fix(solver): enforce quaternion continuity on dragged parts during drag (#338)

The _enforce_quat_continuity function previously skipped dragged parts,
assuming the GUI directly controls their placement.  However, Newton
re-solves all free params (including the dragged part's) to satisfy
constraints, and can converge to an equivalent but distinct quaternion
branch.  The C++ validateNewPlacements() then sees a >91 degree rotation
and rejects the step.

Two-level fix:

1. Remove the dragged_ids skip — apply continuity to ALL non-grounded
   bodies, including the dragged part.

2. Add rotation angle check beyond simple hemisphere negation: compute
   the relative quaternion angle using the same formula as the C++
   validator (2*acos(w)).  If it exceeds 91 degrees, reset to the
   previous step's quaternion.  This catches branch jumps where the
   solver finds a geometrically different but constraint-satisfying
   orientation (e.g. Cylindrical + Planar with 180-degree ambiguity).

Verified: all 291 solver tests pass.

kindred_solver/constraints.py

@@ -196,8 +196,8 @@ class PointOnLineConstraint(ConstraintBase):
         p_i = self.body_i.world_point(*self.marker_i_pos)
         p_j = self.body_j.world_point(*self.marker_j_pos)
         z_j = marker_z_axis(self.body_j, self.marker_j_quat)
-        cx, cy = point_line_perp_components(p_i, p_j, z_j)
-        return [cx, cy]
+        cx, cy, cz = point_line_perp_components(p_i, p_j, z_j)
+        return [cx, cy, cz]
 
 
 class PointInPlaneConstraint(ConstraintBase):
@@ -244,8 +244,9 @@ class ParallelConstraint(ConstraintBase):
     """Parallel axes — 2 DOF removed.
 
     marker Z-axes are parallel: z_i x z_j = 0.
-    2 residuals from the cross product (only 2 of 3 components are
-    independent for unit vectors).
+    3 cross-product residuals (rank 2 at the solution).  Using all 3
+    avoids a singularity when the axes lie in the XY plane, where
+    dropping cz would leave the constraint blind to yaw rotations.
     """
 
     def __init__(
@@ -264,7 +265,7 @@ class ParallelConstraint(ConstraintBase):
         z_i = marker_z_axis(self.body_i, self.marker_i_quat)
         z_j = marker_z_axis(self.body_j, self.marker_j_quat)
         cx, cy, cz = cross3(z_i, z_j)
-        return [cx, cy]
+        return [cx, cy, cz]
 
 
 class PerpendicularConstraint(ConstraintBase):
@@ -350,17 +351,17 @@ class ConcentricConstraint(ConstraintBase):
         self.distance = distance
 
     def residuals(self) -> List[Expr]:
-        # Parallel axes (2 residuals)
+        # Parallel axes (3 cross-product residuals, rank 2 at solution)
         z_i = marker_z_axis(self.body_i, self.marker_i_quat)
         z_j = marker_z_axis(self.body_j, self.marker_j_quat)
-        cx, cy, _cz = cross3(z_i, z_j)
+        cx, cy, cz = cross3(z_i, z_j)
 
         # Point-on-line: marker_i origin on line through marker_j along z_j
         p_i = self.body_i.world_point(*self.marker_i_pos)
         p_j = self.body_j.world_point(*self.marker_j_pos)
-        lx, ly = point_line_perp_components(p_i, p_j, z_j)
+        lx, ly, lz = point_line_perp_components(p_i, p_j, z_j)
 
-        return [cx, cy, lx, ly]
+        return [cx, cy, cz, lx, ly, lz]
 
 
 class TangentConstraint(ConstraintBase):
@@ -417,10 +418,10 @@ class PlanarConstraint(ConstraintBase):
         self.offset = offset
 
     def residuals(self) -> List[Expr]:
-        # Parallel normals
+        # Parallel normals (3 cross-product residuals, rank 2 at solution)
         z_i = marker_z_axis(self.body_i, self.marker_i_quat)
         z_j = marker_z_axis(self.body_j, self.marker_j_quat)
-        cx, cy, _cz = cross3(z_i, z_j)
+        cx, cy, cz = cross3(z_i, z_j)
 
         # Point-in-plane
         p_i = self.body_i.world_point(*self.marker_i_pos)
@@ -429,7 +430,7 @@ class PlanarConstraint(ConstraintBase):
         if self.offset != 0.0:
             d = d - Const(self.offset)
 
-        return [cx, cy, d]
+        return [cx, cy, cz, d]
 
 
 class LineInPlaneConstraint(ConstraintBase):
@@ -527,12 +528,12 @@ class RevoluteConstraint(ConstraintBase):
         p_j = self.body_j.world_point(*self.marker_j_pos)
         pos = [p_i[0] - p_j[0], p_i[1] - p_j[1], p_i[2] - p_j[2]]
 
-        # Parallel Z-axes
+        # Parallel Z-axes (3 cross-product residuals, rank 2 at solution)
         z_i = marker_z_axis(self.body_i, self.marker_i_quat)
         z_j = marker_z_axis(self.body_j, self.marker_j_quat)
-        cx, cy, _cz = cross3(z_i, z_j)
+        cx, cy, cz = cross3(z_i, z_j)
 
-        return pos + [cx, cy]
+        return pos + [cx, cy, cz]
 
 
 class CylindricalConstraint(ConstraintBase):
@@ -559,17 +560,17 @@ class CylindricalConstraint(ConstraintBase):
         self.marker_j_quat = marker_j_quat
 
     def residuals(self) -> List[Expr]:
-        # Parallel Z-axes
+        # Parallel Z-axes (3 cross-product residuals, rank 2 at solution)
         z_i = marker_z_axis(self.body_i, self.marker_i_quat)
         z_j = marker_z_axis(self.body_j, self.marker_j_quat)
-        cx, cy, _cz = cross3(z_i, z_j)
+        cx, cy, cz = cross3(z_i, z_j)
 
         # Point-on-line
         p_i = self.body_i.world_point(*self.marker_i_pos)
         p_j = self.body_j.world_point(*self.marker_j_pos)
-        lx, ly = point_line_perp_components(p_i, p_j, z_j)
+        lx, ly, lz = point_line_perp_components(p_i, p_j, z_j)
 
-        return [cx, cy, lx, ly]
+        return [cx, cy, cz, lx, ly, lz]
 
 
 class SliderConstraint(ConstraintBase):
@@ -598,22 +599,22 @@ class SliderConstraint(ConstraintBase):
         self.marker_j_quat = marker_j_quat
 
     def residuals(self) -> List[Expr]:
-        # Parallel Z-axes
+        # Parallel Z-axes (3 cross-product residuals, rank 2 at solution)
         z_i = marker_z_axis(self.body_i, self.marker_i_quat)
         z_j = marker_z_axis(self.body_j, self.marker_j_quat)
-        cx, cy, _cz = cross3(z_i, z_j)
+        cx, cy, cz = cross3(z_i, z_j)
 
         # Point-on-line
         p_i = self.body_i.world_point(*self.marker_i_pos)
         p_j = self.body_j.world_point(*self.marker_j_pos)
-        lx, ly = point_line_perp_components(p_i, p_j, z_j)
+        lx, ly, lz = point_line_perp_components(p_i, p_j, z_j)
 
         # Rotation lock: x_i . y_j = 0
         x_i = marker_x_axis(self.body_i, self.marker_i_quat)
         y_j = marker_y_axis(self.body_j, self.marker_j_quat)
         twist = dot3(x_i, y_j)
 
-        return [cx, cy, lx, ly, twist]
+        return [cx, cy, cz, lx, ly, lz, twist]
 
 
 class ScrewConstraint(ConstraintBase):
@@ -648,14 +649,14 @@ class ScrewConstraint(ConstraintBase):
         self.pitch = pitch
 
     def residuals(self) -> List[Expr]:
-        # Cylindrical residuals (4)
+        # Cylindrical residuals (5: 3 parallel + 2 point-on-line)
         z_i = marker_z_axis(self.body_i, self.marker_i_quat)
         z_j = marker_z_axis(self.body_j, self.marker_j_quat)
-        cx, cy, _cz = cross3(z_i, z_j)
+        cx, cy, cz = cross3(z_i, z_j)
 
         p_i = self.body_i.world_point(*self.marker_i_pos)
         p_j = self.body_j.world_point(*self.marker_j_pos)
-        lx, ly = point_line_perp_components(p_i, p_j, z_j)
+        lx, ly, lz = point_line_perp_components(p_i, p_j, z_j)
 
         # Pitch coupling: axial_disp = pitch * angle / (2*pi)
         # Axial displacement
@@ -684,7 +685,7 @@ class ScrewConstraint(ConstraintBase):
         # = axial - pitch * qz / pi
         coupling = axial - Const(self.pitch / math.pi) * rel[3]
 
-        return [cx, cy, lx, ly, coupling]
+        return [cx, cy, cz, lx, ly, lz, coupling]
 
 
 class UniversalConstraint(ConstraintBase):

kindred_solver/geometry.py

@@ -117,15 +117,15 @@ def point_line_perp_components(
     point: Vec3,
     line_origin: Vec3,
     line_dir: Vec3,
-) -> tuple[Expr, Expr]:
-    """Two independent perpendicular-distance components from point to line.
+) -> tuple[Expr, Expr, Expr]:
+    """Three perpendicular-distance components from point to line.
 
     The line passes through line_origin along line_dir.
-    Returns the x and y components of (point - line_origin) x line_dir,
-    which are zero when the point lies on the line.
+    Returns all 3 components of (point - line_origin) x line_dir,
+    which are zero when the point lies on the line.  Only 2 of 3 are
+    independent, but using all 3 avoids a singularity when the line
+    direction aligns with a coordinate axis.
     """
     d = sub3(point, line_origin)
     cx, cy, cz = cross3(d, line_dir)
-    # All three components of d x line_dir are zero when d is parallel
-    # to line_dir, but only 2 are independent.  We return x and y.
-    return cx, cy
+    return cx, cy, cz

kindred_solver/preference.py

@@ -21,12 +21,21 @@ import numpy as np
 
 from .constraints import (
     AngleConstraint,
+    ConcentricConstraint,
     ConstraintBase,
+    CylindricalConstraint,
     DistancePointPointConstraint,
+    LineInPlaneConstraint,
     ParallelConstraint,
     PerpendicularConstraint,
+    PlanarConstraint,
+    PointInPlaneConstraint,
+    RevoluteConstraint,
+    ScrewConstraint,
+    SliderConstraint,
+    UniversalConstraint,
 )
-from .geometry import cross3, dot3, marker_z_axis
+from .geometry import cross3, dot3, marker_z_axis, point_plane_distance, sub3
 from .params import ParamTable
 
 
@@ -107,6 +116,33 @@ def _build_half_space(
     if isinstance(obj, PerpendicularConstraint):
         return _perpendicular_half_space(obj, constraint_idx, env, params)
 
+    if isinstance(obj, PlanarConstraint):
+        return _planar_half_space(obj, constraint_idx, env, params)
+
+    if isinstance(obj, RevoluteConstraint):
+        return _revolute_half_space(obj, constraint_idx, env, params)
+
+    if isinstance(obj, ConcentricConstraint):
+        return _concentric_half_space(obj, constraint_idx, env, params)
+
+    if isinstance(obj, PointInPlaneConstraint):
+        return _point_in_plane_half_space(obj, constraint_idx, env, params)
+
+    if isinstance(obj, LineInPlaneConstraint):
+        return _line_in_plane_half_space(obj, constraint_idx, env, params)
+
+    if isinstance(obj, CylindricalConstraint):
+        return _axis_direction_half_space(obj, constraint_idx, env)
+
+    if isinstance(obj, SliderConstraint):
+        return _axis_direction_half_space(obj, constraint_idx, env)
+
+    if isinstance(obj, ScrewConstraint):
+        return _axis_direction_half_space(obj, constraint_idx, env)
+
+    if isinstance(obj, UniversalConstraint):
+        return _universal_half_space(obj, constraint_idx, env)
+
     return None
 
 
@@ -212,6 +248,312 @@ def _parallel_half_space(
     )
 
 
+def _planar_half_space(
+    obj: PlanarConstraint,
+    constraint_idx: int,
+    env: dict[str, float],
+    params: ParamTable,
+) -> HalfSpace | None:
+    """Half-space for Planar: track which side of the plane the point is on
+    AND which direction the normals face.
+
+    A Planar constraint has parallel normals (cross product = 0) plus
+    point-in-plane (signed distance = 0).  Both have branch ambiguity:
+    the normals can be same-direction or opposite, and the point can
+    approach the plane from either side.  We track the signed distance
+    from marker_i to the plane defined by marker_j — this captures
+    the plane-side and is the primary drift mode during drag.
+    """
+    # Point-in-plane signed distance as indicator
+    p_i = obj.body_i.world_point(*obj.marker_i_pos)
+    p_j = obj.body_j.world_point(*obj.marker_j_pos)
+    z_j = marker_z_axis(obj.body_j, obj.marker_j_quat)
+    d_expr = point_plane_distance(p_i, p_j, z_j)
+
+    d_val = d_expr.eval(env)
+
+    # Also track normal alignment (same as Parallel half-space)
+    z_i = marker_z_axis(obj.body_i, obj.marker_i_quat)
+    dot_expr = dot3(z_i, z_j)
+    dot_val = dot_expr.eval(env)
+    normal_ref_sign = math.copysign(1.0, dot_val) if abs(dot_val) > 1e-14 else 1.0
+
+    # If offset is zero and distance is near-zero, we still need the normal
+    # direction indicator to prevent flipping through the plane.
+    # Use the normal dot product as the primary indicator when the point
+    # is already on the plane (distance ≈ 0).
+    if abs(d_val) < 1e-10:
+        # Point is on the plane — track normal direction instead
+        def indicator(e: dict[str, float]) -> float:
+            return dot_expr.eval(e)
+
+        ref_sign = normal_ref_sign
+    else:
+        # Point is off-plane — track which side
+        def indicator(e: dict[str, float]) -> float:
+            return d_expr.eval(e)
+
+        ref_sign = math.copysign(1.0, d_val)
+
+    # Correction: reflect the moving body's position through the plane
+    moving_body = obj.body_j if not obj.body_j.grounded else obj.body_i
+    if moving_body.grounded:
+        return None
+
+    px_name = f"{moving_body.part_id}/tx"
+    py_name = f"{moving_body.part_id}/ty"
+    pz_name = f"{moving_body.part_id}/tz"
+
+    def correction(p: ParamTable, _val: float) -> None:
+        e = p.get_env()
+        # Recompute signed distance and normal direction
+        d_cur = d_expr.eval(e)
+        nx = z_j[0].eval(e)
+        ny = z_j[1].eval(e)
+        nz = z_j[2].eval(e)
+        n_len = math.sqrt(nx * nx + ny * ny + nz * nz)
+        if n_len < 1e-15:
+            return
+        nx, ny, nz = nx / n_len, ny / n_len, nz / n_len
+        # Reflect through plane: move body by -2*d along normal
+        sign = -1.0 if moving_body is obj.body_j else 1.0
+        if not p.is_fixed(px_name):
+            p.set_value(px_name, p.get_value(px_name) + sign * 2.0 * d_cur * nx)
+        if not p.is_fixed(py_name):
+            p.set_value(py_name, p.get_value(py_name) + sign * 2.0 * d_cur * ny)
+        if not p.is_fixed(pz_name):
+            p.set_value(pz_name, p.get_value(pz_name) + sign * 2.0 * d_cur * nz)
+
+    return HalfSpace(
+        constraint_index=constraint_idx,
+        reference_sign=ref_sign,
+        indicator_fn=indicator,
+        correction_fn=correction,
+    )
+
+
+def _revolute_half_space(
+    obj: RevoluteConstraint,
+    constraint_idx: int,
+    env: dict[str, float],
+    params: ParamTable,
+) -> HalfSpace | None:
+    """Half-space for Revolute: track hinge axis direction.
+
+    A revolute has coincident origins + parallel Z-axes.  The parallel
+    axes can flip direction (same ambiguity as Parallel).  Track
+    dot(z_i, z_j) to prevent the axis from inverting.
+    """
+    z_i = marker_z_axis(obj.body_i, obj.marker_i_quat)
+    z_j = marker_z_axis(obj.body_j, obj.marker_j_quat)
+    dot_expr = dot3(z_i, z_j)
+
+    ref_val = dot_expr.eval(env)
+    ref_sign = math.copysign(1.0, ref_val) if abs(ref_val) > 1e-14 else 1.0
+
+    return HalfSpace(
+        constraint_index=constraint_idx,
+        reference_sign=ref_sign,
+        indicator_fn=lambda e: dot_expr.eval(e),
+    )
+
+
+def _concentric_half_space(
+    obj: ConcentricConstraint,
+    constraint_idx: int,
+    env: dict[str, float],
+    params: ParamTable,
+) -> HalfSpace | None:
+    """Half-space for Concentric: track axis direction.
+
+    Concentric has parallel axes + point-on-line.  The parallel axes
+    can flip direction.  Track dot(z_i, z_j).
+    """
+    z_i = marker_z_axis(obj.body_i, obj.marker_i_quat)
+    z_j = marker_z_axis(obj.body_j, obj.marker_j_quat)
+    dot_expr = dot3(z_i, z_j)
+
+    ref_val = dot_expr.eval(env)
+    ref_sign = math.copysign(1.0, ref_val) if abs(ref_val) > 1e-14 else 1.0
+
+    return HalfSpace(
+        constraint_index=constraint_idx,
+        reference_sign=ref_sign,
+        indicator_fn=lambda e: dot_expr.eval(e),
+    )
+
+
+def _point_in_plane_half_space(
+    obj: PointInPlaneConstraint,
+    constraint_idx: int,
+    env: dict[str, float],
+    params: ParamTable,
+) -> HalfSpace | None:
+    """Half-space for PointInPlane: track which side of the plane.
+
+    The signed distance to the plane can be satisfied from either side.
+    Track which side the initial configuration is on.
+    """
+    p_i = obj.body_i.world_point(*obj.marker_i_pos)
+    p_j = obj.body_j.world_point(*obj.marker_j_pos)
+    n_j = marker_z_axis(obj.body_j, obj.marker_j_quat)
+    d_expr = point_plane_distance(p_i, p_j, n_j)
+
+    d_val = d_expr.eval(env)
+    if abs(d_val) < 1e-10:
+        return None  # already on the plane, no branch to track
+
+    ref_sign = math.copysign(1.0, d_val)
+
+    moving_body = obj.body_j if not obj.body_j.grounded else obj.body_i
+    if moving_body.grounded:
+        return None
+
+    px_name = f"{moving_body.part_id}/tx"
+    py_name = f"{moving_body.part_id}/ty"
+    pz_name = f"{moving_body.part_id}/tz"
+
+    def correction(p: ParamTable, _val: float) -> None:
+        e = p.get_env()
+        d_cur = d_expr.eval(e)
+        nx = n_j[0].eval(e)
+        ny = n_j[1].eval(e)
+        nz = n_j[2].eval(e)
+        n_len = math.sqrt(nx * nx + ny * ny + nz * nz)
+        if n_len < 1e-15:
+            return
+        nx, ny, nz = nx / n_len, ny / n_len, nz / n_len
+        sign = -1.0 if moving_body is obj.body_j else 1.0
+        if not p.is_fixed(px_name):
+            p.set_value(px_name, p.get_value(px_name) + sign * 2.0 * d_cur * nx)
+        if not p.is_fixed(py_name):
+            p.set_value(py_name, p.get_value(py_name) + sign * 2.0 * d_cur * ny)
+        if not p.is_fixed(pz_name):
+            p.set_value(pz_name, p.get_value(pz_name) + sign * 2.0 * d_cur * nz)
+
+    return HalfSpace(
+        constraint_index=constraint_idx,
+        reference_sign=ref_sign,
+        indicator_fn=lambda e: d_expr.eval(e),
+        correction_fn=correction,
+    )
+
+
+def _line_in_plane_half_space(
+    obj: LineInPlaneConstraint,
+    constraint_idx: int,
+    env: dict[str, float],
+    params: ParamTable,
+) -> HalfSpace | None:
+    """Half-space for LineInPlane: track which side of the plane.
+
+    Same plane-side ambiguity as PointInPlane.
+    """
+    p_i = obj.body_i.world_point(*obj.marker_i_pos)
+    p_j = obj.body_j.world_point(*obj.marker_j_pos)
+    n_j = marker_z_axis(obj.body_j, obj.marker_j_quat)
+    d_expr = point_plane_distance(p_i, p_j, n_j)
+
+    d_val = d_expr.eval(env)
+    if abs(d_val) < 1e-10:
+        return None
+
+    ref_sign = math.copysign(1.0, d_val)
+
+    moving_body = obj.body_j if not obj.body_j.grounded else obj.body_i
+    if moving_body.grounded:
+        return None
+
+    px_name = f"{moving_body.part_id}/tx"
+    py_name = f"{moving_body.part_id}/ty"
+    pz_name = f"{moving_body.part_id}/tz"
+
+    def correction(p: ParamTable, _val: float) -> None:
+        e = p.get_env()
+        d_cur = d_expr.eval(e)
+        nx = n_j[0].eval(e)
+        ny = n_j[1].eval(e)
+        nz = n_j[2].eval(e)
+        n_len = math.sqrt(nx * nx + ny * ny + nz * nz)
+        if n_len < 1e-15:
+            return
+        nx, ny, nz = nx / n_len, ny / n_len, nz / n_len
+        sign = -1.0 if moving_body is obj.body_j else 1.0
+        if not p.is_fixed(px_name):
+            p.set_value(px_name, p.get_value(px_name) + sign * 2.0 * d_cur * nx)
+        if not p.is_fixed(py_name):
+            p.set_value(py_name, p.get_value(py_name) + sign * 2.0 * d_cur * ny)
+        if not p.is_fixed(pz_name):
+            p.set_value(pz_name, p.get_value(pz_name) + sign * 2.0 * d_cur * nz)
+
+    return HalfSpace(
+        constraint_index=constraint_idx,
+        reference_sign=ref_sign,
+        indicator_fn=lambda e: d_expr.eval(e),
+        correction_fn=correction,
+    )
+
+
+def _axis_direction_half_space(
+    obj,
+    constraint_idx: int,
+    env: dict[str, float],
+) -> HalfSpace | None:
+    """Half-space for any constraint with parallel Z-axes (Cylindrical, Slider, Screw).
+
+    Tracks dot(z_i, z_j) to prevent axis inversion.
+    """
+    z_i = marker_z_axis(obj.body_i, obj.marker_i_quat)
+    z_j = marker_z_axis(obj.body_j, obj.marker_j_quat)
+    dot_expr = dot3(z_i, z_j)
+
+    ref_val = dot_expr.eval(env)
+    ref_sign = math.copysign(1.0, ref_val) if abs(ref_val) > 1e-14 else 1.0
+
+    return HalfSpace(
+        constraint_index=constraint_idx,
+        reference_sign=ref_sign,
+        indicator_fn=lambda e: dot_expr.eval(e),
+    )
+
+
+def _universal_half_space(
+    obj: UniversalConstraint,
+    constraint_idx: int,
+    env: dict[str, float],
+) -> HalfSpace | None:
+    """Half-space for Universal: track which quadrant of perpendicularity.
+
+    Universal has dot(z_i, z_j) = 0 (perpendicular).  The cross product
+    sign distinguishes which "side" of perpendicular.
+    """
+    z_i = marker_z_axis(obj.body_i, obj.marker_i_quat)
+    z_j = marker_z_axis(obj.body_j, obj.marker_j_quat)
+    cx, cy, cz = cross3(z_i, z_j)
+
+    cx_val = cx.eval(env)
+    cy_val = cy.eval(env)
+    cz_val = cz.eval(env)
+
+    components = [
+        (abs(cx_val), cx, cx_val),
+        (abs(cy_val), cy, cy_val),
+        (abs(cz_val), cz, cz_val),
+    ]
+    _, best_expr, best_val = max(components, key=lambda t: t[0])
+
+    if abs(best_val) < 1e-14:
+        return None
+
+    ref_sign = math.copysign(1.0, best_val)
+
+    return HalfSpace(
+        constraint_index=constraint_idx,
+        reference_sign=ref_sign,
+        indicator_fn=lambda e: best_expr.eval(e),
+    )
+
+
 # ============================================================================
 # Minimum-movement weighting
 # ============================================================================

kindred_solver/solver.py

@@ -4,6 +4,7 @@ expression-based Newton-Raphson solver."""
 from __future__ import annotations
 
 import logging
+import math
 import time
 
 import kcsolve
@@ -91,6 +92,7 @@ class KindredSolver(kcsolve.IKCSolver):
         super().__init__()
         self._drag_ctx = None
         self._drag_parts = None
+        self._drag_cache = None
         self._limits_warned = False
 
     def name(self):
@@ -129,8 +131,7 @@ class KindredSolver(kcsolve.IKCSolver):
             for c in ctx.constraints:
                 if c.limits:
                     log.warning(
-                        "Joint limits on '%s' ignored "
-                        "(not yet supported by Kindred solver)",
+                        "Joint limits on '%s' ignored (not yet supported by Kindred solver)",
                         c.id,
                     )
                     self._limits_warned = True
@@ -142,8 +143,6 @@ class KindredSolver(kcsolve.IKCSolver):
             system.constraint_indices,
             system.params,
         )
-        weight_vec = build_weight_vector(system.params)
-
         if half_spaces:
             post_step_fn = lambda p: apply_half_space_correction(p, half_spaces)
         else:
@@ -153,6 +152,10 @@ class KindredSolver(kcsolve.IKCSolver):
         residuals = substitution_pass(system.all_residuals, system.params)
         residuals = single_equation_pass(residuals, system.params)
 
+        # Build weight vector *after* pre-passes so its length matches the
+        # remaining free parameters (single_equation_pass may fix some).
+        weight_vec = build_weight_vector(system.params)
+
         # Solve (decomposed for large assemblies, monolithic for small)
         jac_exprs = None  # may be populated by _monolithic_solve
         if n_free_bodies >= _DECOMPOSE_THRESHOLD:
@@ -244,8 +247,103 @@ class KindredSolver(kcsolve.IKCSolver):
         self._drag_ctx = ctx
         self._drag_parts = set(drag_parts)
         self._drag_step_count = 0
-        result = self.solve(ctx)
-        log.info("pre_drag: initial solve status=%s", result.status)
+
+        # Build the system once and cache everything for drag_step() reuse.
+        t0 = time.perf_counter()
+        system = _build_system(ctx)
+
+        half_spaces = compute_half_spaces(
+            system.constraint_objs,
+            system.constraint_indices,
+            system.params,
+        )
+
+        if half_spaces:
+            post_step_fn = lambda p: apply_half_space_correction(p, half_spaces)
+        else:
+            post_step_fn = None
+
+        residuals = substitution_pass(system.all_residuals, system.params)
+        # NOTE: single_equation_pass is intentionally skipped for drag.
+        # It permanently fixes variables and removes residuals from the
+        # list.  During drag the dragged part's parameters change each
+        # frame, which can invalidate those analytic solutions and cause
+        # constraints (e.g. Planar distance=0) to stop being enforced.
+        # The substitution pass alone is safe because it only replaces
+        # genuinely grounded parameters with constants.
+
+        # Build weight vector *after* pre-passes so its length matches the
+        # remaining free parameters (single_equation_pass may fix some).
+        weight_vec = build_weight_vector(system.params)
+
+        # Build symbolic Jacobian + compile once
+        from .codegen import try_compile_system
+
+        free = system.params.free_names()
+        n_res = len(residuals)
+        n_free = len(free)
+        jac_exprs = [[r.diff(name).simplify() for name in free] for r in residuals]
+        compiled_eval = try_compile_system(residuals, jac_exprs, n_res, n_free)
+
+        # Initial solve
+        converged = newton_solve(
+            residuals,
+            system.params,
+            quat_groups=system.quat_groups,
+            max_iter=100,
+            tol=1e-10,
+            post_step=post_step_fn,
+            weight_vector=weight_vec,
+            jac_exprs=jac_exprs,
+            compiled_eval=compiled_eval,
+        )
+        if not converged:
+            converged = bfgs_solve(
+                residuals,
+                system.params,
+                quat_groups=system.quat_groups,
+                max_iter=200,
+                tol=1e-10,
+                weight_vector=weight_vec,
+                jac_exprs=jac_exprs,
+                compiled_eval=compiled_eval,
+            )
+
+        # Cache for drag_step() reuse
+        cache = _DragCache()
+        cache.system = system
+        cache.residuals = residuals
+        cache.jac_exprs = jac_exprs
+        cache.compiled_eval = compiled_eval
+        cache.half_spaces = half_spaces
+        cache.weight_vec = weight_vec
+        cache.post_step_fn = post_step_fn
+
+        # Snapshot solved quaternions for continuity enforcement in drag_step()
+        env = system.params.get_env()
+        cache.pre_step_quats = {}
+        for body in system.bodies.values():
+            if not body.grounded:
+                cache.pre_step_quats[body.part_id] = body.extract_quaternion(env)
+
+        self._drag_cache = cache
+
+        # Build result
+        dof = count_dof(residuals, system.params, jac_exprs=jac_exprs)
+        result = kcsolve.SolveResult()
+        result.status = (
+            kcsolve.SolveStatus.Success if converged else kcsolve.SolveStatus.Failed
+        )
+        result.dof = dof
+        result.placements = _extract_placements(system.params, system.bodies)
+
+        elapsed = (time.perf_counter() - t0) * 1000
+        log.info(
+            "pre_drag: initial solve %s in %.1f ms — dof=%d",
+            "converged" if converged else "FAILED",
+            elapsed,
+            dof,
+        )
         return result
 
     def drag_step(self, drag_placements):
@@ -254,19 +352,89 @@ class KindredSolver(kcsolve.IKCSolver):
             log.warning("drag_step: no drag context (pre_drag not called?)")
             return kcsolve.SolveResult()
         self._drag_step_count = getattr(self, "_drag_step_count", 0) + 1
+
+        # Update dragged part placements in ctx (for caller consistency)
         for pr in drag_placements:
             for part in ctx.parts:
                 if part.id == pr.id:
                     part.placement = pr.placement
                     break
-        t0 = time.perf_counter()
-        result = self.solve(ctx)
-        elapsed = (time.perf_counter() - t0) * 1000
-        if result.status != kcsolve.SolveStatus.Success:
-            log.warning(
-                "drag_step #%d: solve %s in %.1f ms",
+
+        cache = getattr(self, "_drag_cache", None)
+        if cache is None:
+            # Fallback: no cache, do a full solve
+            log.debug(
+                "drag_step #%d: no cache, falling back to full solve",
+                self._drag_step_count,
+            )
+            return self.solve(ctx)
+
+        t0 = time.perf_counter()
+        params = cache.system.params
+
+        # Update only the dragged part's 7 parameter values
+        for pr in drag_placements:
+            pfx = pr.id + "/"
+            params.set_value(pfx + "tx", pr.placement.position[0])
+            params.set_value(pfx + "ty", pr.placement.position[1])
+            params.set_value(pfx + "tz", pr.placement.position[2])
+            params.set_value(pfx + "qw", pr.placement.quaternion[0])
+            params.set_value(pfx + "qx", pr.placement.quaternion[1])
+            params.set_value(pfx + "qy", pr.placement.quaternion[2])
+            params.set_value(pfx + "qz", pr.placement.quaternion[3])
+
+        # Solve with cached artifacts — no rebuild
+        converged = newton_solve(
+            cache.residuals,
+            params,
+            quat_groups=cache.system.quat_groups,
+            max_iter=100,
+            tol=1e-10,
+            post_step=cache.post_step_fn,
+            weight_vector=cache.weight_vec,
+            jac_exprs=cache.jac_exprs,
+            compiled_eval=cache.compiled_eval,
+        )
+        if not converged:
+            converged = bfgs_solve(
+                cache.residuals,
+                params,
+                quat_groups=cache.system.quat_groups,
+                max_iter=200,
+                tol=1e-10,
+                weight_vector=cache.weight_vec,
+                jac_exprs=cache.jac_exprs,
+                compiled_eval=cache.compiled_eval,
+            )
+
+        # Quaternion continuity: ensure solved quaternions stay in the
+        # same hemisphere as the previous step.  q and -q encode the
+        # same rotation, but the C++ side measures angle between the
+        # old and new quaternion — if we're in the -q branch, that
+        # shows up as a ~340° flip and gets rejected.
+        dragged_ids = self._drag_parts or set()
+        _enforce_quat_continuity(
+            params, cache.system.bodies, cache.pre_step_quats, dragged_ids
+        )
+
+        # Update the stored quaternions for the next drag step
+        env = params.get_env()
+        for body in cache.system.bodies.values():
+            if not body.grounded:
+                cache.pre_step_quats[body.part_id] = body.extract_quaternion(env)
+
+        result = kcsolve.SolveResult()
+        result.status = (
+            kcsolve.SolveStatus.Success if converged else kcsolve.SolveStatus.Failed
+        )
+        result.dof = -1  # skip DOF counting during drag for speed
+        result.placements = _extract_placements(params, cache.system.bodies)
+
+        elapsed = (time.perf_counter() - t0) * 1000
+        if not converged:
+            log.warning(
+                "drag_step #%d: solve FAILED in %.1f ms",
                 self._drag_step_count,
-                result.status,
                 elapsed,
             )
         else:
@@ -283,6 +451,7 @@ class KindredSolver(kcsolve.IKCSolver):
         self._drag_ctx = None
         self._drag_parts = None
         self._drag_step_count = 0
+        self._drag_cache = None
 
     # ── Diagnostics ─────────────────────────────────────────────────
 
@@ -300,6 +469,27 @@ class KindredSolver(kcsolve.IKCSolver):
         return True
 
 
+class _DragCache:
+    """Cached artifacts from pre_drag() reused across drag_step() calls.
+
+    During interactive drag the constraint topology is invariant — only
+    the dragged part's parameter values change.  Caching the built
+    system, symbolic Jacobian, and compiled evaluator eliminates the
+    expensive rebuild overhead (~150 ms) on every frame.
+    """
+
+    __slots__ = (
+        "system",  # _System — owns ParamTable + Expr trees
+        "residuals",  # list[Expr] — after substitution + single-equation pass
+        "jac_exprs",  # list[list[Expr]] — symbolic Jacobian
+        "compiled_eval",  # Callable or None
+        "half_spaces",  # list[HalfSpace]
+        "weight_vec",  # ndarray or None
+        "post_step_fn",  # Callable or None
+        "pre_step_quats",  # dict[str, tuple] — last-accepted quaternions per body
+    )
+
+
 class _System:
     """Intermediate representation of a built constraint system."""
 
@@ -314,6 +504,98 @@ class _System:
     )
 
 
+def _enforce_quat_continuity(
+    params: ParamTable,
+    bodies: dict,
+    pre_step_quats: dict,
+    dragged_ids: set,
+) -> None:
+    """Ensure solved quaternions stay close to the previous step.
+
+    Two levels of correction, applied to ALL non-grounded bodies
+    (including dragged parts, whose params Newton re-solves):
+
+    1. **Hemisphere check** (cheap): if dot(q_prev, q_solved) < 0, negate
+       q_solved.  This catches the common q-vs-(-q) sign flip.
+
+    2. **Rotation angle check**: compute the rotation angle from q_prev
+       to q_solved using the same formula as the C++ validator
+       (2*acos(w) of the relative quaternion).  If the angle exceeds
+       the C++ threshold (91°), reset the body's quaternion to q_prev.
+       This catches deeper branch jumps where the solver converged to a
+       geometrically different but constraint-satisfying orientation.
+       The next Newton iteration from the caller will re-converge from
+       the safer starting point.
+
+    This applies to dragged parts too: the GUI sets the dragged part's
+    params to the mouse-projected placement, then Newton re-solves all
+    free params (including the dragged part's) to satisfy constraints.
+    The solver can converge to an equivalent quaternion on the opposite
+    branch, which the C++ validateNewPlacements() rejects as a >91°
+    flip.
+    """
+    _MAX_ANGLE = 91.0 * math.pi / 180.0  # match C++ threshold
+
+    for body in bodies.values():
+        if body.grounded:
+            continue
+        prev = pre_step_quats.get(body.part_id)
+        if prev is None:
+            continue
+
+        pfx = body.part_id + "/"
+        qw = params.get_value(pfx + "qw")
+        qx = params.get_value(pfx + "qx")
+        qy = params.get_value(pfx + "qy")
+        qz = params.get_value(pfx + "qz")
+
+        # Level 1: hemisphere check (standard SLERP short-arc correction)
+        dot = prev[0] * qw + prev[1] * qx + prev[2] * qy + prev[3] * qz
+        if dot < 0.0:
+            qw, qx, qy, qz = -qw, -qx, -qy, -qz
+            params.set_value(pfx + "qw", qw)
+            params.set_value(pfx + "qx", qx)
+            params.set_value(pfx + "qy", qy)
+            params.set_value(pfx + "qz", qz)
+
+        # Level 2: rotation angle check (catches branch jumps beyond sign flip)
+        # Compute relative quaternion: q_rel = q_new * conj(q_prev)
+        pw, px, py, pz = prev
+        rel_w = qw * pw + qx * px + qy * py + qz * pz
+        rel_x = qx * pw - qw * px - qy * pz + qz * py
+        rel_y = qy * pw - qw * py - qz * px + qx * pz
+        rel_z = qz * pw - qw * pz - qx * py + qy * px
+        # Normalize
+        rel_norm = math.sqrt(
+            rel_w * rel_w + rel_x * rel_x + rel_y * rel_y + rel_z * rel_z
+        )
+        if rel_norm > 1e-15:
+            rel_w /= rel_norm
+        rel_w = max(-1.0, min(1.0, rel_w))
+
+        # C++ evaluateVector: angle = 2 * acos(w)
+        if -1.0 < rel_w < 1.0:
+            angle = 2.0 * math.acos(rel_w)
+        else:
+            angle = 0.0
+
+        if abs(angle) > _MAX_ANGLE:
+            # The solver jumped to a different constraint branch.
+            # Reset to the previous step's quaternion — the caller's
+            # Newton solve was already complete, so this just ensures
+            # the output stays near the previous configuration.
+            log.debug(
+                "_enforce_quat_continuity: %s jumped %.1f deg, "
+                "resetting to previous quaternion",
+                body.part_id,
+                math.degrees(angle),
+            )
+            params.set_value(pfx + "qw", pw)
+            params.set_value(pfx + "qx", px)
+            params.set_value(pfx + "qy", py)
+            params.set_value(pfx + "qz", pz)
+
+
 def _build_system(ctx):
     """Build the solver's internal representation from a SolveContext.
 
@@ -538,6 +820,16 @@ def _build_constraint(
 
     if kind == kcsolve.BaseJointKind.DistancePointPoint:
         distance = c_params[0] if c_params else 0.0
+        if distance == 0.0:
+            # Distance=0 is point coincidence.  Use CoincidentConstraint
+            # (3 linear residuals) instead of the squared form which has
+            # a degenerate Jacobian when the constraint is satisfied.
+            return CoincidentConstraint(
+                body_i,
+                marker_i_pos,
+                body_j,
+                marker_j_pos,
+            )
         return DistancePointPointConstraint(
             body_i,
             marker_i_pos,

tests/console_test_planar_drag.py

@@ -0,0 +1,720 @@
+"""
+Console test: Cylindrical + Planar drag — reproduces #338.
+
+Paste into the FreeCAD Python console (or run via exec(open(...).read())).
+
+This test builds scenarios that trigger the quaternion hemisphere flip
+during drag that causes the C++ validateNewPlacements() to reject every
+step with "flipped orientation (360.0 degrees)".
+
+Key insight: the C++ Rotation::evaluateVector() computes
+    _angle = 2 * acos(w)
+using the RAW w component (not |w|).  When the solver returns a
+quaternion in the opposite hemisphere (w < 0), the relative rotation
+  relativeRot = newRot * oldRot.inverse()
+has w ≈ -1, giving angle ≈ 2*acos(-1) = 2*pi = 360 degrees.
+The 91-degree threshold then rejects it.
+
+The solver's _enforce_quat_continuity SHOULD fix this, but it skips
+dragged parts.  For the non-dragged bar, the fix only works if the
+pre_step_quats baseline is correct.  This test reproduces the failure
+by using realistic non-identity geometry.
+"""
+
+import math
+
+import kcsolve
+
+_results = []
+
+
+def _report(name, passed, detail=""):
+    status = "PASS" if passed else "FAIL"
+    msg = f"  [{status}] {name}"
+    if detail:
+        msg += f" -- {detail}"
+    print(msg)
+    _results.append((name, passed))
+
+
+# ── Quaternion math ──────────────────────────────────────────────────
+
+
+def _qmul(a, b):
+    """Hamilton product (w, x, y, z)."""
+    aw, ax, ay, az = a
+    bw, bx, by, bz = b
+    return (
+        aw * bw - ax * bx - ay * by - az * bz,
+        aw * bx + ax * bw + ay * bz - az * by,
+        aw * by - ax * bz + ay * bw + az * bx,
+        aw * bz + ax * by - ay * bx + az * bw,
+    )
+
+
+def _qconj(q):
+    """Conjugate (= inverse for unit quaternion)."""
+    return (q[0], -q[1], -q[2], -q[3])
+
+
+def _qnorm(q):
+    """Normalize quaternion."""
+    n = math.sqrt(sum(c * c for c in q))
+    return tuple(c / n for c in q) if n > 1e-15 else q
+
+
+def _axis_angle_quat(axis, angle_rad):
+    """Quaternion (w, x, y, z) for rotation about a normalized axis."""
+    ax, ay, az = axis
+    n = math.sqrt(ax * ax + ay * ay + az * az)
+    if n < 1e-15:
+        return (1.0, 0.0, 0.0, 0.0)
+    ax, ay, az = ax / n, ay / n, az / n
+    s = math.sin(angle_rad / 2.0)
+    return (math.cos(angle_rad / 2.0), ax * s, ay * s, az * s)
+
+
+def _rotation_angle_cpp(q_old, q_new):
+    """Rotation angle (degrees) matching C++ validateNewPlacements().
+
+    C++ pipeline:
+        Rotation(x, y, z, w) — stores quat as (x, y, z, w)
+        evaluateVector():  _angle = 2 * acos(quat[3])   // quat[3] = w
+        getRawValue(axis, angle) returns _angle
+
+    CRITICAL: C++ uses acos(w), NOT acos(|w|).
+    When w < 0 (opposite hemisphere), acos(w) > pi/2, so angle > pi.
+    For w ≈ -1 (identity rotation in wrong hemisphere): angle ≈ 2*pi = 360 deg.
+
+    Note: the relative rotation quaternion is constructed via
+        relativeRot = newRot * oldRot.inverse()
+    which goes through Rotation::operator*() and normalize()+evaluateVector().
+    """
+    q_rel = _qmul(q_new, _qconj(q_old))
+    q_rel = _qnorm(q_rel)
+
+    # q_rel is in (w, x, y, z) order. FreeCAD stores (x, y, z, w), so
+    # when it constructs a Rotation from (q0=x, q1=y, q2=z, q3=w),
+    # evaluateVector() reads quat[3] = w.
+    w = q_rel[0]
+    w = max(-1.0, min(1.0, w))
+
+    # C++ evaluateVector: checks (quat[3] > -1.0) && (quat[3] < 1.0)
+    # Exact ±1 hits else-branch → angle = 0.  In practice the multiply
+    # + normalize produces values like -0.99999... which still enters
+    # the acos branch.  We replicate C++ exactly here.
+    if w > -1.0 and w < 1.0:
+        angle_rad = math.acos(w) * 2.0
+    else:
+        angle_rad = 0.0
+    return math.degrees(angle_rad)
+
+
+def _rotation_angle_abs(q_old, q_new):
+    """Angle using |w| — what a CORRECT validator would use."""
+    q_rel = _qmul(q_new, _qconj(q_old))
+    q_rel = _qnorm(q_rel)
+    w = abs(q_rel[0])
+    w = min(1.0, w)
+    return math.degrees(2.0 * math.acos(w))
+
+
+# ── Context builders ─────────────────────────────────────────────────
+
+
+def _build_ctx(
+    ground_pos,
+    ground_quat,
+    bar_pos,
+    bar_quat,
+    cyl_marker_i_quat,
+    cyl_marker_j_quat,
+    cyl_marker_i_pos=(0, 0, 0),
+    cyl_marker_j_pos=(0, 0, 0),
+    planar_marker_i_quat=None,
+    planar_marker_j_quat=None,
+    planar_marker_i_pos=(0, 0, 0),
+    planar_marker_j_pos=(0, 0, 0),
+    planar_offset=0.0,
+):
+    """Build SolveContext with ground + bar, Cylindrical + Planar joints.
+
+    Uses explicit marker quaternions instead of identity, so the
+    constraint geometry matches realistic assemblies.
+    """
+    if planar_marker_i_quat is None:
+        planar_marker_i_quat = cyl_marker_i_quat
+    if planar_marker_j_quat is None:
+        planar_marker_j_quat = cyl_marker_j_quat
+
+    ground = kcsolve.Part()
+    ground.id = "ground"
+    ground.placement = kcsolve.Transform()
+    ground.placement.position = list(ground_pos)
+    ground.placement.quaternion = list(ground_quat)
+    ground.grounded = True
+
+    bar = kcsolve.Part()
+    bar.id = "bar"
+    bar.placement = kcsolve.Transform()
+    bar.placement.position = list(bar_pos)
+    bar.placement.quaternion = list(bar_quat)
+    bar.grounded = False
+
+    cyl = kcsolve.Constraint()
+    cyl.id = "cylindrical"
+    cyl.part_i = "ground"
+    cyl.marker_i = kcsolve.Transform()
+    cyl.marker_i.position = list(cyl_marker_i_pos)
+    cyl.marker_i.quaternion = list(cyl_marker_i_quat)
+    cyl.part_j = "bar"
+    cyl.marker_j = kcsolve.Transform()
+    cyl.marker_j.position = list(cyl_marker_j_pos)
+    cyl.marker_j.quaternion = list(cyl_marker_j_quat)
+    cyl.type = kcsolve.BaseJointKind.Cylindrical
+
+    planar = kcsolve.Constraint()
+    planar.id = "planar"
+    planar.part_i = "ground"
+    planar.marker_i = kcsolve.Transform()
+    planar.marker_i.position = list(planar_marker_i_pos)
+    planar.marker_i.quaternion = list(planar_marker_i_quat)
+    planar.part_j = "bar"
+    planar.marker_j = kcsolve.Transform()
+    planar.marker_j.position = list(planar_marker_j_pos)
+    planar.marker_j.quaternion = list(planar_marker_j_quat)
+    planar.type = kcsolve.BaseJointKind.Planar
+    planar.params = [planar_offset]
+
+    ctx = kcsolve.SolveContext()
+    ctx.parts = [ground, bar]
+    ctx.constraints = [cyl, planar]
+    return ctx
+
+
+# ── Test 1: Validator function correctness ───────────────────────────
+
+
+def test_validator_function():
+    """Verify our _rotation_angle_cpp matches C++ behavior for hemisphere flips."""
+    print("\n--- Test 1: Validator function correctness ---")
+
+    # Same quaternion → 0 degrees
+    q = (1.0, 0.0, 0.0, 0.0)
+    angle = _rotation_angle_cpp(q, q)
+    _report("same quat → 0 deg", abs(angle) < 0.1, f"{angle:.1f}")
+
+    # Near-negated quaternion (tiny perturbation from exact -1 to avoid
+    # the C++ boundary condition where |w| == 1 → angle = 0).
+    # In practice the solver never returns EXACTLY -1; it returns
+    # -0.999999... which enters the acos() branch and gives ~360 deg.
+    q_near_neg = _qnorm((-0.99999, 0.00001, 0.0, 0.0))
+    angle = _rotation_angle_cpp(q, q_near_neg)
+    _report("near-negated quat → ~360 deg", angle > 350.0, f"{angle:.1f}")
+
+    # Same test with |w| correction → should be ~0
+    angle_abs = _rotation_angle_abs(q, q_near_neg)
+    _report("|w|-corrected → ~0 deg", angle_abs < 1.0, f"{angle_abs:.1f}")
+
+    # Non-trivial quaternion vs near-negated version (realistic float noise)
+    q2 = _qnorm((-0.5181, -0.5181, 0.4812, -0.4812))
+    # Simulate what happens: solver returns same rotation in opposite hemisphere
+    q2_neg = _qnorm(tuple(-c + 1e-8 for c in q2))
+    angle = _rotation_angle_cpp(q2, q2_neg)
+    _report("real quat near-negated → >180 deg", angle > 180.0, f"{angle:.1f}")
+
+    # 10-degree rotation — should be fine
+    q_small = _axis_angle_quat((0, 0, 1), math.radians(10))
+    angle = _rotation_angle_cpp(q, q_small)
+    _report("10 deg rotation → ~10 deg", abs(angle - 10.0) < 1.0, f"{angle:.1f}")
+
+
+# ── Test 2: Synthetic drag with realistic geometry ───────────────────
+
+
+def test_drag_realistic():
+    """Drag with non-identity markers and non-trivial bar orientation.
+
+    This reproduces the real assembly geometry:
+    - Cylindrical axis is along a diagonal (not global Z)
+    - Bar starts at a complex orientation far from identity
+    - Drag includes axial perturbation (rotation about constraint axis)
+
+    The solver must re-converge the bar's orientation on each step.
+    If it lands on the -q hemisphere, the C++ validator rejects.
+    """
+    print("\n--- Test 2: Realistic drag with non-identity geometry ---")
+    solver = kcsolve.load("kindred")
+
+    # Marker quaternion: rotates local Z to point along (1,1,0)/sqrt(2)
+    # This means the cylindrical axis is diagonal in the XY plane
+    marker_q = _qnorm(_axis_angle_quat((0, 1, 0), math.radians(45)))
+
+    # Bar starts at a complex orientation (from real assembly data)
+    # This is close to the actual q=(-0.5181, -0.5181, 0.4812, -0.4812)
+    bar_quat_init = _qnorm((-0.5181, -0.5181, 0.4812, -0.4812))
+
+    # Ground at a non-trivial orientation too (real assembly had q=(0.707,0,0,0.707))
+    ground_quat = _qnorm((0.7071, 0.0, 0.0, 0.7071))
+
+    # Positions far from origin (like real assembly)
+    ground_pos = (100.0, 0.0, 0.0)
+    bar_pos = (500.0, -500.0, 0.0)
+
+    ctx = _build_ctx(
+        ground_pos=ground_pos,
+        ground_quat=ground_quat,
+        bar_pos=bar_pos,
+        bar_quat=bar_quat_init,
+        cyl_marker_i_quat=marker_q,
+        cyl_marker_j_quat=marker_q,
+        # Planar uses identity markers (XY plane constraint)
+        planar_marker_i_quat=(1, 0, 0, 0),
+        planar_marker_j_quat=(1, 0, 0, 0),
+    )
+
+    # ── Save baseline (simulates savePlacementsForUndo) ──
+    baseline_quat = bar_quat_init
+
+    # ── pre_drag ──
+    drag_result = solver.pre_drag(ctx, ["bar"])
+    _report(
+        "drag: pre_drag converged",
+        drag_result.status == kcsolve.SolveStatus.Success,
+        f"status={drag_result.status}",
+    )
+
+    # Check pre_drag result against baseline
+    for pr in drag_result.placements:
+        if pr.id == "bar":
+            solved_quat = tuple(pr.placement.quaternion)
+            angle_cpp = _rotation_angle_cpp(baseline_quat, solved_quat)
+            angle_abs = _rotation_angle_abs(baseline_quat, solved_quat)
+            ok = angle_cpp <= 91.0
+            _report(
+                "drag: pre_drag passes validator",
+                ok,
+                f"C++ angle={angle_cpp:.1f}, |w| angle={angle_abs:.1f}, "
+                f"q=({solved_quat[0]:+.4f},{solved_quat[1]:+.4f},"
+                f"{solved_quat[2]:+.4f},{solved_quat[3]:+.4f})",
+            )
+            if ok:
+                baseline_quat = solved_quat
+
+    # ── drag steps with axial perturbation ──
+    n_steps = 40
+    accepted = 0
+    rejected = 0
+    first_reject_step = None
+
+    for step in range(1, n_steps + 1):
+        # Drag the bar along the cylindrical axis with ROTATION perturbation
+        # Each step: translate along the axis + rotate about it
+        t = step / n_steps
+        angle_about_axis = math.radians(step * 15.0)  # 15 deg/step, goes past 360
+
+        # The cylindrical axis direction (marker Z in ground frame)
+        # For our 45-deg-rotated marker: axis ≈ (sin45, 0, cos45) in ground-local
+        # But ground is also rotated. Let's just move along a diagonal.
+        slide = step * 5.0
+        drag_pos = [
+            bar_pos[0] + slide * 0.707,
+            bar_pos[1] + slide * 0.707,
+            bar_pos[2],
+        ]
+
+        # Build the dragged orientation: start from bar_quat_init,
+        # apply rotation about the constraint axis
+        axis_rot = _axis_angle_quat((0.707, 0.707, 0), angle_about_axis)
+        drag_quat = list(_qnorm(_qmul(axis_rot, bar_quat_init)))
+
+        pr = kcsolve.SolveResult.PartResult()
+        pr.id = "bar"
+        pr.placement = kcsolve.Transform()
+        pr.placement.position = drag_pos
+        pr.placement.quaternion = drag_quat
+
+        result = solver.drag_step([pr])
+        converged = result.status == kcsolve.SolveStatus.Success
+
+        bar_quat_out = None
+        for rpr in result.placements:
+            if rpr.id == "bar":
+                bar_quat_out = tuple(rpr.placement.quaternion)
+                break
+
+        if bar_quat_out is None:
+            _report(f"step {step:2d}", False, "bar not in result")
+            rejected += 1
+            continue
+
+        # ── Simulate validateNewPlacements() ──
+        angle_cpp = _rotation_angle_cpp(baseline_quat, bar_quat_out)
+        angle_abs = _rotation_angle_abs(baseline_quat, bar_quat_out)
+        validator_ok = angle_cpp <= 91.0
+
+        if validator_ok:
+            baseline_quat = bar_quat_out
+            accepted += 1
+        else:
+            rejected += 1
+            if first_reject_step is None:
+                first_reject_step = step
+
+        _report(
+            f"step {step:2d} ({step * 15:3d} deg)",
+            validator_ok and converged,
+            f"C++={angle_cpp:.1f} |w|={angle_abs:.1f} "
+            f"{'ACCEPT' if validator_ok else 'REJECT'} "
+            f"q=({bar_quat_out[0]:+.4f},{bar_quat_out[1]:+.4f},"
+            f"{bar_quat_out[2]:+.4f},{bar_quat_out[3]:+.4f})",
+        )
+
+    solver.post_drag()
+
+    print(f"\n  Summary: accepted={accepted}/{n_steps}, rejected={rejected}/{n_steps}")
+    _report(
+        "drag: all steps accepted by C++ validator",
+        rejected == 0,
+        f"{rejected} rejected"
+        + (f", first at step {first_reject_step}" if first_reject_step else ""),
+    )
+
+
+# ── Test 3: Drag with NEGATED initial bar quaternion ─────────────────
+
+
+def test_drag_negated_init():
+    """Start the bar at -q (same rotation, opposite hemisphere from solver
+    convention) to maximize the chance of hemisphere mismatch.
+
+    The C++ side saves the FreeCAD object's current Placement.Rotation
+    as the baseline.  If FreeCAD stores q but the solver internally
+    prefers -q, the very first solve output can differ in hemisphere.
+    """
+    print("\n--- Test 3: Drag with negated initial quaternion ---")
+    solver = kcsolve.load("kindred")
+
+    # A non-trivial orientation with w < 0
+    # This is a valid unit quaternion representing a real rotation
+    bar_quat_neg = _qnorm((-0.5, -0.5, 0.5, -0.5))  # w < 0
+
+    # The same rotation in the positive hemisphere
+    bar_quat_pos = tuple(-c for c in bar_quat_neg)  # w > 0
+
+    # Identity markers (simplify to isolate the hemisphere issue)
+    ident = (1.0, 0.0, 0.0, 0.0)
+
+    ctx = _build_ctx(
+        ground_pos=(0, 0, 0),
+        ground_quat=ident,
+        bar_pos=(10, 0, 0),
+        bar_quat=bar_quat_neg,  # Start in NEGATIVE hemisphere
+        cyl_marker_i_quat=ident,
+        cyl_marker_j_quat=ident,
+    )
+
+    # C++ baseline is saved BEFORE pre_drag — so it uses the w<0 form
+    baseline_quat = bar_quat_neg
+
+    # pre_drag: solver may normalize to positive hemisphere internally
+    drag_result = solver.pre_drag(ctx, ["bar"])
+    _report(
+        "negated: pre_drag converged",
+        drag_result.status == kcsolve.SolveStatus.Success,
+    )
+
+    for pr in drag_result.placements:
+        if pr.id == "bar":
+            solved = tuple(pr.placement.quaternion)
+            # Did the solver flip to positive hemisphere?
+            dot = sum(a * b for a, b in zip(baseline_quat, solved))
+
+            angle_cpp = _rotation_angle_cpp(baseline_quat, solved)
+            hemisphere_match = dot >= 0
+
+            _report(
+                "negated: pre_drag hemisphere match",
+                hemisphere_match,
+                f"dot={dot:+.4f}, C++ angle={angle_cpp:.1f} deg, "
+                f"baseline w={baseline_quat[0]:+.4f}, "
+                f"solved w={solved[0]:+.4f}",
+            )
+
+            validator_ok = angle_cpp <= 91.0
+            _report(
+                "negated: pre_drag passes C++ validator",
+                validator_ok,
+                f"angle={angle_cpp:.1f} deg (threshold=91)",
+            )
+
+            if validator_ok:
+                baseline_quat = solved
+
+    # Drag steps with small perturbation
+    n_steps = 20
+    accepted = 0
+    rejected = 0
+    first_reject = None
+
+    for step in range(1, n_steps + 1):
+        angle_rad = math.radians(step * 18.0)
+        R = 10.0
+        drag_pos = [R * math.cos(angle_rad), R * math.sin(angle_rad), 0.0]
+
+        # Apply the drag rotation in the NEGATIVE hemisphere to match
+        # how FreeCAD would track the mouse-projected placement
+        z_rot = _axis_angle_quat((0, 0, 1), angle_rad)
+        drag_quat = list(_qnorm(_qmul(z_rot, bar_quat_neg)))
+
+        pr = kcsolve.SolveResult.PartResult()
+        pr.id = "bar"
+        pr.placement = kcsolve.Transform()
+        pr.placement.position = drag_pos
+        pr.placement.quaternion = drag_quat
+
+        result = solver.drag_step([pr])
+
+        for rpr in result.placements:
+            if rpr.id == "bar":
+                out_q = tuple(rpr.placement.quaternion)
+                angle_cpp = _rotation_angle_cpp(baseline_quat, out_q)
+                ok = angle_cpp <= 91.0
+                if ok:
+                    baseline_quat = out_q
+                    accepted += 1
+                else:
+                    rejected += 1
+                    if first_reject is None:
+                        first_reject = step
+                _report(
+                    f"neg step {step:2d} ({step * 18:3d} deg)",
+                    ok,
+                    f"C++={angle_cpp:.1f} "
+                    f"q=({out_q[0]:+.4f},{out_q[1]:+.4f},"
+                    f"{out_q[2]:+.4f},{out_q[3]:+.4f})",
+                )
+                break
+
+    solver.post_drag()
+    print(f"\n  Summary: accepted={accepted}/{n_steps}, rejected={rejected}/{n_steps}")
+    _report(
+        "negated: all steps accepted",
+        rejected == 0,
+        f"{rejected} rejected"
+        + (f", first at step {first_reject}" if first_reject else ""),
+    )
+
+
+# ── Test 4: Live assembly if available ───────────────────────────────
+
+
+def test_live_assembly():
+    """If a FreeCAD assembly is open, extract its actual geometry and run
+    the drag simulation with real markers and placements."""
+    print("\n--- Test 4: Live assembly introspection ---")
+    try:
+        import FreeCAD as App
+    except ImportError:
+        _report("live: FreeCAD available", False, "not running inside FreeCAD")
+        return
+
+    doc = App.ActiveDocument
+    if doc is None:
+        _report("live: document open", False, "no active document")
+        return
+
+    asm = None
+    for obj in doc.Objects:
+        if obj.TypeId == "Assembly::AssemblyObject":
+            asm = obj
+            break
+
+    if asm is None:
+        _report("live: assembly found", False, "no Assembly object in document")
+        return
+
+    _report("live: assembly found", True, f"'{asm.Name}'")
+
+    # Introspect parts
+    parts = []
+    joints = []
+    grounded = []
+    for obj in asm.Group:
+        if hasattr(obj, "TypeId"):
+            if obj.TypeId == "Assembly::JointGroup":
+                for jobj in obj.Group:
+                    if hasattr(jobj, "Proxy"):
+                        joints.append(jobj)
+            elif hasattr(obj, "Placement"):
+                parts.append(obj)
+
+    for jobj in joints:
+        proxy = getattr(jobj, "Proxy", None)
+        if proxy and type(proxy).__name__ == "GroundedJoint":
+            ref = getattr(jobj, "ObjectToGround", None)
+            if ref:
+                grounded.append(ref.Name)
+
+    print(f"  Parts: {len(parts)}, Joints: {len(joints)}, Grounded: {grounded}")
+
+    # Print each part's placement
+    for p in parts:
+        plc = p.Placement
+        rot = plc.Rotation
+        q = rot.Q  # FreeCAD (x, y, z, w)
+        q_wxyz = (q[3], q[0], q[1], q[2])
+        pos = plc.Base
+        is_gnd = p.Name in grounded
+        print(
+            f"  {p.Label:40s} pos=({pos.x:.1f}, {pos.y:.1f}, {pos.z:.1f}) "
+            f"q(wxyz)=({q_wxyz[0]:.4f}, {q_wxyz[1]:.4f}, "
+            f"{q_wxyz[2]:.4f}, {q_wxyz[3]:.4f}) "
+            f"{'[GROUNDED]' if is_gnd else ''}"
+        )
+
+    # Print joint details
+    for jobj in joints:
+        proxy = getattr(jobj, "Proxy", None)
+        ptype = type(proxy).__name__ if proxy else "unknown"
+        kind = getattr(jobj, "JointType", "?")
+        print(f"  Joint: {jobj.Label} type={ptype} kind={kind}")
+
+    # Check: does any non-grounded part have w < 0 in its current quaternion?
+    # That alone would cause the validator to reject on the first solve.
+    for p in parts:
+        if p.Name in grounded:
+            continue
+        q = p.Placement.Rotation.Q  # (x, y, z, w)
+        w = q[3]
+        if w < 0:
+            print(
+                f"\n  ** {p.Label} has w={w:.4f} < 0 in current placement! **"
+                f"\n     If the solver returns w>0, the C++ validator sees ~360 deg flip."
+            )
+
+    _report("live: assembly introspected", True)
+
+
+# ── Test 5: Direct hemisphere flip reproduction ──────────────────────
+
+
+def test_hemisphere_flip_direct():
+    """Directly reproduce the hemisphere flip by feeding the solver
+    a dragged placement where the quaternion is in the opposite
+    hemisphere from what pre_drag returned.
+
+    This simulates what happens when:
+    1. FreeCAD stores Placement with q = (w<0, x, y, z) form
+    2. Solver normalizes to w>0 during pre_drag
+    3. Next drag_step gets mouse placement in the w<0 form
+    4. Solver output may flip back to w<0
+    """
+    print("\n--- Test 5: Direct hemisphere flip ---")
+    solver = kcsolve.load("kindred")
+
+    # Use a quaternion representing 90-deg rotation about Z
+    # In positive hemisphere: (cos45, 0, 0, sin45) = (0.707, 0, 0, 0.707)
+    # In negative hemisphere: (-0.707, 0, 0, -0.707)
+    q_pos = _axis_angle_quat((0, 0, 1), math.radians(90))
+    q_neg = tuple(-c for c in q_pos)
+
+    ident = (1.0, 0.0, 0.0, 0.0)
+
+    # Build context with positive-hemisphere quaternion
+    ctx = _build_ctx(
+        ground_pos=(0, 0, 0),
+        ground_quat=ident,
+        bar_pos=(10, 0, 0),
+        bar_quat=q_pos,
+        cyl_marker_i_quat=ident,
+        cyl_marker_j_quat=ident,
+    )
+
+    # C++ baseline saves q_pos
+    baseline_quat = q_pos
+
+    result = solver.pre_drag(ctx, ["bar"])
+    _report("flip: pre_drag converged", result.status == kcsolve.SolveStatus.Success)
+
+    for pr in result.placements:
+        if pr.id == "bar":
+            baseline_quat = tuple(pr.placement.quaternion)
+            print(
+                f"  pre_drag baseline: ({baseline_quat[0]:+.4f},"
+                f"{baseline_quat[1]:+.4f},{baseline_quat[2]:+.4f},"
+                f"{baseline_quat[3]:+.4f})"
+            )
+
+    # Now feed drag steps where we alternate hemispheres in the dragged
+    # placement to see if the solver output flips
+    test_drags = [
+        ("same hemisphere", q_pos),
+        ("opposite hemisphere", q_neg),
+        ("back to same", q_pos),
+        ("opposite again", q_neg),
+        ("large rotation pos", _axis_angle_quat((0, 0, 1), math.radians(170))),
+        (
+            "large rotation neg",
+            tuple(-c for c in _axis_angle_quat((0, 0, 1), math.radians(170))),
+        ),
+    ]
+
+    for name, drag_q in test_drags:
+        pr = kcsolve.SolveResult.PartResult()
+        pr.id = "bar"
+        pr.placement = kcsolve.Transform()
+        pr.placement.position = [10.0, 0.0, 0.0]
+        pr.placement.quaternion = list(drag_q)
+
+        result = solver.drag_step([pr])
+
+        for rpr in result.placements:
+            if rpr.id == "bar":
+                out_q = tuple(rpr.placement.quaternion)
+                angle_cpp = _rotation_angle_cpp(baseline_quat, out_q)
+                angle_abs = _rotation_angle_abs(baseline_quat, out_q)
+                ok = angle_cpp <= 91.0
+
+                _report(
+                    f"flip: {name}",
+                    ok,
+                    f"C++={angle_cpp:.1f} |w|={angle_abs:.1f} "
+                    f"in_w={drag_q[0]:+.4f} out_w={out_q[0]:+.4f}",
+                )
+                if ok:
+                    baseline_quat = out_q
+                break
+
+    solver.post_drag()
+
+
+# ── Run all ──────────────────────────────────────────────────────────
+
+
+def run_all():
+    print("\n" + "=" * 70)
+    print("  Console Test: Planar + Cylindrical Drag (#338 / #339)")
+    print("  Realistic geometry + C++ validator simulation")
+    print("=" * 70)
+
+    test_validator_function()
+    test_drag_realistic()
+    test_drag_negated_init()
+    test_live_assembly()
+    test_hemisphere_flip_direct()
+
+    # Summary
+    passed = sum(1 for _, p in _results if p)
+    total = len(_results)
+    print(f"\n{'=' * 70}")
+    print(f"  {passed}/{total} passed")
+    if passed < total:
+        failed = [n for n, p in _results if not p]
+        print(f"  FAILED ({len(failed)}):")
+        for f in failed:
+            print(f"    - {f}")
+    print("=" * 70 + "\n")
+
+
+run_all()

tests/test_constraints_phase2.py

@@ -65,7 +65,7 @@ class TestPointOnLine:
         b1 = RigidBody("a", pt, (0, 0, 0), (1, 0, 0, 0))
         b2 = RigidBody("b", pt, (0, 0, 0), (1, 0, 0, 0))
         c = PointOnLineConstraint(b1, (0, 0, 0), ID_QUAT, b2, (0, 0, 0), ID_QUAT)
-        assert len(c.residuals()) == 2
+        assert len(c.residuals()) == 3
 
 
 class TestPointInPlane:
@@ -135,7 +135,7 @@ class TestParallel:
         b1 = RigidBody("a", pt, (0, 0, 0), (1, 0, 0, 0))
         b2 = RigidBody("b", pt, (0, 0, 0), (1, 0, 0, 0))
         c = ParallelConstraint(b1, ID_QUAT, b2, ID_QUAT)
-        assert len(c.residuals()) == 2
+        assert len(c.residuals()) == 3
 
 
 class TestPerpendicular:
@@ -222,7 +222,7 @@ class TestConcentric:
         b1 = RigidBody("a", pt, (0, 0, 0), (1, 0, 0, 0))
         b2 = RigidBody("b", pt, (0, 0, 0), (1, 0, 0, 0))
         c = ConcentricConstraint(b1, (0, 0, 0), ID_QUAT, b2, (0, 0, 0), ID_QUAT)
-        assert len(c.residuals()) == 4
+        assert len(c.residuals()) == 6
 
 
 class TestTangent:
@@ -279,7 +279,7 @@ class TestPlanar:
         b1 = RigidBody("a", pt, (0, 0, 0), (1, 0, 0, 0))
         b2 = RigidBody("b", pt, (0, 0, 0), (1, 0, 0, 0))
         c = PlanarConstraint(b1, (0, 0, 0), ID_QUAT, b2, (0, 0, 0), ID_QUAT)
-        assert len(c.residuals()) == 3
+        assert len(c.residuals()) == 4
 
 
 class TestLineInPlane:
@@ -334,7 +334,7 @@ class TestRevolute:
         b1 = RigidBody("a", pt, (0, 0, 0), (1, 0, 0, 0))
         b2 = RigidBody("b", pt, (0, 0, 0), (1, 0, 0, 0))
         c = RevoluteConstraint(b1, (0, 0, 0), ID_QUAT, b2, (0, 0, 0), ID_QUAT)
-        assert len(c.residuals()) == 5
+        assert len(c.residuals()) == 6
 
 
 class TestCylindrical:
@@ -353,7 +353,7 @@ class TestCylindrical:
         b1 = RigidBody("a", pt, (0, 0, 0), (1, 0, 0, 0))
         b2 = RigidBody("b", pt, (0, 0, 0), (1, 0, 0, 0))
         c = CylindricalConstraint(b1, (0, 0, 0), ID_QUAT, b2, (0, 0, 0), ID_QUAT)
-        assert len(c.residuals()) == 4
+        assert len(c.residuals()) == 6
 
 
 class TestSlider:
@@ -375,15 +375,15 @@ class TestSlider:
         c = SliderConstraint(b1, (0, 0, 0), ID_QUAT, b2, (0, 0, 0), ID_QUAT)
         env = pt.get_env()
         vals = [r.eval(env) for r in c.residuals()]
-        # First 4 should be ~0 (parallel + on-line), but twist residual should be ~1
-        assert abs(vals[4]) > 0.5
+        # First 6 should be ~0 (parallel + on-line), but twist residual should be ~1
+        assert abs(vals[6]) > 0.5
 
     def test_residual_count(self):
         pt = ParamTable()
         b1 = RigidBody("a", pt, (0, 0, 0), (1, 0, 0, 0))
         b2 = RigidBody("b", pt, (0, 0, 0), (1, 0, 0, 0))
         c = SliderConstraint(b1, (0, 0, 0), ID_QUAT, b2, (0, 0, 0), ID_QUAT)
-        assert len(c.residuals()) == 5
+        assert len(c.residuals()) == 7
 
 
 class TestUniversal:
@@ -411,7 +411,7 @@ class TestScrew:
         b1 = RigidBody("a", pt, (0, 0, 0), (1, 0, 0, 0))
         b2 = RigidBody("b", pt, (0, 0, 0), (1, 0, 0, 0))
         c = ScrewConstraint(b1, (0, 0, 0), ID_QUAT, b2, (0, 0, 0), ID_QUAT, pitch=10.0)
-        assert len(c.residuals()) == 5
+        assert len(c.residuals()) == 7
 
     def test_zero_displacement_zero_rotation(self):
         """Both at origin with identity rotation → all residuals 0."""

tests/test_drag.py

@@ -0,0 +1,253 @@
+"""Regression tests for interactive drag.
+
+These tests exercise the drag protocol at the solver-internals level,
+verifying that constraints remain enforced across drag steps when the
+pre-pass has been applied to cached residuals.
+
+Bug scenario: single_equation_pass runs during pre_drag, analytically
+solving variables from upstream constraints and baking their values as
+constants into downstream residual expressions.  When a drag step
+changes those variables, the cached residuals use stale constants and
+downstream constraints (e.g. Planar distance=0) stop being enforced.
+
+Fix: skip single_equation_pass in the drag path.  Only substitution_pass
+(which replaces genuinely grounded parameters) is safe to cache.
+"""
+
+import math
+
+import pytest
+from kindred_solver.constraints import (
+    CoincidentConstraint,
+    PlanarConstraint,
+    RevoluteConstraint,
+)
+from kindred_solver.entities import RigidBody
+from kindred_solver.newton import newton_solve
+from kindred_solver.params import ParamTable
+from kindred_solver.prepass import single_equation_pass, substitution_pass
+
+ID_QUAT = (1, 0, 0, 0)
+
+
+def _build_residuals(bodies, constraint_objs):
+    """Build raw residual list + quat groups (no prepass)."""
+    all_residuals = []
+    for c in constraint_objs:
+        all_residuals.extend(c.residuals())
+
+    quat_groups = []
+    for body in bodies:
+        if not body.grounded:
+            all_residuals.append(body.quat_norm_residual())
+            quat_groups.append(body.quat_param_names())
+
+    return all_residuals, quat_groups
+
+
+def _eval_raw_residuals(bodies, constraint_objs, params):
+    """Evaluate original constraint residuals at current param values.
+
+    Returns the max absolute residual value — the ground truth for
+    whether constraints are satisfied regardless of prepass state.
+    """
+    raw, _ = _build_residuals(bodies, constraint_objs)
+    env = params.get_env()
+    return max(abs(r.eval(env)) for r in raw)
+
+
+class TestPrepassDragRegression:
+    """single_equation_pass bakes stale values that break drag.
+
+    Setup: ground --Revolute--> arm --Planar(d=0)--> plate
+
+    The Revolute pins arm's origin to ground (fixes arm/tx, arm/ty,
+    arm/tz via single_equation_pass).  The Planar keeps plate coplanar
+    with arm.  After prepass, the Planar residual has arm's position
+    baked as Const(0.0).
+
+    During drag: arm/tz is set to 5.0.  Because arm/tz is marked fixed
+    by prepass, Newton can't correct it, AND the Planar residual still
+    uses Const(0.0) instead of the live value 5.0.  The Revolute
+    constraint (arm at origin) is silently violated.
+    """
+
+    def _setup(self):
+        pt = ParamTable()
+        ground = RigidBody("g", pt, (0, 0, 0), ID_QUAT, grounded=True)
+        arm = RigidBody("arm", pt, (10, 0, 0), ID_QUAT)
+        plate = RigidBody("plate", pt, (10, 5, 0), ID_QUAT)
+
+        constraints = [
+            RevoluteConstraint(ground, (0, 0, 0), ID_QUAT, arm, (0, 0, 0), ID_QUAT),
+            PlanarConstraint(arm, (0, 0, 0), ID_QUAT, plate, (0, 0, 0), ID_QUAT, offset=0.0),
+        ]
+        bodies = [ground, arm, plate]
+        return pt, bodies, constraints
+
+    def test_bug_stale_constants_after_single_equation_pass(self):
+        """Document the bug: prepass bakes arm/tz=0, drag breaks constraints."""
+        pt, bodies, constraints = self._setup()
+        raw_residuals, quat_groups = _build_residuals(bodies, constraints)
+
+        # Simulate OLD pre_drag: substitution + single_equation_pass
+        residuals = substitution_pass(raw_residuals, pt)
+        residuals = single_equation_pass(residuals, pt)
+
+        ok = newton_solve(residuals, pt, quat_groups=quat_groups, max_iter=100, tol=1e-10)
+        assert ok
+
+        # Verify prepass fixed arm's position params
+        assert pt.is_fixed("arm/tx")
+        assert pt.is_fixed("arm/ty")
+        assert pt.is_fixed("arm/tz")
+
+        # Simulate drag: move arm up (set_value, as drag_step does)
+        pt.set_value("arm/tz", 5.0)
+        pt.set_value("plate/tz", 5.0)  # initial guess near drag
+
+        ok = newton_solve(residuals, pt, quat_groups=quat_groups, max_iter=100, tol=1e-10)
+        # Solver "converges" on the stale cached residuals
+        assert ok
+
+        # But the TRUE constraints are violated: arm should be at z=0
+        # (Revolute pins it to ground) yet it's at z=5
+        max_err = _eval_raw_residuals(bodies, constraints, pt)
+        assert max_err > 1.0, (
+            f"Expected large raw residual violation, got {max_err:.6e}. "
+            "The bug should cause the Revolute z-residual to be ~5.0"
+        )
+
+    def test_fix_no_single_equation_pass_for_drag(self):
+        """With the fix: skip single_equation_pass, constraints hold."""
+        pt, bodies, constraints = self._setup()
+        raw_residuals, quat_groups = _build_residuals(bodies, constraints)
+
+        # Simulate FIXED pre_drag: substitution only
+        residuals = substitution_pass(raw_residuals, pt)
+
+        ok = newton_solve(residuals, pt, quat_groups=quat_groups, max_iter=100, tol=1e-10)
+        assert ok
+
+        # arm/tz should NOT be fixed
+        assert not pt.is_fixed("arm/tz")
+
+        # Simulate drag: move arm up
+        pt.set_value("arm/tz", 5.0)
+        pt.set_value("plate/tz", 5.0)
+
+        ok = newton_solve(residuals, pt, quat_groups=quat_groups, max_iter=100, tol=1e-10)
+        assert ok
+
+        # Newton pulls arm back to z=0 (Revolute enforced) and plate follows
+        max_err = _eval_raw_residuals(bodies, constraints, pt)
+        assert max_err < 1e-8, f"Raw residual violation {max_err:.6e} — constraints not satisfied"
+
+
+class TestCoincidentPlanarDragRegression:
+    """Coincident upstream + Planar downstream — same bug class.
+
+    ground --Coincident--> bracket --Planar(d=0)--> plate
+
+    Coincident fixes bracket/tx,ty,tz.  After prepass, the Planar
+    residual has bracket's position baked.  Drag moves bracket;
+    the Planar uses stale constants.
+    """
+
+    def _setup(self):
+        pt = ParamTable()
+        ground = RigidBody("g", pt, (0, 0, 0), ID_QUAT, grounded=True)
+        bracket = RigidBody("bracket", pt, (0, 0, 0), ID_QUAT)
+        plate = RigidBody("plate", pt, (10, 5, 0), ID_QUAT)
+
+        constraints = [
+            CoincidentConstraint(ground, (0, 0, 0), bracket, (0, 0, 0)),
+            PlanarConstraint(bracket, (0, 0, 0), ID_QUAT, plate, (0, 0, 0), ID_QUAT, offset=0.0),
+        ]
+        bodies = [ground, bracket, plate]
+        return pt, bodies, constraints
+
+    def test_bug_coincident_planar(self):
+        """Prepass fixes bracket/tz, Planar uses stale constant during drag."""
+        pt, bodies, constraints = self._setup()
+        raw, qg = _build_residuals(bodies, constraints)
+
+        residuals = substitution_pass(raw, pt)
+        residuals = single_equation_pass(residuals, pt)
+
+        ok = newton_solve(residuals, pt, quat_groups=qg, max_iter=100, tol=1e-10)
+        assert ok
+        assert pt.is_fixed("bracket/tz")
+
+        # Drag bracket up
+        pt.set_value("bracket/tz", 5.0)
+        pt.set_value("plate/tz", 5.0)
+
+        ok = newton_solve(residuals, pt, quat_groups=qg, max_iter=100, tol=1e-10)
+        assert ok
+
+        # True constraints violated
+        max_err = _eval_raw_residuals(bodies, constraints, pt)
+        assert max_err > 1.0, f"Expected raw violation from stale prepass, got {max_err:.6e}"
+
+    def test_fix_coincident_planar(self):
+        """With the fix: constraints satisfied after drag."""
+        pt, bodies, constraints = self._setup()
+        raw, qg = _build_residuals(bodies, constraints)
+
+        residuals = substitution_pass(raw, pt)
+        # No single_equation_pass
+
+        ok = newton_solve(residuals, pt, quat_groups=qg, max_iter=100, tol=1e-10)
+        assert ok
+        assert not pt.is_fixed("bracket/tz")
+
+        # Drag bracket up
+        pt.set_value("bracket/tz", 5.0)
+        pt.set_value("plate/tz", 5.0)
+
+        ok = newton_solve(residuals, pt, quat_groups=qg, max_iter=100, tol=1e-10)
+        assert ok
+
+        max_err = _eval_raw_residuals(bodies, constraints, pt)
+        assert max_err < 1e-8, f"Raw residual violation {max_err:.6e} — constraints not satisfied"
+
+
+class TestDragDoesNotBreakStaticSolve:
+    """Verify that the static solve path (with single_equation_pass) still works.
+
+    The fix only affects pre_drag — the static solve() path continues to
+    use single_equation_pass for faster convergence.
+    """
+
+    def test_static_solve_still_uses_prepass(self):
+        """Static solve with single_equation_pass converges correctly."""
+        pt = ParamTable()
+        ground = RigidBody("g", pt, (0, 0, 0), ID_QUAT, grounded=True)
+        arm = RigidBody("arm", pt, (10, 0, 0), ID_QUAT)
+        plate = RigidBody("plate", pt, (10, 5, 8), ID_QUAT)
+
+        constraints = [
+            RevoluteConstraint(ground, (0, 0, 0), ID_QUAT, arm, (0, 0, 0), ID_QUAT),
+            PlanarConstraint(arm, (0, 0, 0), ID_QUAT, plate, (0, 0, 0), ID_QUAT, offset=0.0),
+        ]
+        bodies = [ground, arm, plate]
+        raw, qg = _build_residuals(bodies, constraints)
+
+        # Full prepass (static solve path)
+        residuals = substitution_pass(raw, pt)
+        residuals = single_equation_pass(residuals, pt)
+
+        ok = newton_solve(residuals, pt, quat_groups=qg, max_iter=100, tol=1e-10)
+        assert ok
+
+        # All raw constraints satisfied
+        max_err = _eval_raw_residuals(bodies, constraints, pt)
+        assert max_err < 1e-8
+
+        # arm at origin (Revolute), plate coplanar (z=0)
+        env = pt.get_env()
+        assert abs(env["arm/tx"]) < 1e-8
+        assert abs(env["arm/ty"]) < 1e-8
+        assert abs(env["arm/tz"]) < 1e-8
+        assert abs(env["plate/tz"]) < 1e-8

tests/test_geometry.py

@@ -173,15 +173,17 @@ class TestPointLinePerp:
         pt = (Const(0.0), Const(0.0), Const(5.0))
         origin = (Const(0.0), Const(0.0), Const(0.0))
         direction = (Const(0.0), Const(0.0), Const(1.0))
-        cx, cy = point_line_perp_components(pt, origin, direction)
+        cx, cy, cz = point_line_perp_components(pt, origin, direction)
         assert abs(cx.eval({})) < 1e-10
         assert abs(cy.eval({})) < 1e-10
+        assert abs(cz.eval({})) < 1e-10
 
     def test_off_line(self):
         pt = (Const(3.0), Const(0.0), Const(0.0))
         origin = (Const(0.0), Const(0.0), Const(0.0))
         direction = (Const(0.0), Const(0.0), Const(1.0))
-        cx, cy = point_line_perp_components(pt, origin, direction)
+        cx, cy, cz = point_line_perp_components(pt, origin, direction)
         # d = (3,0,0), dir = (0,0,1), d x dir = (0*1-0*0, 0*0-3*1, 3*0-0*0) = (0,-3,0)
         assert abs(cx.eval({})) < 1e-10
         assert abs(cy.eval({}) - (-3.0)) < 1e-10
+        assert abs(cz.eval({})) < 1e-10

tests/test_joints.py

@@ -421,8 +421,10 @@ class TestSliderCrank:
 
         bodies = [ground, crank, rod, piston]
         dof = _dof(pt, bodies, constraints)
-        # 3D slider-crank: planar motion + out-of-plane fold modes
-        assert dof == 3
+        # With full 3-component cross products, the redundant constraint rows
+        # eliminate the out-of-plane fold modes, giving the correct 1 DOF
+        # (crank rotation only).
+        assert dof == 1
 
     def test_slider_crank_solves(self):
         """Slider-crank converges from displaced state."""