diff --git a/tests/AcDream.Core.Tests/Physics/ShadowObjectRegistryMultiPartTests.cs b/tests/AcDream.Core.Tests/Physics/ShadowObjectRegistryMultiPartTests.cs
index 1d47a19..303a73e 100644
--- a/tests/AcDream.Core.Tests/Physics/ShadowObjectRegistryMultiPartTests.cs
+++ b/tests/AcDream.Core.Tests/Physics/ShadowObjectRegistryMultiPartTests.cs
@@ -193,4 +193,28 @@ public class ShadowObjectRegistryMultiPartTests
         Assert.True(found0 && found1,
             "Expected both parts at new world positions (50, 10, 50) and (51, 10, 50)");
     }
+
+    [Fact]
+    public void Deregister_ClearsEntityShapesCache_NoStaleUpdatePositionRebuild()
+    {
+        // A6.P4 door fix (2026-05-24) regression: after Deregister, a stray
+        // UpdatePosition with the same entityId must NOT resurrect the entity
+        // via the _entityShapes path. The Deregister cleanup added in Task 4
+        // (which folded Task 6 into the multi-part registration commit) clears
+        // _entityShapes[entityId] alongside the cell-list cleanup.
+        var reg = new ShadowObjectRegistry();
+        const uint doorEntityId = 0x000F4244u;
+        reg.RegisterMultiPart(doorEntityId, new Vector3(132.6f, 17.1f, 94.08f),
+                               Quaternion.Identity, DoorShapes(), 0x10008u,
+                               EntityCollisionFlags.None, OffX, OffY, LbId);
+
+        reg.Deregister(doorEntityId);
+
+        // Stray UpdatePosition should be a no-op now (no entry to find AND
+        // no _entityShapes entry to rebuild from).
+        reg.UpdatePosition(doorEntityId, new Vector3(200f, 200f, 50f),
+                           Quaternion.Identity, OffX, OffY, LbId);
+
+        Assert.Equal(0, reg.TotalRegistered);
+    }
 }