diff --git a/src/AcDream.Core/Physics/CellTransit.cs b/src/AcDream.Core/Physics/CellTransit.cs
index 900157b..bdacceb 100644
--- a/src/AcDream.Core/Physics/CellTransit.cs
+++ b/src/AcDream.Core/Physics/CellTransit.cs
@@ -494,6 +494,11 @@ public static class CellTransit
             }
         }
 
+        if (PhysicsDiagnostics.ProbeCellSetEnabled)
+        {
+            PhysicsDiagnostics.LogCellSetBuild(currentCellId, worldSphereCenter, candidates);
+        }
+
         // Containment test: for each candidate, transform worldSphereCenter to
         // local and test PointInsideCellBsp.
         foreach (uint candId in candidates)
diff --git a/src/AcDream.Core/Physics/PhysicsDiagnostics.cs b/src/AcDream.Core/Physics/PhysicsDiagnostics.cs
index bb9008d..260ed07 100644
--- a/src/AcDream.Core/Physics/PhysicsDiagnostics.cs
+++ b/src/AcDream.Core/Physics/PhysicsDiagnostics.cs
@@ -69,6 +69,35 @@ public static class PhysicsDiagnostics
     public static bool ProbeBuildingEnabled { get; set; } =
         Environment.GetEnvironmentVariable("ACDREAM_PROBE_BUILDING") == "1";
 
+    /// <summary>
+    /// A6.P5 (2026-05-25) — dump the cellSet that
+    /// <c>BuildCellSetAndPickContaining</c> produces. One line per call:
+    /// seed cell, sphere world XY, candidate count, and the full candidate
+    /// list (hex). Pair with <c>[bsp-test]</c> / <c>[resolve]</c> to see
+    /// whether the door's outdoor cell is reachable from the player's
+    /// current indoor cell via the portal-walk.
+    /// </summary>
+    public static bool ProbeCellSetEnabled { get; set; }
+        = Environment.GetEnvironmentVariable("ACDREAM_PROBE_CELLSET") == "1";
+
+    public static void LogCellSetBuild(
+        uint seedCellId,
+        System.Numerics.Vector3 sphereCenter,
+        System.Collections.Generic.IReadOnlyCollection<uint> cellSet)
+    {
+        if (!ProbeCellSetEnabled) return;
+        var ids = new System.Text.StringBuilder();
+        bool first = true;
+        foreach (uint id in cellSet)
+        {
+            if (!first) ids.Append(',');
+            ids.Append(System.FormattableString.Invariant($"0x{id:X8}"));
+            first = false;
+        }
+        Console.WriteLine(System.FormattableString.Invariant(
+            $"[cellset-build] seed=0x{seedCellId:X8} sphere=({sphereCenter.X:F3},{sphereCenter.Y:F3},{sphereCenter.Z:F3}) count={cellSet.Count} ids={ids}"));
+    }
+
     /// <summary>
     /// L.2d slice 1 (2026-05-13). Diagnostic side-channel: the
     /// <see cref="ResolvedPolygon"/> that <see cref="BSPQuery"/>