diff --git a/src/AcDream.Core/Physics/PhysicsDataCache.cs b/src/AcDream.Core/Physics/PhysicsDataCache.cs
index 3f89793..6fd45a4 100644
--- a/src/AcDream.Core/Physics/PhysicsDataCache.cs
+++ b/src/AcDream.Core/Physics/PhysicsDataCache.cs
@@ -168,6 +168,15 @@ public sealed class PhysicsDataCache
             LocalAabbMin = aabbMin,
             LocalAabbMax = aabbMax,
         };
+
+        if (PhysicsDiagnostics.ProbeCellCacheEnabled)
+        {
+            var root = cellStruct.PhysicsBSP?.Root;
+            int bspRootPolyCount   = root?.Polygons?.Count ?? 0;
+            bool bspRootHasChildren = root?.PosNode is not null || root?.NegNode is not null;
+            Console.WriteLine(System.FormattableString.Invariant(
+                $"[cell-cache] envCellId=0x{envCellId:X8} physicsPolyCount={cellStruct.PhysicsPolygons?.Count ?? 0} resolvedCount={resolved.Count} bspRootPolyCount={bspRootPolyCount} bspRootHasChildren={bspRootHasChildren}"));
+        }
     }
 
     /// <summary>
diff --git a/src/AcDream.Core/Physics/PhysicsDiagnostics.cs b/src/AcDream.Core/Physics/PhysicsDiagnostics.cs
index bb49b20..05935e8 100644
--- a/src/AcDream.Core/Physics/PhysicsDiagnostics.cs
+++ b/src/AcDream.Core/Physics/PhysicsDiagnostics.cs
@@ -195,4 +195,30 @@ public static class PhysicsDiagnostics
     /// </summary>
     public static bool ProbeIndoorBspEnabled { get; set; } =
         Environment.GetEnvironmentVariable("ACDREAM_PROBE_INDOOR_BSP") == "1";
+
+    /// <summary>
+    /// Indoor walking Phase D follow-up (2026-05-19). When true, emits one
+    /// <c>[cell-cache]</c> line each time <see cref="PhysicsDataCache.CacheCellStruct"/>
+    /// caches a new EnvCell. Reports per-cell polygon counts and BSP root
+    /// structure so the caller can cross-reference with <c>[indoor-bsp]</c>
+    /// lines to distinguish between:
+    /// <list type="bullet">
+    ///   <item><description>Empty data (physicsPolyCount=0 or resolvedCount=0)
+    ///     — candidate (a)/(c) in the poly=n/a investigation.</description></item>
+    ///   <item><description>Non-zero polygon counts but bspRootPolyCount=0 at
+    ///     root + tree has children — correct structure for non-leaf root,
+    ///     leaves hold the poly refs; not a bug.</description></item>
+    ///   <item><description>Non-zero polygon counts but bspRootPolyCount=0 at
+    ///     root AND root is a leaf (bspRootHasChildren=false) — BSP leaf with
+    ///     zero poly refs, candidate (b)/(d).</description></item>
+    /// </list>
+    /// This diagnostic fires at most once per EnvCell (cache is no-op after
+    /// first population). It does NOT have a DebugPanel mirror yet — this is
+    /// a one-shot capture tool, not a persistent toggle. Promote to full
+    /// infrastructure after the root cause is identified.
+    ///
+    /// <para>Initial state from <c>ACDREAM_PROBE_CELL_CACHE=1</c>.</para>
+    /// </summary>
+    public static bool ProbeCellCacheEnabled { get; set; } =
+        Environment.GetEnvironmentVariable("ACDREAM_PROBE_CELL_CACHE") == "1";
 }