diff --git a/src/AcDream.Core/Physics/PhysicsEngine.cs b/src/AcDream.Core/Physics/PhysicsEngine.cs
index aaf5a48..be3f76c 100644
--- a/src/AcDream.Core/Physics/PhysicsEngine.cs
+++ b/src/AcDream.Core/Physics/PhysicsEngine.cs
@@ -261,6 +261,45 @@ public sealed class PhysicsEngine
             // Indoor branch needs DataCache to look up cells; outdoor uses
             // _landblocks (no DataCache dependency).
             if (DataCache is null) return fallbackCellId;
+
+            // ── 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).
+            //
+            // 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.
+            //
+            // 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.
+            //
+            // 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+            //     function below) — superseded; can be removed after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)
+            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.
+            }
+
+            // Fallback cell no longer valid → re-resolve via portal-graph BFS.
             uint indoorResult = CellTransit.FindCellList(DataCache, worldPos, sphereRadius, fallbackCellId);
 
             // ISSUES #83 / Phase A1.7 (2026-05-21): verify the indoor result