diff --git a/src/AcDream.Core/Physics/TransitionTypes.cs b/src/AcDream.Core/Physics/TransitionTypes.cs
index 18bab97..c941f67 100644
--- a/src/AcDream.Core/Physics/TransitionTypes.cs
+++ b/src/AcDream.Core/Physics/TransitionTypes.cs
@@ -1439,6 +1439,38 @@ public sealed class Transition
 
                 // Use the full 6-path BSP dispatcher for retail-faithful collision.
                 // Use pre-resolved polygons (vertices+planes computed at cache time).
+                //
+                // 2026-05-20 (Bug B fix): pass the cell's world rotation +
+                // translation so the BSP's internal find_walkable /
+                // step_sphere_down / Path-4 land write world-space ContactPlanes
+                // (NOT cell-local). The prior call passed Quaternion.Identity
+                // and defaulted worldOrigin = Vector3.Zero — which corrupted
+                // every Path 3 + Path 4 CP write inside an indoor cell with
+                // D ≈ 0 instead of D = -world_floor_Z. Mirrors the existing
+                // correct pattern at the FindObjCollisions call site (~line
+                // 1808: passes obj.Rotation + worldOrigin: obj.Position).
+                //
+                // Retail oracle: BSPTREE::find_collisions (decomp
+                // acclient_2013_pseudo_c.txt:323924) calls Plane::localtoglobal
+                // (:323921) before set_contact_plane. Our TransformNormal +
+                // TransformVertices + BuildWorldPlane chain is the equivalent
+                // — it just needs the right rotation + origin.
+                Quaternion cellRotation;
+                Vector3    cellOrigin;
+                if (!Matrix4x4.Decompose(cellPhysics.WorldTransform, out _, out cellRotation, out cellOrigin))
+                {
+                    // Matrix has shear or other non-decomposable parts. EnvCell
+                    // WorldTransform is always rigid rotation + translation per
+                    // the dat format, so this branch should never fire. Log a
+                    // warning + fall back to identity rotation + .Translation
+                    // so we degrade to "translation-only" instead of the prior
+                    // "both broken".
+                    Console.WriteLine(System.FormattableString.Invariant(
+                        $"[indoor-bsp] WARN cellPhysics.WorldTransform did not decompose cleanly for cell 0x{sp.CheckCellId:X8} — falling back to identity rotation"));
+                    cellRotation = Quaternion.Identity;
+                    cellOrigin   = cellPhysics.WorldTransform.Translation;
+                }
+
                 var cellState = BSPQuery.FindCollisions(
                     cellPhysics.BSP.Root,
                     cellPhysics.Resolved,
@@ -1448,8 +1480,9 @@ public sealed class Transition
                     localCurrCenter,
                     Vector3.UnitZ,  // local space Z is up
                     1.0f,           // scale = 1.0 for cell geometry
-                    Quaternion.Identity,
-                    engine);        // engine needed for Path 5 step-up
+                    cellRotation,
+                    engine,         // engine needed for Path 5 step-up
+                    worldOrigin: cellOrigin);
 
                 if (PhysicsDiagnostics.ProbeIndoorBspEnabled)
                 {