diff --git a/src/AcDream.Core/Physics/PhysicsEngine.cs b/src/AcDream.Core/Physics/PhysicsEngine.cs
index 5061f34..eab87de 100644
--- a/src/AcDream.Core/Physics/PhysicsEngine.cs
+++ b/src/AcDream.Core/Physics/PhysicsEngine.cs
@@ -261,7 +261,32 @@ public sealed class PhysicsEngine
             // Indoor branch needs DataCache to look up cells; outdoor uses
             // _landblocks (no DataCache dependency).
             if (DataCache is null) return fallbackCellId;
-            return CellTransit.FindCellList(DataCache, worldPos, sphereRadius, fallbackCellId);
+            uint indoorResult = CellTransit.FindCellList(DataCache, worldPos, sphereRadius, fallbackCellId);
+
+            // ISSUES #83 / Phase A1.7 (2026-05-21): verify the indoor result
+            // actually contains the player. CellTransit.FindCellList falls back
+            // to currentCellId when no candidate cell's CellBSP contains the
+            // sphere center — but this happens even when the player has walked
+            // OUTSIDE the entire portal-connected indoor cell graph (e.g.,
+            // exited through an unblocked wall or doorway gap). In that state
+            // the player's CellId is stuck on an indoor cell whose BSP is
+            // far away, every indoor-bsp query returns OK (NodeIntersects
+            // fails at root), and no walls block.
+            //
+            // If the resolved indoor cell's BSP does NOT contain the sphere
+            // center, fall through to the outdoor cell resolution below — it
+            // will compute the correct landcell from the terrain grid and
+            // optionally re-enter an indoor cell via CheckBuildingTransit.
+            var indoorCell = DataCache.GetCellStruct(indoorResult);
+            if (indoorCell?.CellBSP?.Root is null)
+                return indoorResult;  // Can't verify (no CellBSP); trust FindCellList.
+
+            var localCenter = Vector3.Transform(worldPos, indoorCell.InverseWorldTransform);
+            if (BSPQuery.PointInsideCellBsp(indoorCell.CellBSP.Root, localCenter))
+                return indoorResult;
+
+            // Fall through to outdoor resolution: player has left the indoor
+            // portal-connected graph entirely.
         }
 
         // Outdoor seed: use terrain grid to compute the prefixed cell id.