kindred_solver/preference.py

@@ -283,11 +283,18 @@ def _planar_half_space(
     # 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
+        # Point is on the plane — track normal direction only.
+        # No correction: when combined with rotational joints (Cylindrical,
+        # Revolute), the body's marker normal rotates legitimately and
+        # reflecting through the plane would fight the rotation.
         def indicator(e: dict[str, float]) -> float:
             return dot_expr.eval(e)
 
-        ref_sign = normal_ref_sign
+        return HalfSpace(
+            constraint_index=constraint_idx,
+            reference_sign=normal_ref_sign,
+            indicator_fn=indicator,
+        )
     else:
         # Point is off-plane — track which side
         def indicator(e: dict[str, float]) -> float: