diff --git a/src/AcDream.Core/Physics/TransitionTypes.cs b/src/AcDream.Core/Physics/TransitionTypes.cs
index 4131eea..3b8257c 100644
--- a/src/AcDream.Core/Physics/TransitionTypes.cs
+++ b/src/AcDream.Core/Physics/TransitionTypes.cs
@@ -248,8 +248,17 @@ public sealed class CollisionInfo
     public List<uint> CollideObjectGuids = new();
     public uint? LastCollidedObjectGuid;
 
+    /// <summary>
+    /// Test-only counter for ContactPlane writes. Incremented by every
+    /// call to <see cref="SetContactPlane"/>. Used by
+    /// IndoorContactPlaneRetentionTests to assert that CP retention is
+    /// working (A6.P3 slice 1, 2026-05-21).
+    /// </summary>
+    internal int ContactPlaneWriteCount { get; private set; }
+
     public void SetContactPlane(Plane plane, uint cellId, bool isWater = false)
     {
+        ContactPlaneWriteCount++;
         ContactPlaneValid = true;
         ContactPlane = plane;
         ContactPlaneCellId = cellId;