>::With();
+                limit->setName(c.id + "-LimitLenMax");
+                limit->setMarkerI(marker_i);
+                limit->setMarkerJ(marker_j);
+                limit->settype("=<");
+                limit->setlimit(std::to_string(lim.value));
+                limit->settol(std::to_string(lim.tolerance));
+                assembly_->addLimit(limit);
+                break;
+            }
+            case Constraint::Limit::Kind::RotationMin: {
+                auto limit = CREATE<ASMTRotationLimit>::With();
+                limit->setName(c.id + "-LimitRotMin");
+                limit->setMarkerI(marker_i);
+                limit->setMarkerJ(marker_j);
+                limit->settype("=>");
+                limit->setlimit(std::to_string(lim.value) + "*pi/180.0");
+                limit->settol(std::to_string(lim.tolerance));
+                assembly_->addLimit(limit);
+                break;
+            }
+            case Constraint::Limit::Kind::RotationMax: {
+                auto limit = CREATE<ASMTRotationLimit>::With();
+                limit->setName(c.id + "-LimitRotMax");
+                limit->setMarkerI(marker_i);
+                limit->setMarkerJ(marker_j);
+                limit->settype("=<");
+                limit->setlimit(std::to_string(lim.value) + "*pi/180.0");
+                limit->settol(std::to_string(lim.tolerance));
+                assembly_->addLimit(limit);
+                break;
+            }
+        }
+    }
+}
+
+void OndselAdapter::add_motions(const SolveContext& ctx,
+                                 const std::string& marker_i,
+                                 const std::string& marker_j,
+                                 const std::string& joint_id)
+{
+    // Collect motions that target this joint.
+    std::vector<const MotionDef*> joint_motions;
+    for (const auto& m : ctx.motions) {
+        if (m.joint_id == joint_id) {
+            joint_motions.push_back(&m);
+        }
+    }
+
+    if (joint_motions.empty()) {
+        return;
+    }
+
+    // If there are two motions of different kinds on the same joint,
+    // combine them into a GeneralMotion (cylindrical joint case).
+    if (joint_motions.size() == 2
+        && joint_motions[0]->kind != joint_motions[1]->kind) {
+        auto motion = CREATE<ASMTGeneralMotion>::With();
+        motion->setName(joint_id + "-GeneralMotion");
+        motion->setMarkerI(marker_i);
+        motion->setMarkerJ(marker_j);
+
+        for (const auto* m : joint_motions) {
+            if (m->kind == MotionDef::Kind::Rotational) {
+                motion->angIJJ->atiput(2, m->rotation_expr);
+            }
+            else {
+                motion->rIJI->atiput(2, m->translation_expr);
+            }
+        }
+        assembly_->addMotion(motion);
+        return;
+    }
+
+    // Single motion or multiple of the same kind.
+    for (const auto* m : joint_motions) {
+        switch (m->kind) {
+            case MotionDef::Kind::Rotational: {
+                auto motion = CREATE<ASMTRotationalMotion>::With();
+                motion->setName(joint_id + "-AngularMotion");
+                motion->setMarkerI(marker_i);
+                motion->setMarkerJ(marker_j);
+                motion->setRotationZ(m->rotation_expr);
+                assembly_->addMotion(motion);
+                break;
+            }
+            case MotionDef::Kind::Translational: {
+                auto motion = CREATE<ASMTTranslationalMotion>::With();
+                motion->setName(joint_id + "-LinearMotion");
+                motion->setMarkerI(marker_i);
+                motion->setMarkerJ(marker_j);
+                motion->setTranslationZ(m->translation_expr);
+                assembly_->addMotion(motion);
+                break;
+            }
+            case MotionDef::Kind::General: {
+                auto motion = CREATE<ASMTGeneralMotion>::With();
+                motion->setName(joint_id + "-GeneralMotion");
+                motion->setMarkerI(marker_i);
+                motion->setMarkerJ(marker_j);
+                if (!m->rotation_expr.empty()) {
+                    motion->angIJJ->atiput(2, m->rotation_expr);
+                }
+                if (!m->translation_expr.empty()) {
+                    motion->rIJI->atiput(2, m->translation_expr);
+                }
+                assembly_->addMotion(motion);
+                break;
+            }
+        }
+    }
+}
+
+void OndselAdapter::fix_grounded_parts(const SolveContext& ctx)
+{
+    for (const auto& part : ctx.parts) {
+        if (!part.grounded) {
+            continue;
+        }
+
+        auto it = part_map_.find(part.id);
+        if (it == part_map_.end()) {
+            continue;
+        }
+
+        // Assembly-level marker at the part's placement.
+        std::string asmMarkerName = "ground-" + part.id;
+        auto asmMarker = make_marker(asmMarkerName, part.placement);
+        assembly_->addMarker(asmMarker);
+
+        // Part-level marker at identity.
+        std::string partMarkerName = "FixingMarker";
+        auto partMarker = make_marker(partMarkerName, Transform::identity());
+        it->second->addMarker(partMarker);
+
+        // Fixed joint connecting them.
+        auto fixedJoint = CREATE<ASMTFixedJoint>::With();
+        fixedJoint->setName("ground-fix-" + part.id);
+        fixedJoint->setMarkerI("/OndselAssembly/" + asmMarkerName);
+        fixedJoint->setMarkerJ("/OndselAssembly/" + part.id + "/" + partMarkerName);
+        assembly_->addJoint(fixedJoint);
+    }
+}
+
+void OndselAdapter::set_simulation_params(const SimulationParams& params)
+{
+    auto mbdSim = assembly_->simulationParameters;
+    mbdSim->settstart(params.t_start);
+    mbdSim->settend(params.t_end);
+    mbdSim->sethout(params.h_out);
+    mbdSim->sethmin(params.h_min);
+    mbdSim->sethmax(params.h_max);
+    mbdSim->seterrorTol(params.error_tol);
+}
+
+void OndselAdapter::build_assembly(const SolveContext& ctx)
+{
+    assembly_ = CREATE<ASMTAssembly>::With();
+    assembly_->setName("OndselAssembly");
+    part_map_.clear();
+
+    // Do NOT set externalSystem->freecadAssemblyObject — breaking the coupling.
+
+    // Add parts.
+    for (const auto& part : ctx.parts) {
+        auto mbdPart = make_part(part);
+        assembly_->addPart(mbdPart);
+        part_map_[part.id] = mbdPart;
+    }
+
+    // Fix grounded parts.
+    fix_grounded_parts(ctx);
+
+    // Add constraints (joints + limits + motions).
+    for (const auto& c : ctx.constraints) {
+        if (!c.activated) {
+            continue;
+        }
+
+        auto mbdJoint = create_joint(c);
+        if (!mbdJoint) {
+            continue;
+        }
+
+        // Create markers on the respective parts.
+        auto it_i = part_map_.find(c.part_i);
+        auto it_j = part_map_.find(c.part_j);
+        if (it_i == part_map_.end() || it_j == part_map_.end()) {
+            Base::Console().warning(
+                "KCSolve: constraint '%s' references unknown part(s)\n",
+                c.id.c_str());
+            continue;
+        }
+
+        std::string markerNameI = c.id + "-mkrI";
+        std::string markerNameJ = c.id + "-mkrJ";
+
+        auto mkrI = make_marker(markerNameI, c.marker_i);
+        it_i->second->addMarker(mkrI);
+
+        auto mkrJ = make_marker(markerNameJ, c.marker_j);
+        it_j->second->addMarker(mkrJ);
+
+        std::string fullMarkerI = "/OndselAssembly/" + c.part_i + "/" + markerNameI;
+        std::string fullMarkerJ = "/OndselAssembly/" + c.part_j + "/" + markerNameJ;
+
+        mbdJoint->setName(c.id);
+        mbdJoint->setMarkerI(fullMarkerI);
+        mbdJoint->setMarkerJ(fullMarkerJ);
+        assembly_->addJoint(mbdJoint);
+
+        // Add limits (only when not in simulation mode).
+        if (!ctx.simulation.has_value() && !c.limits.empty()) {
+            add_limits(c, fullMarkerI, fullMarkerJ);
+        }
+
+        // Add motions.
+        if (!ctx.motions.empty()) {
+            add_motions(ctx, fullMarkerI, fullMarkerJ, c.id);
+        }
+    }
+
+    // Set simulation parameters if present.
+    if (ctx.simulation.has_value()) {
+        set_simulation_params(*ctx.simulation);
+    }
+}
+
+
+// ── Result extraction ──────────────────────────────────────────────
+
+Transform OndselAdapter::extract_part_transform(
+    const std::shared_ptr<ASMTPart>& part) const
+{
+    Transform tf;
+    double x, y, z;
+    part->getPosition3D(x, y, z);
+    tf.position = {x, y, z};
+
+    double q0, q1, q2, q3;
+    part->getQuarternions(q0, q1, q2, q3);
+    // OndselSolver returns (w, x, y, z) — matches our convention.
+    tf.quaternion = {q0, q1, q2, q3};
+
+    return tf;
+}
+
+SolveResult OndselAdapter::extract_result() const
+{
+    SolveResult result;
+    result.status = SolveStatus::Success;
+
+    for (const auto& [id, mbdPart] : part_map_) {
+        SolveResult::PartResult pr;
+        pr.id = id;
+        pr.placement = extract_part_transform(mbdPart);
+        result.placements.push_back(std::move(pr));
+    }
+
+    return result;
+}
+
+std::vector<ConstraintDiagnostic> OndselAdapter::extract_diagnostics() const
+{
+    std::vector<ConstraintDiagnostic> diags;
+
+    if (!assembly_ || !assembly_->mbdSystem) {
+        return diags;
+    }
+
+    assembly_->mbdSystem->jointsMotionsDo([&](std::shared_ptr<Joint> jm) {
+        if (!jm) {
+            return;
+        }
+
+        bool isRedundant = false;
+        jm->constraintsDo([&](std::shared_ptr<MbD::Constraint> con) {
+            if (!con) {
+                return;
+            }
+            std::string spec = con->constraintSpec();
+            if (spec.rfind("Redundant", 0) == 0) {
+                isRedundant = true;
+            }
+        });
+
+        if (isRedundant) {
+            // Extract the constraint name from the solver's joint name.
+            // Format: "/OndselAssembly/ground_moves#Joint001" → "Joint001"
+            std::string fullName = jm->name;
+            std::size_t hashPos = fullName.find_last_of('#');
+            std::string cleanName = (hashPos != std::string::npos)
+                ? fullName.substr(hashPos + 1)
+                : fullName;
+
+            ConstraintDiagnostic diag;
+            diag.constraint_id = cleanName;
+            diag.kind = ConstraintDiagnostic::Kind::Redundant;
+            diag.detail = "Constraint is redundant";
+            diags.push_back(std::move(diag));
+        }
+    });
+
+    return diags;
+}
+
+
+// ── Solve operations ───────────────────────────────────────────────
+
+SolveResult OndselAdapter::solve(const SolveContext& ctx)
+{
+    try {
+        build_assembly(ctx);
+        assembly_->runPreDrag();
+        return extract_result();
+    }
+    catch (const std::exception& e) {
+        Base::Console().warning("KCSolve: OndselAdapter solve failed: %s\n", e.what());
+        return SolveResult {SolveStatus::Failed, {}, -1, {}, 0};
+    }
+    catch (...) {
+        Base::Console().warning("KCSolve: OndselAdapter solve failed: unknown exception\n");
+        return SolveResult {SolveStatus::Failed, {}, -1, {}, 0};
+    }
+}
+
+SolveResult OndselAdapter::update(const SolveContext& ctx)
+{
+    return solve(ctx);
+}
+
+
+// ── Drag protocol ──────────────────────────────────────────────────
+
+SolveResult OndselAdapter::pre_drag(const SolveContext& ctx,
+                                     const std::vector<std::string>& drag_parts)
+{
+    drag_part_ids_ = drag_parts;
+
+    try {
+        build_assembly(ctx);
+        assembly_->runPreDrag();
+        return extract_result();
+    }
+    catch (const std::exception& e) {
+        Base::Console().warning("KCSolve: OndselAdapter pre_drag failed: %s\n", e.what());
+        return SolveResult {SolveStatus::Failed, {}, -1, {}, 0};
+    }
+    catch (...) {
+        Base::Console().warning("KCSolve: OndselAdapter pre_drag failed: unknown exception\n");
+        return SolveResult {SolveStatus::Failed, {}, -1, {}, 0};
+    }
+}
+
+SolveResult OndselAdapter::drag_step(
+    const std::vector<SolveResult::PartResult>& drag_placements)
+{
+    if (!assembly_) {
+        return SolveResult {SolveStatus::Failed, {}, -1, {}, 0};
+    }
+
+    try {
+        auto dragParts = std::make_shared<std::vector<std::shared_ptr<ASMTPart>>>();
+
+        for (const auto& dp : drag_placements) {
+            auto it = part_map_.find(dp.id);
+            if (it == part_map_.end()) {
+                continue;
+            }
+            auto& mbdPart = it->second;
+
+            // Update position.
+            const auto& pos = dp.placement.position;
+            mbdPart->updateMbDFromPosition3D(pos[0], pos[1], pos[2]);
+
+            // Update rotation.
+            double mat[3][3];
+            quat_to_matrix(dp.placement.quaternion, mat);
+            mbdPart->updateMbDFromRotationMatrix(
+                mat[0][0], mat[0][1], mat[0][2],
+                mat[1][0], mat[1][1], mat[1][2],
+                mat[2][0], mat[2][1], mat[2][2]);
+
+            dragParts->push_back(mbdPart);
+        }
+
+        assembly_->runDragStep(dragParts);
+        return extract_result();
+    }
+    catch (...) {
+        // Drag step failures are non-fatal — caller will skip this frame.
+        return SolveResult {SolveStatus::Failed, {}, -1, {}, 0};
+    }
+}
+
+void OndselAdapter::post_drag()
+{
+    if (assembly_) {
+        assembly_->runPostDrag();
+    }
+    drag_part_ids_.clear();
+}
+
+
+// ── Kinematic simulation ───────────────────────────────────────────
+
+SolveResult OndselAdapter::run_kinematic(const SolveContext& ctx)
+{
+    try {
+        build_assembly(ctx);
+        assembly_->runKINEMATIC();
+
+        auto result = extract_result();
+        result.num_frames = assembly_->numberOfFrames();
+        return result;
+    }
+    catch (const std::exception& e) {
+        Base::Console().warning("KCSolve: OndselAdapter run_kinematic failed: %s\n", e.what());
+        return SolveResult {SolveStatus::Failed, {}, -1, {}, 0};
+    }
+    catch (...) {
+        Base::Console().warning(
+            "KCSolve: OndselAdapter run_kinematic failed: unknown exception\n");
+        return SolveResult {SolveStatus::Failed, {}, -1, {}, 0};
+    }
+}
+
+std::size_t OndselAdapter::num_frames() const
+{
+    if (!assembly_) {
+        return 0;
+    }
+    return assembly_->numberOfFrames();
+}
+
+SolveResult OndselAdapter::update_for_frame(std::size_t index)
+{
+    if (!assembly_) {
+        return SolveResult {SolveStatus::Failed, {}, -1, {}, 0};
+    }
+
+    if (index >= assembly_->numberOfFrames()) {
+        return SolveResult {SolveStatus::Failed, {}, -1, {}, 0};
+    }
+
+    assembly_->updateForFrame(index);
+    return extract_result();
+}
+
+
+// ── Diagnostics ────────────────────────────────────────────────────
+
+std::vector<ConstraintDiagnostic> OndselAdapter::diagnose(const SolveContext& ctx)
+{
+    // Ensure we have a solved assembly to inspect.
+    if (!assembly_ || !assembly_->mbdSystem) {
+        solve(ctx);
+    }
+    return extract_diagnostics();
+}
+
+// ── Native export ──────────────────────────────────────────────────
+
+void OndselAdapter::export_native(const std::string& path)
+{
+    if (assembly_) {
+        assembly_->outputFile(path);
+    }
+}
+
+}  // namespace KCSolve

src/Mod/Assembly/Solver/OndselAdapter.h

@@ -0,0 +1,129 @@
+// SPDX-License-Identifier: LGPL-2.1-or-later
+/****************************************************************************
+ *                                                                          *
+ *   Copyright (c) 2025 Kindred Systems <development@kindred-systems.com>   *
+ *                                                                          *
+ *   This file is part of FreeCAD.                                          *
+ *                                                                          *
+ *   FreeCAD is free software: you can redistribute it and/or modify it     *
+ *   under the terms of the GNU Lesser General Public License as            *
+ *   published by the Free Software Foundation, either version 2.1 of the   *
+ *   License, or (at your option) any later version.                        *
+ *                                                                          *
+ *   FreeCAD is distributed in the hope that it will be useful, but         *
+ *   WITHOUT ANY WARRANTY; without even the implied warranty of             *
+ *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU       *
+ *   Lesser General Public License for more details.                        *
+ *                                                                          *
+ *   You should have received a copy of the GNU Lesser General Public       *
+ *   License along with FreeCAD. If not, see                                *
+ *   <https://www.gnu.org/licenses/>.                                       *
+ *                                                                          *
+ ***************************************************************************/
+
+#ifndef KCSOLVE_ONDSELADAPTER_H
+#define KCSOLVE_ONDSELADAPTER_H
+
+#include <memory>
+#include <string>
+#include <unordered_map>
+#include <vector>
+
+#include "IKCSolver.h"
+#include "KCSolveGlobal.h"
+
+namespace MbD
+{
+class ASMTAssembly;
+class ASMTJoint;
+class ASMTMarker;
+class ASMTPart;
+}  // namespace MbD
+
+namespace KCSolve
+{
+
+/// IKCSolver implementation wrapping OndselSolver's Lagrangian MBD engine.
+///
+/// Translates KCSolve types (SolveContext, BaseJointKind, Transform) to
+/// OndselSolver's ASMT hierarchy (ASMTAssembly, ASMTPart, ASMTJoint, etc.)
+/// and extracts results back into SolveResult.
+///
+/// All OndselSolver #includes are confined to OndselAdapter.cpp.
+
+class KCSolveExport OndselAdapter : public IKCSolver
+{
+public:
+    OndselAdapter() = default;
+
+    // ── IKCSolver pure virtuals ────────────────────────────────────
+
+    std::string name() const override;
+    std::vector<BaseJointKind> supported_joints() const override;
+    SolveResult solve(const SolveContext& ctx) override;
+
+    // ── IKCSolver overrides ────────────────────────────────────────
+
+    SolveResult update(const SolveContext& ctx) override;
+
+    SolveResult pre_drag(const SolveContext& ctx,
+                         const std::vector<std::string>& drag_parts) override;
+    SolveResult drag_step(
+        const std::vector<SolveResult::PartResult>& drag_placements) override;
+    void post_drag() override;
+
+    SolveResult run_kinematic(const SolveContext& ctx) override;
+    std::size_t num_frames() const override;
+    SolveResult update_for_frame(std::size_t index) override;
+
+    std::vector<ConstraintDiagnostic> diagnose(const SolveContext& ctx) override;
+
+    bool is_deterministic() const override;
+    bool supports_bundle_fixed() const override;
+    void export_native(const std::string& path) override;
+
+    /// Register OndselAdapter as "ondsel" in the SolverRegistry.
+    /// Call once at module init time.
+    static void register_solver();
+
+private:
+    // ── Assembly building ──────────────────────────────────────────
+
+    void build_assembly(const SolveContext& ctx);
+    std::shared_ptr<MbD::ASMTPart> make_part(const Part& part);
+    std::shared_ptr<MbD::ASMTMarker> make_marker(const std::string& name,
+                                                   const Transform& tf);
+    std::shared_ptr<MbD::ASMTJoint> create_joint(const Constraint& c);
+    void add_limits(const Constraint& c,
+                    const std::string& marker_i,
+                    const std::string& marker_j);
+    void add_motions(const SolveContext& ctx,
+                     const std::string& marker_i,
+                     const std::string& marker_j,
+                     const std::string& joint_id);
+    void fix_grounded_parts(const SolveContext& ctx);
+    void set_simulation_params(const SimulationParams& params);
+
+    // ── Result extraction ──────────────────────────────────────────
+
+    SolveResult extract_result() const;
+    std::vector<ConstraintDiagnostic> extract_diagnostics() const;
+    Transform extract_part_transform(
+        const std::shared_ptr<MbD::ASMTPart>& part) const;
+
+    // ── Quaternion ↔ rotation matrix conversion ────────────────────
+
+    /// Convert unit quaternion (w,x,y,z) to 3×3 rotation matrix (row-major).
+    static void quat_to_matrix(const std::array<double, 4>& q,
+                                double (&mat)[3][3]);
+
+    // ── Internal state ─────────────────────────────────────────────
+
+    std::shared_ptr<MbD::ASMTAssembly> assembly_;
+    std::unordered_map<std::string, std::shared_ptr<MbD::ASMTPart>> part_map_;
+    std::vector<std::string> drag_part_ids_;
+};
+
+}  // namespace KCSolve
+
+#endif  // KCSOLVE_ONDSELADAPTER_H

src/Mod/Assembly/Solver/SolverRegistry.cpp

@@ -0,0 +1,346 @@
+// SPDX-License-Identifier: LGPL-2.1-or-later
+/****************************************************************************
+ *                                                                          *
+ *   Copyright (c) 2025 Kindred Systems <development@kindred-systems.com>   *
+ *                                                                          *
+ *   This file is part of FreeCAD.                                          *
+ *                                                                          *
+ *   FreeCAD is free software: you can redistribute it and/or modify it     *
+ *   under the terms of the GNU Lesser General Public License as            *
+ *   published by the Free Software Foundation, either version 2.1 of the   *
+ *   License, or (at your option) any later version.                        *
+ *                                                                          *
+ *   FreeCAD is distributed in the hope that it will be useful, but         *
+ *   WITHOUT ANY WARRANTY; without even the implied warranty of             *
+ *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU       *
+ *   Lesser General Public License for more details.                        *
+ *                                                                          *
+ *   You should have received a copy of the GNU Lesser General Public       *
+ *   License along with FreeCAD. If not, see                                *
+ *   <https://www.gnu.org/licenses/>.                                       *
+ *                                                                          *
+ ***************************************************************************/
+
+#include "SolverRegistry.h"
+
+#include <Base/Console.h>
+
+#include <cstdlib>
+#include <cstring>
+#include <filesystem>
+#include <sstream>
+
+#ifdef _WIN32
+# define WIN32_LEAN_AND_MEAN
+# include <windows.h>
+#else
+# include <dlfcn.h>
+#endif
+
+namespace fs = std::filesystem;
+
+namespace
+{
+
+// Platform extension for shared libraries.
+#ifdef _WIN32
+constexpr const char* PLUGIN_EXT = ".dll";
+constexpr char PATH_SEP = ';';
+#elif defined(__APPLE__)
+constexpr const char* PLUGIN_EXT = ".dylib";
+constexpr char PATH_SEP = ':';
+#else
+constexpr const char* PLUGIN_EXT = ".so";
+constexpr char PATH_SEP = ':';
+#endif
+
+// Dynamic library loading wrappers.
+void* open_library(const char* path)
+{
+#ifdef _WIN32
+    return static_cast<void*>(LoadLibraryA(path));
+#else
+    return dlopen(path, RTLD_LAZY);
+#endif
+}
+
+void* get_symbol(void* handle, const char* symbol)
+{
+#ifdef _WIN32
+    return reinterpret_cast<void*>(
+        GetProcAddress(static_cast<HMODULE>(handle), symbol));
+#else
+    return dlsym(handle, symbol);
+#endif
+}
+
+std::string load_error()
+{
+#ifdef _WIN32
+    DWORD err = GetLastError();
+    char* msg = nullptr;
+    FormatMessageA(FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_SYSTEM,
+                   nullptr, err, 0, reinterpret_cast<char*>(&msg), 0, nullptr);
+    std::string result = msg ? msg : "unknown error";
+    LocalFree(msg);
+    return result;
+#else
+    const char* err = dlerror();
+    return err ? err : "unknown error";
+#endif
+}
+
+/// Parse major version from a version string like "1.0" or "2.1.3".
+/// Returns -1 on failure.
+int parse_major_version(const char* version_str)
+{
+    if (!version_str) {
+        return -1;
+    }
+    char* end = nullptr;
+    long major = std::strtol(version_str, &end, 10);
+    if (end == version_str || major < 0) {
+        return -1;
+    }
+    return static_cast<int>(major);
+}
+
+}  // anonymous namespace
+
+
+namespace KCSolve
+{
+
+// Plugin C entry point types.
+using ApiVersionFn = const char* (*)();
+using CreateFn = IKCSolver* (*)();
+
+
+// ── Singleton ──────────────────────────────────────────────────────
+
+SolverRegistry& SolverRegistry::instance()
+{
+    static SolverRegistry reg;
+    return reg;
+}
+
+SolverRegistry::SolverRegistry() = default;
+
+SolverRegistry::~SolverRegistry()
+{
+    for (void* handle : handles_) {
+        close_handle(handle);
+    }
+}
+
+void SolverRegistry::close_handle(void* handle)
+{
+    if (!handle) {
+        return;
+    }
+#ifdef _WIN32
+    FreeLibrary(static_cast<HMODULE>(handle));
+#else
+    dlclose(handle);
+#endif
+}
+
+
+// ── Registration ───────────────────────────────────────────────────
+
+bool SolverRegistry::register_solver(const std::string& name, CreateSolverFn factory)
+{
+    std::lock_guard<std::mutex> lock(mutex_);
+    auto [it, inserted] = factories_.emplace(name, std::move(factory));
+    if (!inserted) {
+        Base::Console().warning("KCSolve: solver '%s' already registered, skipping\n",
+                                name.c_str());
+        return false;
+    }
+    if (default_name_.empty()) {
+        default_name_ = name;
+    }
+    Base::Console().log("KCSolve: registered solver '%s'\n", name.c_str());
+    return true;
+}
+
+
+// ── Lookup ─────────────────────────────────────────────────────────
+
+std::unique_ptr<IKCSolver> SolverRegistry::get(const std::string& name) const
+{
+    std::lock_guard<std::mutex> lock(mutex_);
+    const std::string& key = name.empty() ? default_name_ : name;
+    if (key.empty()) {
+        return nullptr;
+    }
+    auto it = factories_.find(key);
+    if (it == factories_.end()) {
+        return nullptr;
+    }
+    return it->second();
+}
+
+std::vector<std::string> SolverRegistry::available() const
+{
+    std::lock_guard<std::mutex> lock(mutex_);
+    std::vector<std::string> names;
+    names.reserve(factories_.size());
+    for (const auto& [name, _] : factories_) {
+        names.push_back(name);
+    }
+    return names;
+}
+
+std::vector<BaseJointKind> SolverRegistry::joints_for(const std::string& name) const
+{
+    auto solver = get(name);
+    if (!solver) {
+        return {};
+    }
+    return solver->supported_joints();
+}
+
+bool SolverRegistry::set_default(const std::string& name)
+{
+    std::lock_guard<std::mutex> lock(mutex_);
+    if (factories_.find(name) == factories_.end()) {
+        return false;
+    }
+    default_name_ = name;
+    return true;
+}
+
+std::string SolverRegistry::get_default() const
+{
+    std::lock_guard<std::mutex> lock(mutex_);
+    return default_name_;
+}
+
+
+// ── Plugin scanning ────────────────────────────────────────────────
+
+void SolverRegistry::scan(const std::string& directory)
+{
+    std::error_code ec;
+    if (!fs::is_directory(directory, ec)) {
+        // Non-existent directories are not an error — just skip.
+        return;
+    }
+
+    Base::Console().log("KCSolve: scanning '%s' for plugins\n", directory.c_str());
+
+    for (const auto& entry : fs::directory_iterator(directory, ec)) {
+        if (ec) {
+            Base::Console().warning("KCSolve: error iterating '%s': %s\n",
+                                    directory.c_str(), ec.message().c_str());
+            break;
+        }
+
+        if (!entry.is_regular_file(ec)) {
+            continue;
+        }
+
+        const auto& path = entry.path();
+        if (path.extension() != PLUGIN_EXT) {
+            continue;
+        }
+
+        const std::string path_str = path.string();
+
+        // Load the shared library.
+        void* handle = open_library(path_str.c_str());
+        if (!handle) {
+            Base::Console().warning("KCSolve: failed to load '%s': %s\n",
+                                    path_str.c_str(), load_error().c_str());
+            continue;
+        }
+
+        // Check API version.
+        auto version_fn = reinterpret_cast<ApiVersionFn>(
+            get_symbol(handle, "kcsolve_api_version"));
+        if (!version_fn) {
+            // Not a KCSolve plugin — silently skip.
+            close_handle(handle);
+            continue;
+        }
+
+        const char* version_str = version_fn();
+        int major = parse_major_version(version_str);
+        if (major != API_VERSION_MAJOR) {
+            Base::Console().warning(
+                "KCSolve: plugin '%s' has incompatible API version '%s' "
+                "(expected major %d)\n",
+                path_str.c_str(),
+                version_str ? version_str : "(null)",
+                API_VERSION_MAJOR);
+            close_handle(handle);
+            continue;
+        }
+
+        // Get the factory symbol.
+        auto create_fn = reinterpret_cast<CreateFn>(
+            get_symbol(handle, "kcsolve_create"));
+        if (!create_fn) {
+            Base::Console().warning(
+                "KCSolve: plugin '%s' missing kcsolve_create() symbol\n",
+                path_str.c_str());
+            close_handle(handle);
+            continue;
+        }
+
+        // Create a temporary instance to get the solver name.
+        std::unique_ptr<IKCSolver> probe(create_fn());
+        if (!probe) {
+            Base::Console().warning(
+                "KCSolve: plugin '%s' kcsolve_create() returned null\n",
+                path_str.c_str());
+            close_handle(handle);
+            continue;
+        }
+
+        std::string solver_name = probe->name();
+        probe.reset();
+
+        // Wrap the C function pointer in a factory lambda.
+        CreateSolverFn factory = [create_fn]() -> std::unique_ptr<IKCSolver> {
+            return std::unique_ptr<IKCSolver>(create_fn());
+        };
+
+        if (register_solver(solver_name, std::move(factory))) {
+            handles_.push_back(handle);
+            Base::Console().log("KCSolve: loaded plugin '%s' from '%s'\n",
+                                solver_name.c_str(), path_str.c_str());
+        }
+        else {
+            // Duplicate name — close the handle.
+            close_handle(handle);
+        }
+    }
+}
+
+void SolverRegistry::scan_default_paths()
+{
+    // 1. KCSOLVE_PLUGIN_PATH environment variable.
+    const char* env_path = std::getenv("KCSOLVE_PLUGIN_PATH");
+    if (env_path && env_path[0] != '\0') {
+        std::istringstream stream(env_path);
+        std::string dir;
+        while (std::getline(stream, dir, PATH_SEP)) {
+            if (!dir.empty()) {
+                scan(dir);
+            }
+        }
+    }
+
+    // 2. System install path: <install_prefix>/lib/kcsolve/
+    //    Derive from the executable location or use a compile-time path.
+    //    For now, use a path relative to the FreeCAD lib directory.
+    std::error_code ec;
+    fs::path system_dir = fs::path(CMAKE_INSTALL_PREFIX) / "lib" / "kcsolve";
+    if (fs::is_directory(system_dir, ec)) {
+        scan(system_dir.string());
+    }
+}
+
+}  // namespace KCSolve

src/Mod/Assembly/Solver/SolverRegistry.h

@@ -0,0 +1,124 @@
+// SPDX-License-Identifier: LGPL-2.1-or-later
+/****************************************************************************
+ *                                                                          *
+ *   Copyright (c) 2025 Kindred Systems <development@kindred-systems.com>   *
+ *                                                                          *
+ *   This file is part of FreeCAD.                                          *
+ *                                                                          *
+ *   FreeCAD is free software: you can redistribute it and/or modify it     *
+ *   under the terms of the GNU Lesser General Public License as            *
+ *   published by the Free Software Foundation, either version 2.1 of the   *
+ *   License, or (at your option) any later version.                        *
+ *                                                                          *
+ *   FreeCAD is distributed in the hope that it will be useful, but         *
+ *   WITHOUT ANY WARRANTY; without even the implied warranty of             *
+ *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU       *
+ *   Lesser General Public License for more details.                        *
+ *                                                                          *
+ *   You should have received a copy of the GNU Lesser General Public       *
+ *   License along with FreeCAD. If not, see                                *
+ *   <https://www.gnu.org/licenses/>.                                       *
+ *                                                                          *
+ ***************************************************************************/
+
+#ifndef KCSOLVE_SOLVERREGISTRY_H
+#define KCSOLVE_SOLVERREGISTRY_H
+
+#include <functional>
+#include <memory>
+#include <mutex>
+#include <string>
+#include <unordered_map>
+#include <vector>
+
+#include "IKCSolver.h"
+#include "KCSolveGlobal.h"
+
+namespace KCSolve
+{
+
+/// Factory function that creates a solver instance.
+using CreateSolverFn = std::function<std::unique_ptr<IKCSolver>()>;
+
+/// Current KCSolve API major version. Plugins must match this to load.
+constexpr int API_VERSION_MAJOR = 1;
+
+/// Singleton registry for pluggable solver backends.
+///
+/// Solver plugins register themselves at module load time via
+/// register_solver(). The Assembly module retrieves solvers via get().
+///
+/// Thread safety: all public methods are internally synchronized.
+///
+/// Usage:
+///   // Registration (at module init):
+///   KCSolve::SolverRegistry::instance().register_solver(
+///       "ondsel", []() { return std::make_unique<OndselAdapter>(); });
+///
+///   // Retrieval:
+///   auto solver = KCSolve::SolverRegistry::instance().get();  // default
+///   auto solver = KCSolve::SolverRegistry::instance().get("ondsel");
+
+class KCSolveExport SolverRegistry
+{
+public:
+    /// Access the singleton instance.
+    static SolverRegistry& instance();
+
+    ~SolverRegistry();
+
+    /// Register a solver backend.
+    /// @param name     Unique solver name (e.g. "ondsel").
+    /// @param factory  Factory function that creates solver instances.
+    /// @return         true if registration succeeded, false if name taken.
+    bool register_solver(const std::string& name, CreateSolverFn factory);
+
+    /// Create an instance of the named solver.
+    /// @param name  Solver name. If empty, uses the default solver.
+    /// @return      Solver instance, or nullptr if not found.
+    std::unique_ptr<IKCSolver> get(const std::string& name = {}) const;
+
+    /// Return the names of all registered solvers.
+    std::vector<std::string> available() const;
+
+    /// Query which BaseJointKind values a named solver supports.
+    /// Creates a temporary instance to call supported_joints().
+    std::vector<BaseJointKind> joints_for(const std::string& name) const;
+
+    /// Set the default solver name.
+    /// @return  true if the name is registered, false otherwise.
+    bool set_default(const std::string& name);
+
+    /// Get the default solver name.
+    std::string get_default() const;
+
+    /// Scan a directory for solver plugin shared libraries.
+    /// Each plugin must export kcsolve_api_version() and kcsolve_create().
+    /// Non-existent or empty directories are handled gracefully.
+    void scan(const std::string& directory);
+
+    /// Scan all default plugin discovery paths:
+    ///   1. KCSOLVE_PLUGIN_PATH env var (colon-separated, semicolon on Windows)
+    ///   2. <install_prefix>/lib/kcsolve/
+    void scan_default_paths();
+
+private:
+    SolverRegistry();
+
+    SolverRegistry(const SolverRegistry&) = delete;
+    SolverRegistry& operator=(const SolverRegistry&) = delete;
+    SolverRegistry(SolverRegistry&&) = delete;
+    SolverRegistry& operator=(SolverRegistry&&) = delete;
+
+    /// Close a single plugin handle (platform-specific).
+    static void close_handle(void* handle);
+
+    mutable std::mutex mutex_;
+    std::unordered_map<std::string, CreateSolverFn> factories_;
+    std::string default_name_;
+    std::vector<void*> handles_;  // loaded plugin library handles
+};
+
+}  // namespace KCSolve
+
+#endif  // KCSOLVE_SOLVERREGISTRY_H

src/Mod/Assembly/Solver/Types.h

@@ -0,0 +1,286 @@
+// SPDX-License-Identifier: LGPL-2.1-or-later
+/****************************************************************************
+ *                                                                          *
+ *   Copyright (c) 2025 Kindred Systems <development@kindred-systems.com>   *
+ *                                                                          *
+ *   This file is part of FreeCAD.                                          *
+ *                                                                          *
+ *   FreeCAD is free software: you can redistribute it and/or modify it     *
+ *   under the terms of the GNU Lesser General Public License as            *
+ *   published by the Free Software Foundation, either version 2.1 of the   *
+ *   License, or (at your option) any later version.                        *
+ *                                                                          *
+ *   FreeCAD is distributed in the hope that it will be useful, but         *
+ *   WITHOUT ANY WARRANTY; without even the implied warranty of             *
+ *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU       *
+ *   Lesser General Public License for more details.                        *
+ *                                                                          *
+ *   You should have received a copy of the GNU Lesser General Public       *
+ *   License along with FreeCAD. If not, see                                *
+ *   <https://www.gnu.org/licenses/>.                                       *
+ *                                                                          *
+ ***************************************************************************/
+
+#ifndef KCSOLVE_TYPES_H
+#define KCSOLVE_TYPES_H
+
+#include <array>
+#include <cstddef>
+#include <cstdint>
+#include <optional>
+#include <string>
+#include <vector>
+
+namespace KCSolve
+{
+
+// ── Transform ──────────────────────────────────────────────────────
+//
+// Rigid-body transform: position (x, y, z) + unit quaternion (w, x, y, z).
+// Semantically equivalent to Base::Placement but free of FreeCAD dependencies
+// so that KCSolve headers remain standalone (for future server worker use).
+//
+// Quaternion convention: (w, x, y, z) — mathematical standard.
+// Note: Base::Rotation(q0,q1,q2,q3) uses (x, y, z, w) ordering.
+// The adapter layer handles this swap.
+
+struct Transform
+{
+    std::array<double, 3> position {0.0, 0.0, 0.0};
+    std::array<double, 4> quaternion {1.0, 0.0, 0.0, 0.0};  // w, x, y, z
+
+    static Transform identity()
+    {
+        return {};
+    }
+};
+
+// ── BaseJointKind ──────────────────────────────────────────────────
+//
+// Decomposed primitive constraint types. Uses SOLIDWORKS-inspired vocabulary
+// from the INTER_SOLVER.md spec rather than OndselSolver internal names.
+//
+// The existing Assembly::JointType (13 values) and Assembly::DistanceType
+// (35+ values) map to these via the adapter layer. In particular, the
+// "Distance" JointType is decomposed based on geometry classification
+// (see makeMbdJointDistance in AssemblyObject.cpp).
+
+enum class BaseJointKind : std::uint8_t
+{
+    // Point constraints (decomposed from JointType::Distance)
+    Coincident,          // PointOnPoint, d=0 — 3 DOF removed
+    PointOnLine,         // Point constrained to a line — 2 DOF removed
+    PointInPlane,        // Point constrained to a plane — 1 DOF removed
+
+    // Axis/surface constraints (decomposed from JointType::Distance)
+    Concentric,          // Coaxial (line-line, circle-circle, cyl-cyl) — 4 DOF removed
+    Tangent,             // Face-on-face tangency — 1 DOF removed
+    Planar,              // Coplanar faces — 3 DOF removed
+    LineInPlane,         // Line constrained to a plane — 2 DOF removed
+
+    // Axis orientation constraints (direct from JointType)
+    Parallel,            // Parallel axes — 2 DOF removed
+    Perpendicular,       // 90-degree axes — 1 DOF removed
+    Angle,               // Arbitrary axis angle — 1 DOF removed
+
+    // Standard kinematic joints (direct 1:1 from JointType)
+    Fixed,               // Rigid weld — 6 DOF removed
+    Revolute,            // Hinge — 5 DOF removed
+    Cylindrical,         // Rotation + sliding on axis — 4 DOF removed
+    Slider,              // Linear translation — 5 DOF removed
+    Ball,                // Spherical — 3 DOF removed
+    Screw,               // Helical (rotation + coupled translation) — 5 DOF removed
+    Universal,           // U-joint / Cardan — 4 DOF removed (future)
+
+    // Mechanical element constraints
+    Gear,                // Gear pair or belt (sign determines direction)
+    RackPinion,          // Rack-and-pinion
+    Cam,                 // Cam-follower (future)
+    Slot,                // Slot constraint (future)
+
+    // Distance variants with non-zero offset
+    DistancePointPoint,  // Point-to-point with offset — 2 DOF removed
+    DistanceCylSph,      // Cylinder-sphere distance — varies
+
+    Custom,              // Solver-specific extension point
+};
+
+// ── Part ───────────────────────────────────────────────────────────
+
+struct Part
+{
+    std::string id;
+    Transform placement;
+    double mass {1.0};
+    bool grounded {false};
+};
+
+// ── Constraint ─────────────────────────────────────────────────────
+//
+// A constraint between two parts. Built from a FreeCAD JointObject by
+// the adapter layer (classifying geometry into the specific BaseJointKind).
+
+struct Constraint
+{
+    std::string id;         // FreeCAD document object name (e.g. "Joint001")
+
+    std::string part_i;     // solver-side part ID for first reference
+    Transform marker_i;     // coordinate system on part_i
+
+    std::string part_j;     // solver-side part ID for second reference
+    Transform marker_j;     // coordinate system on part_j
+
+    BaseJointKind type {};
+
+    // Scalar parameters (interpretation depends on type):
+    //   Angle:              params[0] = angle in radians
+    //   RackPinion:         params[0] = pitch radius
+    //   Screw:              params[0] = pitch
+    //   Gear:               params[0] = radiusI, params[1] = radiusJ (negative for belt)
+    //   DistancePointPoint: params[0] = distance
+    //   DistanceCylSph:     params[0] = distance
+    //   Planar:             params[0] = offset
+    //   Concentric:         params[0] = distance
+    //   PointInPlane:       params[0] = offset
+    //   LineInPlane:        params[0] = offset
+    std::vector<double> params;
+
+    // Joint limits (length or angle bounds)
+    struct Limit
+    {
+        enum class Kind : std::uint8_t
+        {
+            TranslationMin,
+            TranslationMax,
+            RotationMin,
+            RotationMax,
+        };
+
+        Kind kind {};
+        double value {0.0};
+        double tolerance {1.0e-9};
+    };
+    std::vector<Limit> limits;
+
+    bool activated {true};
+};
+
+// ── MotionDef ──────────────────────────────────────────────────────
+//
+// A motion driver for kinematic simulation.
+
+struct MotionDef
+{
+    enum class Kind : std::uint8_t
+    {
+        Rotational,
+        Translational,
+        General,
+    };
+
+    Kind kind {};
+    std::string joint_id;       // which constraint this drives
+    std::string marker_i;
+    std::string marker_j;
+
+    // Motion law expressions (function of time 't').
+    // For General: both are set. Otherwise only the relevant one.
+    std::string rotation_expr;
+    std::string translation_expr;
+};
+
+// ── SimulationParams ───────────────────────────────────────────────
+//
+// Parameters for kinematic simulation (run_kinematic).
+// Maps to create_mbdSimulationParameters() in AssemblyObject.cpp.
+
+struct SimulationParams
+{
+    double t_start {0.0};
+    double t_end {1.0};
+    double h_out {0.01};        // output time step
+    double h_min {1.0e-9};
+    double h_max {1.0};
+    double error_tol {1.0e-6};
+};
+
+// ── SolveContext ───────────────────────────────────────────────────
+//
+// Complete input to a solve operation. Built by the adapter layer
+// from FreeCAD document objects.
+
+struct SolveContext
+{
+    std::vector<Part> parts;
+    std::vector<Constraint> constraints;
+    std::vector<MotionDef> motions;
+
+    // Present when running kinematic simulation via run_kinematic().
+    std::optional<SimulationParams> simulation;
+
+    // Hint: bundle parts connected by Fixed joints into single rigid bodies.
+    // When true and the solver does not support_bundle_fixed(), the adapter
+    // layer pre-bundles before passing to the solver.
+    bool bundle_fixed {false};
+};
+
+// ── SolveStatus ────────────────────────────────────────────────────
+//
+// Matches the return codes from AssemblyObject::solve().
+
+enum class SolveStatus : std::int8_t
+{
+    Success = 0,
+    Failed = -1,
+    InvalidFlip = -2,       // orientation flipped past threshold
+    NoGroundedParts = -6,   // no grounded parts in assembly
+};
+
+// ── ConstraintDiagnostic ───────────────────────────────────────────
+//
+// Per-constraint diagnostic information from updateSolveStatus().
+
+struct ConstraintDiagnostic
+{
+    enum class Kind : std::uint8_t
+    {
+        Redundant,
+        Conflicting,
+        PartiallyRedundant,
+        Malformed,
+    };
+
+    std::string constraint_id;  // FreeCAD object name
+    Kind kind {};
+    std::string detail;         // human-readable description
+};
+
+// ── SolveResult ────────────────────────────────────────────────────
+//
+// Output of a solve operation.
+
+struct SolveResult
+{
+    SolveStatus status {SolveStatus::Success};
+
+    // Updated placements for each part (only parts that moved).
+    struct PartResult
+    {
+        std::string id;
+        Transform placement;
+    };
+    std::vector<PartResult> placements;
+
+    // Degrees of freedom remaining (-1 = unknown).
+    int dof {-1};
+
+    // Constraint diagnostics (redundant, conflicting, etc.).
+    std::vector<ConstraintDiagnostic> diagnostics;
+
+    // For kinematic simulation: number of computed frames.
+    std::size_t num_frames {0};
+};
+
+}  // namespace KCSolve
+
+#endif  // KCSOLVE_TYPES_H

src/Mod/Assembly/Solver/bindings/CMakeLists.txt

@@ -0,0 +1,31 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+
+set(KCSolvePy_SRCS
+    PyIKCSolver.h
+    kcsolve_py.cpp
+)
+
+add_library(kcsolve_py SHARED ${KCSolvePy_SRCS})
+
+target_include_directories(kcsolve_py
+    PRIVATE
+        ${CMAKE_SOURCE_DIR}/src
+        ${CMAKE_BINARY_DIR}/src
+        ${pybind11_INCLUDE_DIR}
+)
+
+target_link_libraries(kcsolve_py
+    PRIVATE
+        pybind11::module
+        Python3::Python
+        KCSolve
+)
+
+if(FREECAD_WARN_ERROR)
+    target_compile_warn_error(kcsolve_py)
+endif()
+
+SET_BIN_DIR(kcsolve_py kcsolve /Mod/Assembly)
+SET_PYTHON_PREFIX_SUFFIX(kcsolve_py)
+
+INSTALL(TARGETS kcsolve_py DESTINATION ${CMAKE_INSTALL_LIBDIR})

src/Mod/Assembly/Solver/bindings/PyIKCSolver.h

@@ -0,0 +1,121 @@
+// SPDX-License-Identifier: LGPL-2.1-or-later
+/****************************************************************************
+ *                                                                          *
+ *   Copyright (c) 2025 Kindred Systems <development@kindred-systems.com>   *
+ *                                                                          *
+ *   This file is part of FreeCAD.                                          *
+ *                                                                          *
+ *   FreeCAD is free software: you can redistribute it and/or modify it     *
+ *   under the terms of the GNU Lesser General Public License as            *
+ *   published by the Free Software Foundation, either version 2.1 of the   *
+ *   License, or (at your option) any later version.                        *
+ *                                                                          *
+ *   FreeCAD is distributed in the hope that it will be useful, but         *
+ *   WITHOUT ANY WARRANTY; without even the implied warranty of             *
+ *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU       *
+ *   Lesser General Public License for more details.                        *
+ *                                                                          *
+ *   You should have received a copy of the GNU Lesser General Public       *
+ *   License along with FreeCAD. If not, see                                *
+ *   <https://www.gnu.org/licenses/>.                                       *
+ *                                                                          *
+ ***************************************************************************/
+
+#ifndef KCSOLVE_PYIKCSOLVER_H
+#define KCSOLVE_PYIKCSOLVER_H
+
+#include <pybind11/pybind11.h>
+#include <pybind11/stl.h>
+
+#include <Mod/Assembly/Solver/IKCSolver.h>
+
+namespace KCSolve
+{
+
+/// pybind11 trampoline class for IKCSolver.
+/// Enables Python subclasses that override virtual methods.
+class PyIKCSolver : public IKCSolver
+{
+public:
+    using IKCSolver::IKCSolver;
+
+    // ── Pure virtuals ──────────────────────────────────────────────
+
+    std::string name() const override
+    {
+        PYBIND11_OVERRIDE_PURE(std::string, IKCSolver, name);
+    }
+
+    std::vector<BaseJointKind> supported_joints() const override
+    {
+        PYBIND11_OVERRIDE_PURE(std::vector<BaseJointKind>, IKCSolver, supported_joints);
+    }
+
+    SolveResult solve(const SolveContext& ctx) override
+    {
+        PYBIND11_OVERRIDE_PURE(SolveResult, IKCSolver, solve, ctx);
+    }
+
+    // ── Virtuals with defaults ─────────────────────────────────────
+
+    SolveResult update(const SolveContext& ctx) override
+    {
+        PYBIND11_OVERRIDE(SolveResult, IKCSolver, update, ctx);
+    }
+
+    SolveResult pre_drag(const SolveContext& ctx,
+                         const std::vector<std::string>& drag_parts) override
+    {
+        PYBIND11_OVERRIDE(SolveResult, IKCSolver, pre_drag, ctx, drag_parts);
+    }
+
+    SolveResult drag_step(
+        const std::vector<SolveResult::PartResult>& drag_placements) override
+    {
+        PYBIND11_OVERRIDE(SolveResult, IKCSolver, drag_step, drag_placements);
+    }
+
+    void post_drag() override
+    {
+        PYBIND11_OVERRIDE(void, IKCSolver, post_drag);
+    }
+
+    SolveResult run_kinematic(const SolveContext& ctx) override
+    {
+        PYBIND11_OVERRIDE(SolveResult, IKCSolver, run_kinematic, ctx);
+    }
+
+    std::size_t num_frames() const override
+    {
+        PYBIND11_OVERRIDE(std::size_t, IKCSolver, num_frames);
+    }
+
+    SolveResult update_for_frame(std::size_t index) override
+    {
+        PYBIND11_OVERRIDE(SolveResult, IKCSolver, update_for_frame, index);
+    }
+
+    std::vector<ConstraintDiagnostic> diagnose(const SolveContext& ctx) override
+    {
+        PYBIND11_OVERRIDE(std::vector<ConstraintDiagnostic>, IKCSolver, diagnose, ctx);
+    }
+
+    bool is_deterministic() const override
+    {
+        PYBIND11_OVERRIDE(bool, IKCSolver, is_deterministic);
+    }
+
+    void export_native(const std::string& path) override
+    {
+        PYBIND11_OVERRIDE(void, IKCSolver, export_native, path);
+    }
+
+    bool supports_bundle_fixed() const override
+    {
+        PYBIND11_OVERRIDE(bool, IKCSolver, supports_bundle_fixed);
+    }
+};
+
+}  // namespace KCSolve
+
+#endif  // KCSOLVE_PYIKCSOLVER_H

src/Mod/Assembly/Solver/bindings/kcsolve_py.cpp

@@ -0,0 +1,830 @@
+// SPDX-License-Identifier: LGPL-2.1-or-later
+/****************************************************************************
+ *                                                                          *
+ *   Copyright (c) 2025 Kindred Systems <development@kindred-systems.com>   *
+ *                                                                          *
+ *   This file is part of FreeCAD.                                          *
+ *                                                                          *
+ *   FreeCAD is free software: you can redistribute it and/or modify it     *
+ *   under the terms of the GNU Lesser General Public License as            *
+ *   published by the Free Software Foundation, either version 2.1 of the   *
+ *   License, or (at your option) any later version.                        *
+ *                                                                          *
+ *   FreeCAD is distributed in the hope that it will be useful, but         *
+ *   WITHOUT ANY WARRANTY; without even the implied warranty of             *
+ *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU       *
+ *   Lesser General Public License for more details.                        *
+ *                                                                          *
+ *   You should have received a copy of the GNU Lesser General Public       *
+ *   License along with FreeCAD. If not, see                                *
+ *   <https://www.gnu.org/licenses/>.                                       *
+ *                                                                          *
+ ***************************************************************************/
+
+#include <pybind11/pybind11.h>
+#include <pybind11/stl.h>
+
+#include <Mod/Assembly/Solver/IKCSolver.h>
+#include <Mod/Assembly/Solver/OndselAdapter.h>
+#include <Mod/Assembly/Solver/SolverRegistry.h>
+#include <Mod/Assembly/Solver/Types.h>
+
+#include "PyIKCSolver.h"
+
+#include <cstddef>
+#include <memory>
+#include <string>
+
+namespace py = pybind11;
+using namespace KCSolve;
+
+
+// ── Enum string mapping ────────────────────────────────────────────
+//
+// Constexpr tables for bidirectional enum <-> string conversion.
+// String values match the py::enum_ .value("Name", ...) names exactly,
+// which is also the JSON wire format specified in SOLVER.md §3.
+
+namespace
+{
+
+template<typename E>
+struct EnumEntry
+{
+    E value;
+    const char* name;
+};
+
+static constexpr EnumEntry<BaseJointKind> kBaseJointKindEntries[] = {
+    {BaseJointKind::Coincident,         "Coincident"},
+    {BaseJointKind::PointOnLine,        "PointOnLine"},
+    {BaseJointKind::PointInPlane,       "PointInPlane"},
+    {BaseJointKind::Concentric,         "Concentric"},
+    {BaseJointKind::Tangent,            "Tangent"},
+    {BaseJointKind::Planar,             "Planar"},
+    {BaseJointKind::LineInPlane,        "LineInPlane"},
+    {BaseJointKind::Parallel,           "Parallel"},
+    {BaseJointKind::Perpendicular,      "Perpendicular"},
+    {BaseJointKind::Angle,              "Angle"},
+    {BaseJointKind::Fixed,              "Fixed"},
+    {BaseJointKind::Revolute,           "Revolute"},
+    {BaseJointKind::Cylindrical,        "Cylindrical"},
+    {BaseJointKind::Slider,             "Slider"},
+    {BaseJointKind::Ball,               "Ball"},
+    {BaseJointKind::Screw,              "Screw"},
+    {BaseJointKind::Universal,          "Universal"},
+    {BaseJointKind::Gear,               "Gear"},
+    {BaseJointKind::RackPinion,         "RackPinion"},
+    {BaseJointKind::Cam,                "Cam"},
+    {BaseJointKind::Slot,               "Slot"},
+    {BaseJointKind::DistancePointPoint, "DistancePointPoint"},
+    {BaseJointKind::DistanceCylSph,     "DistanceCylSph"},
+    {BaseJointKind::Custom,             "Custom"},
+};
+
+static constexpr EnumEntry<SolveStatus> kSolveStatusEntries[] = {
+    {SolveStatus::Success,         "Success"},
+    {SolveStatus::Failed,          "Failed"},
+    {SolveStatus::InvalidFlip,     "InvalidFlip"},
+    {SolveStatus::NoGroundedParts, "NoGroundedParts"},
+};
+
+static constexpr EnumEntry<ConstraintDiagnostic::Kind> kDiagnosticKindEntries[] = {
+    {ConstraintDiagnostic::Kind::Redundant,          "Redundant"},
+    {ConstraintDiagnostic::Kind::Conflicting,        "Conflicting"},
+    {ConstraintDiagnostic::Kind::PartiallyRedundant, "PartiallyRedundant"},
+    {ConstraintDiagnostic::Kind::Malformed,          "Malformed"},
+};
+
+static constexpr EnumEntry<MotionDef::Kind> kMotionKindEntries[] = {
+    {MotionDef::Kind::Rotational,    "Rotational"},
+    {MotionDef::Kind::Translational, "Translational"},
+    {MotionDef::Kind::General,       "General"},
+};
+
+static constexpr EnumEntry<Constraint::Limit::Kind> kLimitKindEntries[] = {
+    {Constraint::Limit::Kind::TranslationMin, "TranslationMin"},
+    {Constraint::Limit::Kind::TranslationMax, "TranslationMax"},
+    {Constraint::Limit::Kind::RotationMin,    "RotationMin"},
+    {Constraint::Limit::Kind::RotationMax,    "RotationMax"},
+};
+
+template<typename E, std::size_t N>
+const char* enum_to_str(E val, const EnumEntry<E> (&table)[N])
+{
+    for (std::size_t i = 0; i < N; ++i) {
+        if (table[i].value == val) {
+            return table[i].name;
+        }
+    }
+    throw py::value_error("Unknown enum value: " + std::to_string(static_cast<int>(val)));
+}
+
+template<typename E, std::size_t N>
+E str_to_enum(const std::string& name, const EnumEntry<E> (&table)[N],
+              const char* enum_type_name)
+{
+    for (std::size_t i = 0; i < N; ++i) {
+        if (name == table[i].name) {
+            return table[i].value;
+        }
+    }
+    throw py::value_error(
+        std::string("Invalid ") + enum_type_name + " value: '" + name + "'");
+}
+
+
+// ── Dict conversion helpers ────────────────────────────────────────
+//
+// Standalone functions for each type so SolveContext/SolveResult can
+// reuse them without duplicating serialization logic.
+
+py::dict transform_to_dict(const Transform& t)
+{
+    py::dict d;
+    d["position"] = py::make_tuple(t.position[0], t.position[1], t.position[2]);
+    d["quaternion"] = py::make_tuple(
+        t.quaternion[0], t.quaternion[1], t.quaternion[2], t.quaternion[3]);
+    return d;
+}
+
+Transform transform_from_dict(const py::dict& d)
+{
+    Transform t;
+    auto pos = d["position"].cast<py::sequence>();
+    if (py::len(pos) != 3) {
+        throw py::value_error("position must have exactly 3 elements");
+    }
+    for (int i = 0; i < 3; ++i) {
+        t.position[static_cast<std::size_t>(i)] = pos[i].cast<double>();
+    }
+    auto quat = d["quaternion"].cast<py::sequence>();
+    if (py::len(quat) != 4) {
+        throw py::value_error("quaternion must have exactly 4 elements");
+    }
+    for (int i = 0; i < 4; ++i) {
+        t.quaternion[static_cast<std::size_t>(i)] = quat[i].cast<double>();
+    }
+    return t;
+}
+
+py::dict part_to_dict(const Part& p)
+{
+    py::dict d;
+    d["id"] = p.id;
+    d["placement"] = transform_to_dict(p.placement);
+    d["mass"] = p.mass;
+    d["grounded"] = p.grounded;
+    return d;
+}
+
+Part part_from_dict(const py::dict& d)
+{
+    Part p;
+    p.id = d["id"].cast<std::string>();
+    p.placement = transform_from_dict(d["placement"].cast<py::dict>());
+    if (d.contains("mass")) {
+        p.mass = d["mass"].cast<double>();
+    }
+    if (d.contains("grounded")) {
+        p.grounded = d["grounded"].cast<bool>();
+    }
+    return p;
+}
+
+py::dict limit_to_dict(const Constraint::Limit& lim)
+{
+    py::dict d;
+    d["kind"] = enum_to_str(lim.kind, kLimitKindEntries);
+    d["value"] = lim.value;
+    d["tolerance"] = lim.tolerance;
+    return d;
+}
+
+Constraint::Limit limit_from_dict(const py::dict& d)
+{
+    Constraint::Limit lim;
+    lim.kind = str_to_enum(d["kind"].cast<std::string>(),
+                           kLimitKindEntries, "LimitKind");
+    lim.value = d["value"].cast<double>();
+    if (d.contains("tolerance")) {
+        lim.tolerance = d["tolerance"].cast<double>();
+    }
+    return lim;
+}
+
+py::dict constraint_to_dict(const Constraint& c)
+{
+    py::dict d;
+    d["id"] = c.id;
+    d["part_i"] = c.part_i;
+    d["marker_i"] = transform_to_dict(c.marker_i);
+    d["part_j"] = c.part_j;
+    d["marker_j"] = transform_to_dict(c.marker_j);
+    d["type"] = enum_to_str(c.type, kBaseJointKindEntries);
+    d["params"] = py::cast(c.params);
+    py::list lims;
+    for (const auto& lim : c.limits) {
+        lims.append(limit_to_dict(lim));
+    }
+    d["limits"] = lims;
+    d["activated"] = c.activated;
+    return d;
+}
+
+Constraint constraint_from_dict(const py::dict& d)
+{
+    Constraint c;
+    c.id = d["id"].cast<std::string>();
+    c.part_i = d["part_i"].cast<std::string>();
+    c.marker_i = transform_from_dict(d["marker_i"].cast<py::dict>());
+    c.part_j = d["part_j"].cast<std::string>();
+    c.marker_j = transform_from_dict(d["marker_j"].cast<py::dict>());
+    c.type = str_to_enum(d["type"].cast<std::string>(),
+                         kBaseJointKindEntries, "BaseJointKind");
+    if (d.contains("params")) {
+        c.params = d["params"].cast<std::vector<double>>();
+    }
+    if (d.contains("limits")) {
+        for (auto item : d["limits"]) {
+            c.limits.push_back(limit_from_dict(item.cast<py::dict>()));
+        }
+    }
+    if (d.contains("activated")) {
+        c.activated = d["activated"].cast<bool>();
+    }
+    return c;
+}
+
+py::dict motion_to_dict(const MotionDef& m)
+{
+    py::dict d;
+    d["kind"] = enum_to_str(m.kind, kMotionKindEntries);
+    d["joint_id"] = m.joint_id;
+    d["marker_i"] = m.marker_i;
+    d["marker_j"] = m.marker_j;
+    d["rotation_expr"] = m.rotation_expr;
+    d["translation_expr"] = m.translation_expr;
+    return d;
+}
+
+MotionDef motion_from_dict(const py::dict& d)
+{
+    MotionDef m;
+    m.kind = str_to_enum(d["kind"].cast<std::string>(),
+                         kMotionKindEntries, "MotionKind");
+    m.joint_id = d["joint_id"].cast<std::string>();
+    if (d.contains("marker_i")) {
+        m.marker_i = d["marker_i"].cast<std::string>();
+    }
+    if (d.contains("marker_j")) {
+        m.marker_j = d["marker_j"].cast<std::string>();
+    }
+    if (d.contains("rotation_expr")) {
+        m.rotation_expr = d["rotation_expr"].cast<std::string>();
+    }
+    if (d.contains("translation_expr")) {
+        m.translation_expr = d["translation_expr"].cast<std::string>();
+    }
+    return m;
+}
+
+py::dict sim_to_dict(const SimulationParams& s)
+{
+    py::dict d;
+    d["t_start"] = s.t_start;
+    d["t_end"] = s.t_end;
+    d["h_out"] = s.h_out;
+    d["h_min"] = s.h_min;
+    d["h_max"] = s.h_max;
+    d["error_tol"] = s.error_tol;
+    return d;
+}
+
+SimulationParams sim_from_dict(const py::dict& d)
+{
+    SimulationParams s;
+    if (d.contains("t_start")) {
+        s.t_start = d["t_start"].cast<double>();
+    }
+    if (d.contains("t_end")) {
+        s.t_end = d["t_end"].cast<double>();
+    }
+    if (d.contains("h_out")) {
+        s.h_out = d["h_out"].cast<double>();
+    }
+    if (d.contains("h_min")) {
+        s.h_min = d["h_min"].cast<double>();
+    }
+    if (d.contains("h_max")) {
+        s.h_max = d["h_max"].cast<double>();
+    }
+    if (d.contains("error_tol")) {
+        s.error_tol = d["error_tol"].cast<double>();
+    }
+    return s;
+}
+
+py::dict diagnostic_to_dict(const ConstraintDiagnostic& diag)
+{
+    py::dict d;
+    d["constraint_id"] = diag.constraint_id;
+    d["kind"] = enum_to_str(diag.kind, kDiagnosticKindEntries);
+    d["detail"] = diag.detail;
+    return d;
+}
+
+ConstraintDiagnostic diagnostic_from_dict(const py::dict& d)
+{
+    ConstraintDiagnostic diag;
+    diag.constraint_id = d["constraint_id"].cast<std::string>();
+    diag.kind = str_to_enum(d["kind"].cast<std::string>(),
+                            kDiagnosticKindEntries, "DiagnosticKind");
+    if (d.contains("detail")) {
+        diag.detail = d["detail"].cast<std::string>();
+    }
+    return diag;
+}
+
+py::dict part_result_to_dict(const SolveResult::PartResult& pr)
+{
+    py::dict d;
+    d["id"] = pr.id;
+    d["placement"] = transform_to_dict(pr.placement);
+    return d;
+}
+
+SolveResult::PartResult part_result_from_dict(const py::dict& d)
+{
+    SolveResult::PartResult pr;
+    pr.id = d["id"].cast<std::string>();
+    pr.placement = transform_from_dict(d["placement"].cast<py::dict>());
+    return pr;
+}
+
+py::dict solve_context_to_dict(const SolveContext& ctx)
+{
+    py::dict d;
+    d["api_version"] = API_VERSION_MAJOR;
+
+    py::list parts;
+    for (const auto& p : ctx.parts) {
+        parts.append(part_to_dict(p));
+    }
+    d["parts"] = parts;
+
+    py::list constraints;
+    for (const auto& c : ctx.constraints) {
+        constraints.append(constraint_to_dict(c));
+    }
+    d["constraints"] = constraints;
+
+    py::list motions;
+    for (const auto& m : ctx.motions) {
+        motions.append(motion_to_dict(m));
+    }
+    d["motions"] = motions;
+
+    if (ctx.simulation.has_value()) {
+        d["simulation"] = sim_to_dict(*ctx.simulation);
+    }
+    else {
+        d["simulation"] = py::none();
+    }
+
+    d["bundle_fixed"] = ctx.bundle_fixed;
+    return d;
+}
+
+SolveContext solve_context_from_dict(const py::dict& d)
+{
+    SolveContext ctx;
+
+    if (d.contains("api_version")) {
+        int v = d["api_version"].cast<int>();
+        if (v != API_VERSION_MAJOR) {
+            throw py::value_error(
+                "Unsupported api_version " + std::to_string(v)
+                + ", expected " + std::to_string(API_VERSION_MAJOR));
+        }
+    }
+
+    for (auto item : d["parts"]) {
+        ctx.parts.push_back(part_from_dict(item.cast<py::dict>()));
+    }
+
+    for (auto item : d["constraints"]) {
+        ctx.constraints.push_back(constraint_from_dict(item.cast<py::dict>()));
+    }
+
+    if (d.contains("motions")) {
+        for (auto item : d["motions"]) {
+            ctx.motions.push_back(motion_from_dict(item.cast<py::dict>()));
+        }
+    }
+
+    if (d.contains("simulation") && !d["simulation"].is_none()) {
+        ctx.simulation = sim_from_dict(d["simulation"].cast<py::dict>());
+    }
+
+    if (d.contains("bundle_fixed")) {
+        ctx.bundle_fixed = d["bundle_fixed"].cast<bool>();
+    }
+
+    return ctx;
+}
+
+py::dict solve_result_to_dict(const SolveResult& r)
+{
+    py::dict d;
+    d["status"] = enum_to_str(r.status, kSolveStatusEntries);
+
+    py::list placements;
+    for (const auto& pr : r.placements) {
+        placements.append(part_result_to_dict(pr));
+    }
+    d["placements"] = placements;
+
+    d["dof"] = r.dof;
+
+    py::list diagnostics;
+    for (const auto& diag : r.diagnostics) {
+        diagnostics.append(diagnostic_to_dict(diag));
+    }
+    d["diagnostics"] = diagnostics;
+
+    d["num_frames"] = r.num_frames;
+    return d;
+}
+
+SolveResult solve_result_from_dict(const py::dict& d)
+{
+    SolveResult r;
+    r.status = str_to_enum(d["status"].cast<std::string>(),
+                           kSolveStatusEntries, "SolveStatus");
+
+    if (d.contains("placements")) {
+        for (auto item : d["placements"]) {
+            r.placements.push_back(part_result_from_dict(item.cast<py::dict>()));
+        }
+    }
+
+    if (d.contains("dof")) {
+        r.dof = d["dof"].cast<int>();
+    }
+
+    if (d.contains("diagnostics")) {
+        for (auto item : d["diagnostics"]) {
+            r.diagnostics.push_back(diagnostic_from_dict(item.cast<py::dict>()));
+        }
+    }
+
+    if (d.contains("num_frames")) {
+        r.num_frames = d["num_frames"].cast<std::size_t>();
+    }
+
+    return r;
+}
+
+}  // anonymous namespace
+
+
+// ── PySolverHolder ─────────────────────────────────────────────────
+//
+// Wraps a Python IKCSolver subclass instance so it can live inside a
+// std::unique_ptr<IKCSolver> returned by SolverRegistry::get().
+// Prevents Python GC by holding a py::object reference and acquires
+// the GIL before every forwarded call.
+
+class PySolverHolder : public IKCSolver
+{
+public:
+    explicit PySolverHolder(py::object obj)
+        : obj_(std::move(obj))
+    {
+        solver_ = obj_.cast<IKCSolver*>();
+    }
+
+    std::string name() const override
+    {
+        py::gil_scoped_acquire gil;
+        return solver_->name();
+    }
+
+    std::vector<BaseJointKind> supported_joints() const override
+    {
+        py::gil_scoped_acquire gil;
+        return solver_->supported_joints();
+    }
+
+    SolveResult solve(const SolveContext& ctx) override
+    {
+        py::gil_scoped_acquire gil;
+        return solver_->solve(ctx);
+    }
+
+    SolveResult update(const SolveContext& ctx) override
+    {
+        py::gil_scoped_acquire gil;
+        return solver_->update(ctx);
+    }
+
+    SolveResult pre_drag(const SolveContext& ctx,
+                         const std::vector<std::string>& drag_parts) override
+    {
+        py::gil_scoped_acquire gil;
+        return solver_->pre_drag(ctx, drag_parts);
+    }
+
+    SolveResult drag_step(
+        const std::vector<SolveResult::PartResult>& drag_placements) override
+    {
+        py::gil_scoped_acquire gil;
+        return solver_->drag_step(drag_placements);
+    }
+
+    void post_drag() override
+    {
+        py::gil_scoped_acquire gil;
+        solver_->post_drag();
+    }
+
+    SolveResult run_kinematic(const SolveContext& ctx) override
+    {
+        py::gil_scoped_acquire gil;
+        return solver_->run_kinematic(ctx);
+    }
+
+    std::size_t num_frames() const override
+    {
+        py::gil_scoped_acquire gil;
+        return solver_->num_frames();
+    }
+
+    SolveResult update_for_frame(std::size_t index) override
+    {
+        py::gil_scoped_acquire gil;
+        return solver_->update_for_frame(index);
+    }
+
+    std::vector<ConstraintDiagnostic> diagnose(const SolveContext& ctx) override
+    {
+        py::gil_scoped_acquire gil;
+        return solver_->diagnose(ctx);
+    }
+
+    bool is_deterministic() const override
+    {
+        py::gil_scoped_acquire gil;
+        return solver_->is_deterministic();
+    }
+
+    void export_native(const std::string& path) override
+    {
+        py::gil_scoped_acquire gil;
+        solver_->export_native(path);
+    }
+
+    bool supports_bundle_fixed() const override
+    {
+        py::gil_scoped_acquire gil;
+        return solver_->supports_bundle_fixed();
+    }
+
+private:
+    py::object obj_;     // prevents Python GC
+    IKCSolver* solver_;  // raw pointer into the trampoline inside obj_
+};
+
+
+// ── Module definition ──────────────────────────────────────────────
+
+PYBIND11_MODULE(kcsolve, m)
+{
+    m.doc() = "KCSolve — pluggable assembly constraint solver API";
+    m.attr("API_VERSION_MAJOR") = API_VERSION_MAJOR;
+
+    // ── Enums ──────────────────────────────────────────────────────
+
+    py::enum_<BaseJointKind>(m, "BaseJointKind")
+        .value("Coincident", BaseJointKind::Coincident)
+        .value("PointOnLine", BaseJointKind::PointOnLine)
+        .value("PointInPlane", BaseJointKind::PointInPlane)
+        .value("Concentric", BaseJointKind::Concentric)
+        .value("Tangent", BaseJointKind::Tangent)
+        .value("Planar", BaseJointKind::Planar)
+        .value("LineInPlane", BaseJointKind::LineInPlane)
+        .value("Parallel", BaseJointKind::Parallel)
+        .value("Perpendicular", BaseJointKind::Perpendicular)
+        .value("Angle", BaseJointKind::Angle)
+        .value("Fixed", BaseJointKind::Fixed)
+        .value("Revolute", BaseJointKind::Revolute)
+        .value("Cylindrical", BaseJointKind::Cylindrical)
+        .value("Slider", BaseJointKind::Slider)
+        .value("Ball", BaseJointKind::Ball)
+        .value("Screw", BaseJointKind::Screw)
+        .value("Universal", BaseJointKind::Universal)
+        .value("Gear", BaseJointKind::Gear)
+        .value("RackPinion", BaseJointKind::RackPinion)
+        .value("Cam", BaseJointKind::Cam)
+        .value("Slot", BaseJointKind::Slot)
+        .value("DistancePointPoint", BaseJointKind::DistancePointPoint)
+        .value("DistanceCylSph", BaseJointKind::DistanceCylSph)
+        .value("Custom", BaseJointKind::Custom);
+
+    py::enum_<SolveStatus>(m, "SolveStatus")
+        .value("Success", SolveStatus::Success)
+        .value("Failed", SolveStatus::Failed)
+        .value("InvalidFlip", SolveStatus::InvalidFlip)
+        .value("NoGroundedParts", SolveStatus::NoGroundedParts);
+
+    py::enum_<ConstraintDiagnostic::Kind>(m, "DiagnosticKind")
+        .value("Redundant", ConstraintDiagnostic::Kind::Redundant)
+        .value("Conflicting", ConstraintDiagnostic::Kind::Conflicting)
+        .value("PartiallyRedundant", ConstraintDiagnostic::Kind::PartiallyRedundant)
+        .value("Malformed", ConstraintDiagnostic::Kind::Malformed);
+
+    py::enum_<MotionDef::Kind>(m, "MotionKind")
+        .value("Rotational", MotionDef::Kind::Rotational)
+        .value("Translational", MotionDef::Kind::Translational)
+        .value("General", MotionDef::Kind::General);
+
+    py::enum_<Constraint::Limit::Kind>(m, "LimitKind")
+        .value("TranslationMin", Constraint::Limit::Kind::TranslationMin)
+        .value("TranslationMax", Constraint::Limit::Kind::TranslationMax)
+        .value("RotationMin", Constraint::Limit::Kind::RotationMin)
+        .value("RotationMax", Constraint::Limit::Kind::RotationMax);
+
+    // ── Struct bindings ────────────────────────────────────────────
+
+    py::class_<Transform>(m, "Transform")
+        .def(py::init<>())
+        .def_readwrite("position", &Transform::position)
+        .def_readwrite("quaternion", &Transform::quaternion)
+        .def_static("identity", &Transform::identity)
+        .def("__repr__", [](const Transform& t) {
+            return "<kcsolve.Transform pos=["
+                + std::to_string(t.position[0]) + ", "
+                + std::to_string(t.position[1]) + ", "
+                + std::to_string(t.position[2]) + "]>";
+        })
+        .def("to_dict", [](const Transform& t) { return transform_to_dict(t); })
+        .def_static("from_dict", [](const py::dict& d) { return transform_from_dict(d); });
+
+    py::class_<Part>(m, "Part")
+        .def(py::init<>())
+        .def_readwrite("id", &Part::id)
+        .def_readwrite("placement", &Part::placement)
+        .def_readwrite("mass", &Part::mass)
+        .def_readwrite("grounded", &Part::grounded)
+        .def("to_dict", [](const Part& p) { return part_to_dict(p); })
+        .def_static("from_dict", [](const py::dict& d) { return part_from_dict(d); });
+
+    auto constraint_class = py::class_<Constraint>(m, "Constraint");
+
+    py::class_<Constraint::Limit>(constraint_class, "Limit")
+        .def(py::init<>())
+        .def_readwrite("kind", &Constraint::Limit::kind)
+        .def_readwrite("value", &Constraint::Limit::value)
+        .def_readwrite("tolerance", &Constraint::Limit::tolerance)
+        .def("to_dict", [](const Constraint::Limit& l) { return limit_to_dict(l); })
+        .def_static("from_dict", [](const py::dict& d) { return limit_from_dict(d); });
+
+    constraint_class
+        .def(py::init<>())
+        .def_readwrite("id", &Constraint::id)
+        .def_readwrite("part_i", &Constraint::part_i)
+        .def_readwrite("marker_i", &Constraint::marker_i)
+        .def_readwrite("part_j", &Constraint::part_j)
+        .def_readwrite("marker_j", &Constraint::marker_j)
+        .def_readwrite("type", &Constraint::type)
+        .def_readwrite("params", &Constraint::params)
+        .def_readwrite("limits", &Constraint::limits)
+        .def_readwrite("activated", &Constraint::activated)
+        .def("to_dict", [](const Constraint& c) { return constraint_to_dict(c); })
+        .def_static("from_dict", [](const py::dict& d) { return constraint_from_dict(d); });
+
+    py::class_<MotionDef>(m, "MotionDef")
+        .def(py::init<>())
+        .def_readwrite("kind", &MotionDef::kind)
+        .def_readwrite("joint_id", &MotionDef::joint_id)
+        .def_readwrite("marker_i", &MotionDef::marker_i)
+        .def_readwrite("marker_j", &MotionDef::marker_j)
+        .def_readwrite("rotation_expr", &MotionDef::rotation_expr)
+        .def_readwrite("translation_expr", &MotionDef::translation_expr)
+        .def("to_dict", [](const MotionDef& m) { return motion_to_dict(m); })
+        .def_static("from_dict", [](const py::dict& d) { return motion_from_dict(d); });
+
+    py::class_<SimulationParams>(m, "SimulationParams")
+        .def(py::init<>())
+        .def_readwrite("t_start", &SimulationParams::t_start)
+        .def_readwrite("t_end", &SimulationParams::t_end)
+        .def_readwrite("h_out", &SimulationParams::h_out)
+        .def_readwrite("h_min", &SimulationParams::h_min)
+        .def_readwrite("h_max", &SimulationParams::h_max)
+        .def_readwrite("error_tol", &SimulationParams::error_tol)
+        .def("to_dict", [](const SimulationParams& s) { return sim_to_dict(s); })
+        .def_static("from_dict", [](const py::dict& d) { return sim_from_dict(d); });
+
+    py::class_<SolveContext>(m, "SolveContext")
+        .def(py::init<>())
+        .def_readwrite("parts", &SolveContext::parts)
+        .def_readwrite("constraints", &SolveContext::constraints)
+        .def_readwrite("motions", &SolveContext::motions)
+        .def_readwrite("simulation", &SolveContext::simulation)
+        .def_readwrite("bundle_fixed", &SolveContext::bundle_fixed)
+        .def("to_dict", [](const SolveContext& ctx) { return solve_context_to_dict(ctx); })
+        .def_static("from_dict", [](const py::dict& d) { return solve_context_from_dict(d); });
+
+    py::class_<ConstraintDiagnostic>(m, "ConstraintDiagnostic")
+        .def(py::init<>())
+        .def_readwrite("constraint_id", &ConstraintDiagnostic::constraint_id)
+        .def_readwrite("kind", &ConstraintDiagnostic::kind)
+        .def_readwrite("detail", &ConstraintDiagnostic::detail)
+        .def("to_dict", [](const ConstraintDiagnostic& d) { return diagnostic_to_dict(d); })
+        .def_static("from_dict", [](const py::dict& d) { return diagnostic_from_dict(d); });
+
+    auto result_class = py::class_<SolveResult>(m, "SolveResult");
+
+    py::class_<SolveResult::PartResult>(result_class, "PartResult")
+        .def(py::init<>())
+        .def_readwrite("id", &SolveResult::PartResult::id)
+        .def_readwrite("placement", &SolveResult::PartResult::placement)
+        .def("to_dict", [](const SolveResult::PartResult& pr) { return part_result_to_dict(pr); })
+        .def_static("from_dict", [](const py::dict& d) { return part_result_from_dict(d); });
+
+    result_class
+        .def(py::init<>())
+        .def_readwrite("status", &SolveResult::status)
+        .def_readwrite("placements", &SolveResult::placements)
+        .def_readwrite("dof", &SolveResult::dof)
+        .def_readwrite("diagnostics", &SolveResult::diagnostics)
+        .def_readwrite("num_frames", &SolveResult::num_frames)
+        .def("to_dict", [](const SolveResult& r) { return solve_result_to_dict(r); })
+        .def_static("from_dict", [](const py::dict& d) { return solve_result_from_dict(d); });
+
+    // ── IKCSolver (with trampoline for Python subclassing) ─────────
+
+    py::class_<IKCSolver, PyIKCSolver>(m, "IKCSolver")
+        .def(py::init<>())
+        .def("name", &IKCSolver::name)
+        .def("supported_joints", &IKCSolver::supported_joints)
+        .def("solve", &IKCSolver::solve, py::arg("ctx"))
+        .def("update", &IKCSolver::update, py::arg("ctx"))
+        .def("pre_drag", &IKCSolver::pre_drag,
+             py::arg("ctx"), py::arg("drag_parts"))
+        .def("drag_step", &IKCSolver::drag_step,
+             py::arg("drag_placements"))
+        .def("post_drag", &IKCSolver::post_drag)
+        .def("run_kinematic", &IKCSolver::run_kinematic, py::arg("ctx"))
+        .def("num_frames", &IKCSolver::num_frames)
+        .def("update_for_frame", &IKCSolver::update_for_frame,
+             py::arg("index"))
+        .def("diagnose", &IKCSolver::diagnose, py::arg("ctx"))
+        .def("is_deterministic", &IKCSolver::is_deterministic)
+        .def("export_native", &IKCSolver::export_native, py::arg("path"))
+        .def("supports_bundle_fixed", &IKCSolver::supports_bundle_fixed);
+
+    // ── OndselAdapter ──────────────────────────────────────────────
+
+    py::class_<OndselAdapter, IKCSolver>(m, "OndselAdapter")
+        .def(py::init<>());
+
+    // ── Module-level functions (SolverRegistry wrapper) ────────────
+
+    m.def("available", []() {
+        return SolverRegistry::instance().available();
+    }, "Return names of all registered solvers.");
+
+    m.def("load", [](const std::string& name) {
+        return SolverRegistry::instance().get(name);
+    }, py::arg("name") = "",
+       "Create an instance of the named solver (default if empty).\n"
+       "Returns None if the solver is not found.");
+
+    m.def("joints_for", [](const std::string& name) {
+        return SolverRegistry::instance().joints_for(name);
+    }, py::arg("name"),
+       "Query supported joint types for the named solver.");
+
+    m.def("set_default", [](const std::string& name) {
+        return SolverRegistry::instance().set_default(name);
+    }, py::arg("name"),
+       "Set the default solver name. Returns True if the name is registered.");
+
+    m.def("get_default", []() {
+        return SolverRegistry::instance().get_default();
+    }, "Get the current default solver name.");
+
+    m.def("register_solver", [](const std::string& name, py::object py_solver_class) {
+        auto cls = std::make_shared<py::object>(std::move(py_solver_class));
+        CreateSolverFn factory = [cls]() -> std::unique_ptr<IKCSolver> {
+            py::gil_scoped_acquire gil;
+            py::object instance = (*cls)();
+            return std::make_unique<PySolverHolder>(std::move(instance));
+        };
+        return SolverRegistry::instance().register_solver(name, std::move(factory));
+    }, py::arg("name"), py::arg("solver_class"),
+       "Register a Python solver class with the SolverRegistry.\n"
+       "solver_class must be a callable that returns an IKCSolver subclass.");
+}

src/Mod/Assembly/TestAssemblyWorkbench.py

@@ -22,11 +22,19 @@
 # **************************************************************************/
 
 import TestApp
-
-from AssemblyTests.TestCore import TestCore
 from AssemblyTests.TestCommandInsertLink import TestCommandInsertLink
-
+from AssemblyTests.TestCore import TestCore
+from AssemblyTests.TestKCSolvePy import (
+    TestKCSolveImport,  # noqa: F401
+    TestKCSolveRegistry,  # noqa: F401
+    TestKCSolveTypes,  # noqa: F401
+    TestPySolver,  # noqa: F401
+)
+from AssemblyTests.TestKindredSolverIntegration import TestKindredSolverIntegration
+from AssemblyTests.TestSolverIntegration import TestSolverIntegration
 
 # Use the modules so that code checkers don't complain (flake8)
 True if TestCore else False
 True if TestCommandInsertLink else False
+True if TestSolverIntegration else False
+True if TestKindredSolverIntegration else False

src/Mod/Create/CMakeLists.txt

@@ -84,3 +84,18 @@ install(
     DESTINATION
         mods/sdk
 )
+
+# Install Kindred Solver addon
+install(
+    DIRECTORY
+        ${CMAKE_SOURCE_DIR}/mods/solver/kindred_solver
+    DESTINATION
+        mods/solver
+)
+install(
+    FILES
+        ${CMAKE_SOURCE_DIR}/mods/solver/package.xml
+        ${CMAKE_SOURCE_DIR}/mods/solver/Init.py
+    DESTINATION
+        mods/solver
+)

src/Mod/Create/kc_format.py

@@ -90,6 +90,24 @@ def _manifest_enrich_hook(doc, filename, entries):
 register_pre_reinject(_manifest_enrich_hook)
 
 
+def _solver_context_hook(doc, filename, entries):
+    """Pack solver context into silo/solver/context.json for assemblies."""
+    try:
+        for obj in doc.Objects:
+            if obj.TypeId == "Assembly::AssemblyObject":
+                ctx = obj.getSolveContext()
+                if ctx:  # non-empty means we have grounded parts
+                    entries["silo/solver/context.json"] = (
+                        json.dumps(ctx, indent=2) + "\n"
+                    ).encode("utf-8")
+                break  # one assembly per document
+    except Exception as exc:
+        FreeCAD.Console.PrintWarning(f"kc_format: solver context hook failed: {exc}\n")
+
+
+register_pre_reinject(_solver_context_hook)
+
+
 KC_VERSION = "1.0"
 
 

src/Mod/Create/silo_tree.py

@@ -50,6 +50,12 @@ _KNOWN_ENTRIES = [
         "Dependencies",
         ("links", lambda v: isinstance(v, list) and len(v) > 0),
     ),
+    (
+        "silo/solver/context.json",
+        "SiloSolverContext",
+        "Solver Context",
+        ("parts", lambda v: isinstance(v, list) and len(v) > 0),
+    ),
 ]
 
 

tests/CMakeLists.txt

@@ -95,6 +95,7 @@ if(BUILD_GUI)
 endif()
 if(BUILD_ASSEMBLY)
     list (APPEND TestExecutables Assembly_tests_run)
+    list (APPEND TestExecutables KCSolve_tests_run)
 endif(BUILD_ASSEMBLY)
 if(BUILD_MATERIAL)
     list (APPEND TestExecutables Material_tests_run)

tests/src/Mod/Assembly/CMakeLists.txt

@@ -1,6 +1,7 @@
 # SPDX-License-Identifier: LGPL-2.1-or-later
 
 add_subdirectory(App)
+add_subdirectory(Solver)
 
 if (NOT FREECAD_USE_EXTERNAL_ONDSELSOLVER)
 	target_include_directories(Assembly_tests_run PUBLIC

tests/src/Mod/Assembly/Solver/CMakeLists.txt

@@ -0,0 +1,13 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+
+add_executable(KCSolve_tests_run
+    SolverRegistry.cpp
+    OndselAdapter.cpp
+)
+
+target_link_libraries(KCSolve_tests_run
+    gtest_main
+    ${Google_Tests_LIBS}
+    KCSolve
+    FreeCADApp
+)

tests/src/Mod/Assembly/Solver/OndselAdapter.cpp

@@ -0,0 +1,251 @@
+// SPDX-License-Identifier: LGPL-2.1-or-later
+
+#include <gtest/gtest.h>
+
+#include <FCConfig.h>
+
+#include <App/Application.h>
+#include <Mod/Assembly/Solver/IKCSolver.h>
+#include <Mod/Assembly/Solver/OndselAdapter.h>
+#include <Mod/Assembly/Solver/Types.h>
+#include <src/App/InitApplication.h>
+
+#include <algorithm>
+#include <cmath>
+
+using namespace KCSolve;
+
+// ── Fixture ────────────────────────────────────────────────────────
+
+class OndselAdapterTest : public ::testing::Test
+{
+protected:
+    static void SetUpTestSuite()
+    {
+        tests::initApplication();
+    }
+
+    void SetUp() override
+    {
+        adapter_ = std::make_unique<OndselAdapter>();
+    }
+
+    /// Build a minimal two-part context with a single constraint.
+    static SolveContext twoPartContext(BaseJointKind jointKind,
+                                      bool groundFirst = true)
+    {
+        SolveContext ctx;
+
+        Part p1;
+        p1.id = "Part1";
+        p1.placement = Transform::identity();
+        p1.grounded = groundFirst;
+        ctx.parts.push_back(p1);
+
+        Part p2;
+        p2.id = "Part2";
+        p2.placement = Transform::identity();
+        p2.placement.position = {100.0, 0.0, 0.0};
+        p2.grounded = false;
+        ctx.parts.push_back(p2);
+
+        Constraint c;
+        c.id = "Joint1";
+        c.part_i = "Part1";
+        c.marker_i = Transform::identity();
+        c.part_j = "Part2";
+        c.marker_j = Transform::identity();
+        c.type = jointKind;
+        ctx.constraints.push_back(c);
+
+        return ctx;
+    }
+
+    std::unique_ptr<OndselAdapter> adapter_;
+};
+
+
+// ── Identity / capability tests ────────────────────────────────────
+
+TEST_F(OndselAdapterTest, Name)  // NOLINT
+{
+    auto n = adapter_->name();
+    EXPECT_FALSE(n.empty());
+    EXPECT_NE(n.find("Ondsel"), std::string::npos);
+}
+
+TEST_F(OndselAdapterTest, SupportedJoints)  // NOLINT
+{
+    auto joints = adapter_->supported_joints();
+    EXPECT_FALSE(joints.empty());
+
+    // Must include core kinematic joints.
+    EXPECT_NE(std::find(joints.begin(), joints.end(), BaseJointKind::Fixed), joints.end());
+    EXPECT_NE(std::find(joints.begin(), joints.end(), BaseJointKind::Revolute), joints.end());
+    EXPECT_NE(std::find(joints.begin(), joints.end(), BaseJointKind::Cylindrical), joints.end());
+    EXPECT_NE(std::find(joints.begin(), joints.end(), BaseJointKind::Ball), joints.end());
+
+    // Must exclude unsupported types.
+    EXPECT_EQ(std::find(joints.begin(), joints.end(), BaseJointKind::Universal), joints.end());
+    EXPECT_EQ(std::find(joints.begin(), joints.end(), BaseJointKind::Cam), joints.end());
+    EXPECT_EQ(std::find(joints.begin(), joints.end(), BaseJointKind::Slot), joints.end());
+}
+
+TEST_F(OndselAdapterTest, IsDeterministic)  // NOLINT
+{
+    EXPECT_TRUE(adapter_->is_deterministic());
+}
+
+TEST_F(OndselAdapterTest, SupportsBundleFixed)  // NOLINT
+{
+    EXPECT_FALSE(adapter_->supports_bundle_fixed());
+}
+
+
+// ── Solve round-trips ──────────────────────────────────────────────
+
+TEST_F(OndselAdapterTest, SolveFixedJoint)  // NOLINT
+{
+    auto ctx = twoPartContext(BaseJointKind::Fixed);
+    auto result = adapter_->solve(ctx);
+
+    EXPECT_EQ(result.status, SolveStatus::Success);
+    EXPECT_FALSE(result.placements.empty());
+
+    // Both parts should end up at the same position (fixed joint).
+    const auto* pr1 = &result.placements[0];
+    const auto* pr2 = &result.placements[1];
+    if (pr1->id == "Part2") {
+        std::swap(pr1, pr2);
+    }
+
+    // Part1 is grounded — should remain at origin.
+    EXPECT_NEAR(pr1->placement.position[0], 0.0, 1e-3);
+    EXPECT_NEAR(pr1->placement.position[1], 0.0, 1e-3);
+    EXPECT_NEAR(pr1->placement.position[2], 0.0, 1e-3);
+
+    // Part2 should be pulled to Part1's position by the fixed joint
+    // (markers are both identity, so the parts are welded at the same point).
+    EXPECT_NEAR(pr2->placement.position[0], 0.0, 1e-3);
+    EXPECT_NEAR(pr2->placement.position[1], 0.0, 1e-3);
+    EXPECT_NEAR(pr2->placement.position[2], 0.0, 1e-3);
+}
+
+TEST_F(OndselAdapterTest, SolveRevoluteJoint)  // NOLINT
+{
+    auto ctx = twoPartContext(BaseJointKind::Revolute);
+    auto result = adapter_->solve(ctx);
+
+    EXPECT_EQ(result.status, SolveStatus::Success);
+    EXPECT_FALSE(result.placements.empty());
+}
+
+TEST_F(OndselAdapterTest, SolveNoGroundedParts)  // NOLINT
+{
+    // OndselAdapter itself doesn't require grounded parts — that check
+    // lives in AssemblyObject. The solver should still attempt to solve.
+    auto ctx = twoPartContext(BaseJointKind::Fixed, /*groundFirst=*/false);
+    auto result = adapter_->solve(ctx);
+
+    // May succeed or fail depending on OndselSolver's behavior, but must not crash.
+    EXPECT_TRUE(result.status == SolveStatus::Success
+                || result.status == SolveStatus::Failed);
+}
+
+TEST_F(OndselAdapterTest, SolveCatchesException)  // NOLINT
+{
+    // Malformed context: constraint references non-existent parts.
+    SolveContext ctx;
+
+    Part p;
+    p.id = "LonePart";
+    p.placement = Transform::identity();
+    p.grounded = true;
+    ctx.parts.push_back(p);
+
+    Constraint c;
+    c.id = "BadJoint";
+    c.part_i = "DoesNotExist";
+    c.marker_i = Transform::identity();
+    c.part_j = "AlsoDoesNotExist";
+    c.marker_j = Transform::identity();
+    c.type = BaseJointKind::Fixed;
+    ctx.constraints.push_back(c);
+
+    // Should not crash — returns Failed or succeeds with warnings.
+    auto result = adapter_->solve(ctx);
+    SUCCEED();  // If we get here without crashing, the test passes.
+}
+
+
+// ── Drag protocol ──────────────────────────────────────────────────
+
+TEST_F(OndselAdapterTest, DragProtocol)  // NOLINT
+{
+    auto ctx = twoPartContext(BaseJointKind::Revolute);
+
+    auto preResult = adapter_->pre_drag(ctx, {"Part2"});
+    EXPECT_EQ(preResult.status, SolveStatus::Success);
+
+    // Move Part2 slightly.
+    SolveResult::PartResult dragPlc;
+    dragPlc.id = "Part2";
+    dragPlc.placement = Transform::identity();
+    dragPlc.placement.position = {10.0, 5.0, 0.0};
+
+    auto stepResult = adapter_->drag_step({dragPlc});
+    // drag_step may fail if the solver can't converge — that's OK.
+    EXPECT_TRUE(stepResult.status == SolveStatus::Success
+                || stepResult.status == SolveStatus::Failed);
+
+    // post_drag must not crash.
+    adapter_->post_drag();
+    SUCCEED();
+}
+
+
+// ── Diagnostics ────────────────────────────────────────────────────
+
+TEST_F(OndselAdapterTest, DiagnoseRedundant)  // NOLINT
+{
+    // Over-constrained: two fixed joints between the same two parts.
+    SolveContext ctx;
+
+    Part p1;
+    p1.id = "PartA";
+    p1.placement = Transform::identity();
+    p1.grounded = true;
+    ctx.parts.push_back(p1);
+
+    Part p2;
+    p2.id = "PartB";
+    p2.placement = Transform::identity();
+    p2.placement.position = {50.0, 0.0, 0.0};
+    p2.grounded = false;
+    ctx.parts.push_back(p2);
+
+    Constraint c1;
+    c1.id = "FixedJoint1";
+    c1.part_i = "PartA";
+    c1.marker_i = Transform::identity();
+    c1.part_j = "PartB";
+    c1.marker_j = Transform::identity();
+    c1.type = BaseJointKind::Fixed;
+    ctx.constraints.push_back(c1);
+
+    Constraint c2;
+    c2.id = "FixedJoint2";
+    c2.part_i = "PartA";
+    c2.marker_i = Transform::identity();
+    c2.part_j = "PartB";
+    c2.marker_j = Transform::identity();
+    c2.type = BaseJointKind::Fixed;
+    ctx.constraints.push_back(c2);
+
+    auto diags = adapter_->diagnose(ctx);
+    // With two identical fixed joints, one must be redundant.
+    bool hasRedundant = std::any_of(diags.begin(), diags.end(), [](const auto& d) {
+        return d.kind == ConstraintDiagnostic::Kind::Redundant;
+    });
+    EXPECT_TRUE(hasRedundant);
+}

tests/src/Mod/Assembly/Solver/SolverRegistry.cpp

@@ -0,0 +1,131 @@
+// SPDX-License-Identifier: LGPL-2.1-or-later
+
+#include <gtest/gtest.h>
+
+#include <Mod/Assembly/Solver/IKCSolver.h>
+#include <Mod/Assembly/Solver/SolverRegistry.h>
+#include <Mod/Assembly/Solver/Types.h>
+
+#include <algorithm>
+
+using namespace KCSolve;
+
+// ── Minimal mock solver for registry tests ─────────────────────────
+
+namespace
+{
+
+class MockSolver : public IKCSolver
+{
+public:
+    std::string name() const override
+    {
+        return "MockSolver";
+    }
+
+    std::vector<BaseJointKind> supported_joints() const override
+    {
+        return {BaseJointKind::Fixed, BaseJointKind::Revolute};
+    }
+
+    SolveResult solve(const SolveContext& /*ctx*/) override
+    {
+        return SolveResult {SolveStatus::Success, {}, 0, {}, 0};
+    }
+};
+
+}  // namespace
+
+
+// ── Tests ──────────────────────────────────────────────────────────
+//
+// SolverRegistry is a singleton — tests use unique names to avoid
+// interference across test cases.
+
+TEST(SolverRegistryTest, GetUnknownReturnsNull)  // NOLINT
+{
+    auto solver = SolverRegistry::instance().get("nonexistent_solver_xyz");
+    EXPECT_EQ(solver, nullptr);
+}
+
+TEST(SolverRegistryTest, RegisterAndGet)  // NOLINT
+{
+    auto& reg = SolverRegistry::instance();
+
+    bool ok = reg.register_solver("test_reg_get",
+                                  []() { return std::make_unique<MockSolver>(); });
+    EXPECT_TRUE(ok);
+
+    auto solver = reg.get("test_reg_get");
+    ASSERT_NE(solver, nullptr);
+    EXPECT_EQ(solver->name(), "MockSolver");
+}
+
+TEST(SolverRegistryTest, DuplicateRegistrationFails)  // NOLINT
+{
+    auto& reg = SolverRegistry::instance();
+
+    bool first = reg.register_solver("test_dup",
+                                     []() { return std::make_unique<MockSolver>(); });
+    EXPECT_TRUE(first);
+
+    bool second = reg.register_solver("test_dup",
+                                      []() { return std::make_unique<MockSolver>(); });
+    EXPECT_FALSE(second);
+}
+
+TEST(SolverRegistryTest, AvailableListsSolvers)  // NOLINT
+{
+    auto& reg = SolverRegistry::instance();
+
+    reg.register_solver("test_avail_1",
+                        []() { return std::make_unique<MockSolver>(); });
+    reg.register_solver("test_avail_2",
+                        []() { return std::make_unique<MockSolver>(); });
+
+    auto names = reg.available();
+    EXPECT_NE(std::find(names.begin(), names.end(), "test_avail_1"), names.end());
+    EXPECT_NE(std::find(names.begin(), names.end(), "test_avail_2"), names.end());
+}
+
+TEST(SolverRegistryTest, SetDefaultAndGet)  // NOLINT
+{
+    auto& reg = SolverRegistry::instance();
+
+    reg.register_solver("test_default",
+                        []() { return std::make_unique<MockSolver>(); });
+
+    bool ok = reg.set_default("test_default");
+    EXPECT_TRUE(ok);
+
+    // get() with no arg should return the default.
+    auto solver = reg.get();
+    ASSERT_NE(solver, nullptr);
+    EXPECT_EQ(solver->name(), "MockSolver");
+}
+
+TEST(SolverRegistryTest, SetDefaultUnknownFails)  // NOLINT
+{
+    auto& reg = SolverRegistry::instance();
+    bool ok = reg.set_default("totally_unknown_solver");
+    EXPECT_FALSE(ok);
+}
+
+TEST(SolverRegistryTest, JointsForReturnsCapabilities)  // NOLINT
+{
+    auto& reg = SolverRegistry::instance();
+
+    reg.register_solver("test_joints",
+                        []() { return std::make_unique<MockSolver>(); });
+
+    auto joints = reg.joints_for("test_joints");
+    EXPECT_EQ(joints.size(), 2u);
+    EXPECT_NE(std::find(joints.begin(), joints.end(), BaseJointKind::Fixed), joints.end());
+    EXPECT_NE(std::find(joints.begin(), joints.end(), BaseJointKind::Revolute), joints.end());
+}
+
+TEST(SolverRegistryTest, JointsForUnknownReturnsEmpty)  // NOLINT
+{
+    auto joints = SolverRegistry::instance().joints_for("totally_unknown_solver_2");
+    EXPECT_TRUE(joints.empty());
+}