diff --git a/src/AcDream.Core/Physics/BSPQuery.cs b/src/AcDream.Core/Physics/BSPQuery.cs
index bdff8f4..4e045aa 100644
--- a/src/AcDream.Core/Physics/BSPQuery.cs
+++ b/src/AcDream.Core/Physics/BSPQuery.cs
@@ -394,6 +394,14 @@ public static class BSPQuery
                 applied: true);
         }
 
+        if (PhysicsDiagnostics.ProbePolyDumpEnabled)
+        {
+            // A6.P3 slice 4 (2026-05-22): dump the polygon geometry to
+            // cross-reference against WorldBuilder's straight-from-dat
+            // read. CellId taken from path.CheckCellId.
+            PhysicsDiagnostics.LogPolyDump(path.CheckCellId, poly);
+        }
+
         return true;
     }
 
@@ -2212,6 +2220,7 @@ public static class BSPQuery
                 Plane     = new Plane(normal, planeD),
                 NumPoints = n,
                 SidesType = poly.SidesType,
+                Id        = id,
             };
         }
         return resolved;
diff --git a/src/AcDream.Core/Physics/PhysicsDataCache.cs b/src/AcDream.Core/Physics/PhysicsDataCache.cs
index 934b3a1..9bcda0b 100644
--- a/src/AcDream.Core/Physics/PhysicsDataCache.cs
+++ b/src/AcDream.Core/Physics/PhysicsDataCache.cs
@@ -289,6 +289,7 @@ public sealed class PhysicsDataCache
                 Plane = new Plane(normal, d),
                 NumPoints = numVerts,
                 SidesType = poly.SidesType,
+                Id = id,
             };
         }
         return resolved;
@@ -383,6 +384,15 @@ public sealed class ResolvedPolygon
     public required Plane Plane { get; init; }
     public required int NumPoints { get; init; }
     public required CullMode SidesType { get; init; }
+    /// <summary>
+    /// Polygon index within its parent (cell or GfxObj). Used by the
+    /// `ACDREAM_PROBE_POLY_DUMP` probe (A6.P3 slice 4 investigation,
+    /// 2026-05-22) to identify which dat polygon a push-back hit so we
+    /// can compare our extracted vertices/plane against WorldBuilder's
+    /// straight-from-dat read. Defaults to 0 for test fixtures that
+    /// don't care about polygon identity.
+    /// </summary>
+    public ushort Id { get; init; }
 }
 
 /// <summary>Cached physics data for a single GfxObj part.</summary>
diff --git a/src/AcDream.Core/Physics/PhysicsDiagnostics.cs b/src/AcDream.Core/Physics/PhysicsDiagnostics.cs
index 5942a19..731eb05 100644
--- a/src/AcDream.Core/Physics/PhysicsDiagnostics.cs
+++ b/src/AcDream.Core/Physics/PhysicsDiagnostics.cs
@@ -304,6 +304,51 @@ public static class PhysicsDiagnostics
     public static bool ProbePushBackEnabled { get; set; } =
         Environment.GetEnvironmentVariable("ACDREAM_PROBE_PUSH_BACK") == "1";
 
+    /// <summary>
+    /// A6.P3 slice 4 investigation (2026-05-22) — dumps the polygon's
+    /// vertices + plane + sidesType + cell id whenever a push-back fires.
+    /// Lets us compare our extracted polygon (from our cache) against
+    /// WorldBuilder's straight-from-dat read for the same poly index, to
+    /// falsify the "is our dat-read producing wrong polygon geometry?"
+    /// hypothesis for issue #98 (cellar-up stuck at top step).
+    ///
+    /// <para>
+    /// Initial state from <c>ACDREAM_PROBE_POLY_DUMP=1</c>.
+    /// Heavy output (one dump per AdjustSphereToPlane call); use briefly
+    /// to capture a specific scenario, then turn off.
+    /// </para>
+    /// </summary>
+    public static bool ProbePolyDumpEnabled { get; set; } =
+        Environment.GetEnvironmentVariable("ACDREAM_PROBE_POLY_DUMP") == "1";
+
+    /// <summary>
+    /// Emit one <c>[poly-dump]</c> line with the full polygon geometry:
+    /// cell id, polygon index, num vertices, sides flag, plane normal+D,
+    /// and all vertex coordinates. Used in conjunction with the push-back
+    /// probe to identify which dat polygon a push-back hit so we can
+    /// cross-reference against WorldBuilder's straight-from-dat read.
+    ///
+    /// <para>Caller MUST guard with <c>if (!ProbePolyDumpEnabled) return;</c>.</para>
+    /// </summary>
+    public static void LogPolyDump(uint cellId, ResolvedPolygon poly)
+    {
+        var ci = System.Globalization.CultureInfo.InvariantCulture;
+        var sb = new System.Text.StringBuilder(256);
+        sb.AppendFormat(ci,
+            "[poly-dump] cell=0x{0:X8} polyId=0x{1:X4} numPts={2} sides={3} " +
+            "n=({4:F6},{5:F6},{6:F6}) d={7:F6} verts=[",
+            cellId, poly.Id, poly.NumPoints, poly.SidesType,
+            poly.Plane.Normal.X, poly.Plane.Normal.Y, poly.Plane.Normal.Z, poly.Plane.D);
+        for (int i = 0; i < poly.Vertices.Length; i++)
+        {
+            if (i > 0) sb.Append(',');
+            sb.AppendFormat(ci, "({0:F6},{1:F6},{2:F6})",
+                poly.Vertices[i].X, poly.Vertices[i].Y, poly.Vertices[i].Z);
+        }
+        sb.Append(']');
+        Console.WriteLine(sb.ToString());
+    }
+
     /// <summary>
     /// A6.P1 emission helper for the <c>AdjustSphereToPlane</c> site.
     /// One line per call: input sphere center, plane geometry, push-back