diff --git a/kindred_solver/solver.py b/kindred_solver/solver.py
index f6467b5..c91a679 100644
--- a/kindred_solver/solver.py
+++ b/kindred_solver/solver.py
@@ -143,8 +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:
@@ -154,6 +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:
@@ -255,7 +257,6 @@ 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)
@@ -265,6 +266,10 @@ 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)
+
         # Build symbolic Jacobian + compile once
         from .codegen import try_compile_system