diff --git a/src/AcDream.Core/Physics/PhysicsEngine.cs b/src/AcDream.Core/Physics/PhysicsEngine.cs
index be3f76c..f8eefb4 100644
--- a/src/AcDream.Core/Physics/PhysicsEngine.cs
+++ b/src/AcDream.Core/Physics/PhysicsEngine.cs
@@ -264,39 +264,43 @@ public sealed class PhysicsEngine
 
             // ── Cell-stickiness check (A6.P3 slice 3, 2026-05-22) ──
             // Before re-resolving via FindCellList, check if the fallback
-            // CellId still validly contains the sphere. If yes, prefer it
-            // over any FindCellList result that might pick a different
-            // overlapping cell — fixes issue #98 (cellar-up stuck at last
-            // step due to CellId ping-ponging between adjacent cells whose
-            // BSPs all overlap the sphere).
+            // CellId still validly contains the sphere CENTER (point-in).
+            // If yes, prefer it over any FindCellList result that might
+            // pick a different cell whose BSP also contains the point —
+            // fixes issue #98 (cellar-up stuck at last step due to CellId
+            // ping-ponging between adjacent cells in iteration-order races).
             //
-            // Mechanism: when the sphere is on a cell boundary where it
-            // overlaps multiple cells, FindCellList's candidate-iteration
-            // order (HashSet, implementation-defined) determines which
-            // cell wins. That order may shift tick-to-tick → ping-pong.
-            // Stickiness ensures we keep the CURRENT cell as long as the
-            // sphere still validly overlaps it, only switching when the
-            // sphere has truly left.
+            // Mechanism: when the sphere is on a cell boundary where the
+            // CENTER is in multiple cells geometrically (overlapping BSPs),
+            // FindCellList's candidate-iteration order (HashSet,
+            // implementation-defined) determines which cell wins. That
+            // order may shift tick-to-tick → ping-pong. Stickiness keeps
+            // the CURRENT cell as long as the sphere center is still
+            // inside it, only switching when the center has moved out.
+            //
+            // Uses POINT-IN (not sphere-overlap). Sphere-overlap stickiness
+            // (the first slice 3 attempt) over-corrected — held the player
+            // in the fallback cell even when the center had transitioned to
+            // an adjacent cell, blocking legitimate cell transitions at
+            // stair tops + portal exits. Point-in matches FindCellList's
+            // own semantics for "which cell are you in."
             //
             // Retail oracle: cell-array hysteresis pattern from
             // CObjCell::find_cell_list Position-variant at
-            // acclient_2013_pseudo_c.txt:308742-308783. Retail's cell
-            // resolution preserves cell membership when the sphere is
-            // close to (but slightly past) cell boundaries.
+            // acclient_2013_pseudo_c.txt:308742-308783.
             //
             // Likely closes/obsoletes:
             //   - #98 (cellar ascent stuck at last step) — direct target
             //   - #97 (phantom collisions + fall-through on 2nd floor) —
             //     same instability family hypothesized
-            //   - #90 (sphere-overlap stickiness workaround in this same
-            //     function below) — superseded; can be removed after
-            //     visual verification (deferred to A6.P4)
+            //   - #90 (sphere-overlap workaround below) — superseded;
+            //     can be removed after visual verification (A6.P4)
             var fallbackCell = DataCache.GetCellStruct(fallbackCellId);
             if (fallbackCell?.CellBSP?.Root is not null)
             {
                 var fallbackLocal = Vector3.Transform(worldPos, fallbackCell.InverseWorldTransform);
-                if (BSPQuery.SphereIntersectsCellBsp(fallbackCell.CellBSP.Root, fallbackLocal, sphereRadius))
-                    return fallbackCellId;  // sphere still overlaps; stick.
+                if (BSPQuery.PointInsideCellBsp(fallbackCell.CellBSP.Root, fallbackLocal))
+                    return fallbackCellId;  // center still inside; stick.
             }
 
             // Fallback cell no longer valid → re-resolve via portal-graph BFS.