diff --git a/src/AcDream.Core/Physics/TransitionTypes.cs b/src/AcDream.Core/Physics/TransitionTypes.cs
index 50dc718..ae2b3de 100644
--- a/src/AcDream.Core/Physics/TransitionTypes.cs
+++ b/src/AcDream.Core/Physics/TransitionTypes.cs
@@ -1383,10 +1383,31 @@ public sealed class Transition
                 // Retail: CEnvCell::find_env_collisions returns from the cell
                 // branch with the cell's walkable plane set — no fall-through
                 // to terrain.
-                if (TryFindIndoorWalkablePlane(cellPhysics, localCenter, sphereRadius,
-                        out var indoorPlane,
-                        out var indoorVertices,
-                        out uint _))
+                bool walkableHit = TryFindIndoorWalkablePlane(
+                    cellPhysics, localCenter, sphereRadius,
+                    out var indoorPlane,
+                    out var indoorVertices,
+                    out uint hitPolyId);
+
+                if (PhysicsDiagnostics.ProbeIndoorBspEnabled)
+                {
+                    if (walkableHit)
+                    {
+                        // dz = signed gap between foot and synthesized plane.
+                        // Plane: N·p + D = 0  ⇒  pZ_on_plane = -D/N.z (for upward-facing planes)
+                        // gap   = foot.Z - pZ_on_plane = foot.Z - (-D/N.z) = foot.Z + D/N.z
+                        float dz = footCenter.Z + indoorPlane.D / indoorPlane.Normal.Z;
+                        Console.WriteLine(System.FormattableString.Invariant(
+                            $"[indoor-walkable] cell=0x{sp.CheckCellId:X8} wpos=({footCenter.X:F3},{footCenter.Y:F3},{footCenter.Z:F3}) probe={INDOOR_WALKABLE_PROBE_DISTANCE:F2} result=HIT poly=0x{hitPolyId:X4} wn=({indoorPlane.Normal.X:F3},{indoorPlane.Normal.Y:F3},{indoorPlane.Normal.Z:F3}) wD={indoorPlane.D:F3} dz={dz:+0.00;-0.00;+0.00}"));
+                    }
+                    else
+                    {
+                        Console.WriteLine(System.FormattableString.Invariant(
+                            $"[indoor-walkable] cell=0x{sp.CheckCellId:X8} wpos=({footCenter.X:F3},{footCenter.Y:F3},{footCenter.Z:F3}) probe={INDOOR_WALKABLE_PROBE_DISTANCE:F2} result=MISS"));
+                    }
+                }
+
+                if (walkableHit)
                 {
                     return ValidateWalkable(
                         footCenter,