diff --git a/tests/AcDream.Core.Tests/Physics/Issue185OutdoorStairsSeamReplayTests.cs b/tests/AcDream.Core.Tests/Physics/Issue185OutdoorStairsSeamReplayTests.cs
index de6bf12b..89495a1b 100644
--- a/tests/AcDream.Core.Tests/Physics/Issue185OutdoorStairsSeamReplayTests.cs
+++ b/tests/AcDream.Core.Tests/Physics/Issue185OutdoorStairsSeamReplayTests.cs
@@ -193,11 +193,7 @@ public class Issue185OutdoorStairsSeamReplayTests
     public void OutdoorStairs_SideWallContact_DoesNotReverseDownhill()
     {
         var engine = BuildStairEngine();
-        var body = GroundedOnTread();
-        body.Position = new Vector3(133.03775f, 75.53931f, 59.608147f);
-        body.ContactPlane = new Plane(
-            new Vector3(3.2782555e-07f, -0.62469506f, 0.78086877f),
-            0.75193405f);
+        var body = CapturedSideWallBody(x: 133.03775f);
 
         ResolveResult result = engine.ResolveWithTransition(
             currentPos: body.Position,
@@ -225,6 +221,64 @@ public class Issue185OutdoorStairsSeamReplayTests
             $"{body.Position.Z:F6} -> {result.Position.Z:F6}.");
     }
 
+    /// <summary>
+    /// Control for the outside-support capture above. Moving the same center
+    /// 8.8 cm inward puts it 0.20 m beyond the tread edge, inside retail's
+    /// 0.24 m half-radius support allowance. The side-wall response must then
+    /// preserve meaningful uphill progress rather than treating every contact
+    /// near the edge as unsupported.
+    /// </summary>
+    [Fact]
+    public void OutdoorStairs_SideWallContact_InsideHalfRadius_AdvancesUphill()
+    {
+        var engine = BuildStairEngine();
+        var body = CapturedSideWallBody(x: 132.95f);
+
+        ResolveResult result = engine.ResolveWithTransition(
+            currentPos: body.Position,
+            targetPos: body.Position + new Vector3(0.30007935f, 0.8837738f, 0f),
+            cellId: StairCellId,
+            sphereRadius: 0.48f,
+            sphereHeight: 1.835f,
+            stepUpHeight: 0.6f,
+            stepDownHeight: 1.5f,
+            isOnGround: true,
+            body: body,
+            moverFlags: ObjectInfoState.IsPlayer | ObjectInfoState.EdgeSlide,
+            movingEntityId: 0x01000000u);
+
+        _out.WriteLine(
+            $"inside-support out=({result.Position.X:F6},{result.Position.Y:F6},"
+            + $"{result.Position.Z:F6}) collision={result.CollisionNormalValid} "
+            + $"normal=({result.CollisionNormal.X:F3},{result.CollisionNormal.Y:F3},"
+            + $"{result.CollisionNormal.Z:F3})");
+
+        Assert.True(result.Position.Y > body.Position.Y + 0.25f,
+            $"Inside-half-radius support failed to preserve meaningful uphill motion: "
+            + $"{body.Position.Y:F6} -> {result.Position.Y:F6}.");
+        Assert.True(result.Position.Z > body.Position.Z + 0.10f,
+            $"Inside-half-radius support failed to climb the tread: "
+            + $"{body.Position.Z:F6} -> {result.Position.Z:F6}.");
+    }
+
+    private static PhysicsBody CapturedSideWallBody(float x)
+    {
+        PhysicsBody body = GroundedOnTread();
+        body.Position = new Vector3(x, 75.53931f, 59.608147f);
+        var tread = new Plane(
+            new Vector3(3.2782555e-07f, -0.62469506f, 0.78086877f),
+            0.75193405f);
+        body.ContactPlane = tread;
+        body.WalkablePlane = tread;
+
+        // GroundedOnTread describes k=4. The live #271 capture is on k=1:
+        // three authored 0.5 m Y / 0.4 m Z stair increments lower.
+        Vector3 delta = new(0f, 1.5f, 1.2f);
+        Vector3[] vertices = Assert.IsType<Vector3[]>(body.WalkableVertices);
+        body.WalkableVertices = Array.ConvertAll(vertices, vertex => vertex - delta);
+        return body;
+    }
+
     private static string SolutionRoot()
     {
         var dir = AppContext.BaseDirectory;
diff --git a/tests/AcDream.Core.Tests/Physics/Issue273HoltburgTightGapReplayTests.cs b/tests/AcDream.Core.Tests/Physics/Issue273HoltburgTightGapReplayTests.cs
index 66abfcc7..73692eb8 100644
--- a/tests/AcDream.Core.Tests/Physics/Issue273HoltburgTightGapReplayTests.cs
+++ b/tests/AcDream.Core.Tests/Physics/Issue273HoltburgTightGapReplayTests.cs
@@ -44,7 +44,10 @@ public sealed class Issue273HoltburgTightGapReplayTests
         new FlatCollisionSphere(new Vector3(0f, 0f, 1.350f), 0.480f),
     ];
 
-    private static PhysicsEngine BuildEngine()
+    private static PhysicsEngine BuildEngine(
+        bool preparedFlat = false,
+        bool includeShell = true,
+        bool includeBlockingPost = true)
     {
         var cache = new PhysicsDataCache();
         var engine = new PhysicsEngine { DataCache = cache };
@@ -57,23 +60,38 @@ public sealed class Issue273HoltburgTightGapReplayTests
             "issue273",
             "0x01000F69.gfxobj.json");
         Assert.True(File.Exists(dumpPath), $"Missing issue #273 fixture: {dumpPath}");
-        cache.RegisterGfxObjForTest(
-            ShellGfxObj,
-            GfxObjDumpSerializer.Hydrate(GfxObjDumpSerializer.Read(dumpPath)));
+        GfxObjPhysics physics = GfxObjDumpSerializer.Hydrate(
+            GfxObjDumpSerializer.Read(dumpPath));
+        if (preparedFlat)
+        {
+            cache.CollisionTraversalMode = CollisionTraversalMode.Flat;
+            cache.CacheGfxObj(
+                ShellGfxObj,
+                FlatCollisionAssetBuilder.FlattenGfxObj(physics));
+        }
+        else
+        {
+            cache.RegisterGfxObjForTest(ShellGfxObj, physics);
+        }
 
         // The shell is registered through retail's building channel, not as a
         // shadow object. Its one portal is irrelevant to this exterior sweep.
-        cache.CacheBuilding(
-            Cell,
-            Array.Empty<BldPortalInfo>(),
-            BuildingTransform,
-            ShellGfxObj);
+        if (includeShell)
+        {
+            cache.CacheBuilding(
+                Cell,
+                Array.Empty<BldPortalInfo>(),
+                BuildingTransform,
+                ShellGfxObj);
+        }
 
-        // Terrain is deliberately below the shell. The player stands on the
-        // shell's authored z=2 ledge (world z=96), not synthetic terrain.
+        // Terrain is deliberately below the shell in the real fixture. The
+        // no-shell counterfactual substitutes a featureless floor at the same
+        // height so it isolates the removed shell wall/ledge while preserving
+        // grounded movement around the remaining post.
         var heights = new byte[81];
         var heightTable = new float[256];
-        Array.Fill(heightTable, -1000f);
+        Array.Fill(heightTable, includeShell ? -1000f : 96f);
         engine.AddLandblock(
             Landblock,
             new TerrainSurface(heights, heightTable),
@@ -82,11 +100,14 @@ public sealed class Issue273HoltburgTightGapReplayTests
             0f,
             0f);
 
-        RegisterPost(
-            engine,
-            0xCA9B4027u,
-            new Vector3(160.173f, 34.487f, 95.975f),
-            radius: 0.282f);
+        if (includeBlockingPost)
+        {
+            RegisterPost(
+                engine,
+                0xCA9B4027u,
+                new Vector3(160.173f, 34.487f, 95.975f),
+                radius: 0.282f);
+        }
         RegisterPost(
             engine,
             0xCA9B402Eu,
@@ -185,9 +206,66 @@ public sealed class Issue273HoltburgTightGapReplayTests
     [Fact]
     public void CapturedRun_DoesNotSqueezeBetweenPostAndBuilding()
     {
-        PhysicsEngine engine = BuildEngine();
+        ReplayFrame[] trace = RunCapturedReplay(BuildEngine());
+        Vector3 position = trace[^1].Result.Position;
+
+        // The captured broken run reached (163.234, 36.982) by this point,
+        // already beyond the post and sliding along the building. Retail
+        // blocks the passage before the player can cross the post's Y.
+        Assert.True(
+            position.Y < 34.487f,
+            $"Player squeezed through the retail-blocked gap: "
+            + $"final=({position.X:F3},{position.Y:F3},{position.Z:F3}).");
+    }
+
+    [Fact]
+    public void CapturedRun_PreparedFlatMatchesGraphForEveryFrame()
+    {
+        ReplayFrame[] graph = RunCapturedReplay(BuildEngine());
+        ReplayFrame[] preparedFlat = RunCapturedReplay(
+            BuildEngine(preparedFlat: true));
+
+        Assert.Equal(graph.Length, preparedFlat.Length);
+        for (int frame = 0; frame < graph.Length; frame++)
+            AssertFrameBitwise(graph[frame], preparedFlat[frame], frame);
+    }
+
+    [Theory]
+    [InlineData(false, true, "shell")]
+    [InlineData(true, false, "blocking post")]
+    public void CapturedRun_RequiresBothShellAndBlockingPost(
+        bool includeShell,
+        bool includeBlockingPost,
+        string omittedGeometry)
+    {
+        PhysicsEngine engine = BuildEngine(
+            includeShell: includeShell,
+            includeBlockingPost: includeBlockingPost);
+        PhysicsBody body = GroundedBody(new Vector3(160.016f, 33.562f, 96.005f));
+        if (!includeShell)
+        {
+            // Do not retain the removed shell through the body's remembered
+            // support polygon. The contact plane keeps the counterfactual at
+            // the captured height while the shell geometry itself is absent.
+            body.WalkablePolygonValid = false;
+            body.WalkableVertices = null;
+        }
+
+        ReplayFrame[] trace = RunCapturedReplay(engine, body);
+        Vector3 final = trace[^1].Result.Position;
+
+        Assert.True(
+            final.Y > 34.487f,
+            $"Omitting the {omittedGeometry} still reproduced the complete "
+            + $"fixture's block: final=({final.X:F3},{final.Y:F3},{final.Z:F3}).");
+    }
+
+    private ReplayFrame[] RunCapturedReplay(
+        PhysicsEngine engine,
+        PhysicsBody? body = null)
+    {
         Vector3 position = new(160.016f, 33.562f, 96.005f);
-        var body = GroundedBody(position);
+        body ??= GroundedBody(position);
         uint cell = Cell;
 
         // First frame is copied verbatim from the live capture. Subsequent
@@ -204,6 +282,7 @@ public sealed class Issue273HoltburgTightGapReplayTests
             new(0.727f, 0.585f, 0f),
             new(0.728f, 0.585f, 0f),
         ];
+        var trace = new ReplayFrame[offsets.Length];
 
         for (int frame = 0; frame < offsets.Length; frame++)
         {
@@ -232,17 +311,118 @@ public sealed class Issue273HoltburgTightGapReplayTests
             position = result.Position;
             cell = result.CellId;
             body.Position = position;
+            trace[frame] = new ReplayFrame(
+                result,
+                body.TransientState,
+                body.ContactPlaneValid,
+                body.ContactPlane,
+                body.ContactPlaneCellId,
+                body.ContactPlaneIsWater,
+                body.WalkablePolygonValid,
+                body.WalkablePlane,
+                body.WalkableUp,
+                body.SlidingNormal);
         }
 
-        // The captured broken run reached (163.234, 36.982) by this point,
-        // already beyond the post and sliding along the building. Retail
-        // blocks the passage before the player can cross the post's Y.
-        Assert.True(
-            position.Y < 34.487f,
-            $"Player squeezed through the retail-blocked gap: "
-            + $"final=({position.X:F3},{position.Y:F3},{position.Z:F3}).");
+        return trace;
     }
 
+    private static void AssertFrameBitwise(
+        ReplayFrame expected,
+        ReplayFrame actual,
+        int frame)
+    {
+        string context = $"frame {frame}";
+        AssertVectorBitwise(expected.Result.Position, actual.Result.Position, context);
+        Assert.Equal(expected.Result.CellId, actual.Result.CellId);
+        Assert.Equal(expected.Result.IsOnGround, actual.Result.IsOnGround);
+        Assert.Equal(
+            expected.Result.CollisionNormalValid,
+            actual.Result.CollisionNormalValid);
+        AssertVectorBitwise(
+            expected.Result.CollisionNormal,
+            actual.Result.CollisionNormal,
+            context);
+        Assert.Equal(expected.Result.Ok, actual.Result.Ok);
+        AssertQuaternionBitwise(
+            expected.Result.Orientation,
+            actual.Result.Orientation,
+            context);
+        Assert.Equal(expected.Result.InContact, actual.Result.InContact);
+        Assert.Equal(expected.Result.OnWalkable, actual.Result.OnWalkable);
+        AssertPlaneBitwise(
+            expected.Result.ContactPlane,
+            actual.Result.ContactPlane,
+            context);
+        Assert.Equal(
+            expected.Result.ContactPlaneCellId,
+            actual.Result.ContactPlaneCellId);
+        Assert.Equal(
+            expected.Result.ContactPlaneIsWater,
+            actual.Result.ContactPlaneIsWater);
+        Assert.Equal(expected.TransientState, actual.TransientState);
+        Assert.Equal(expected.ContactPlaneValid, actual.ContactPlaneValid);
+        AssertPlaneBitwise(expected.ContactPlane, actual.ContactPlane, context);
+        Assert.Equal(expected.ContactPlaneCellId, actual.ContactPlaneCellId);
+        Assert.Equal(expected.ContactPlaneIsWater, actual.ContactPlaneIsWater);
+        Assert.Equal(expected.WalkablePolygonValid, actual.WalkablePolygonValid);
+        AssertPlaneBitwise(expected.WalkablePlane, actual.WalkablePlane, context);
+        AssertVectorBitwise(expected.WalkableUp, actual.WalkableUp, context);
+        AssertVectorBitwise(expected.SlidingNormal, actual.SlidingNormal, context);
+    }
+
+    private static void AssertVectorBitwise(
+        Vector3 expected,
+        Vector3 actual,
+        string context)
+    {
+        AssertFloatBitwise(expected.X, actual.X, $"{context}.X");
+        AssertFloatBitwise(expected.Y, actual.Y, $"{context}.Y");
+        AssertFloatBitwise(expected.Z, actual.Z, $"{context}.Z");
+    }
+
+    private static void AssertQuaternionBitwise(
+        Quaternion expected,
+        Quaternion actual,
+        string context)
+    {
+        AssertFloatBitwise(expected.X, actual.X, $"{context}.X");
+        AssertFloatBitwise(expected.Y, actual.Y, $"{context}.Y");
+        AssertFloatBitwise(expected.Z, actual.Z, $"{context}.Z");
+        AssertFloatBitwise(expected.W, actual.W, $"{context}.W");
+    }
+
+    private static void AssertPlaneBitwise(
+        Plane expected,
+        Plane actual,
+        string context)
+    {
+        AssertVectorBitwise(expected.Normal, actual.Normal, $"{context}.Normal");
+        AssertFloatBitwise(expected.D, actual.D, $"{context}.D");
+    }
+
+    private static void AssertFloatBitwise(
+        float expected,
+        float actual,
+        string context)
+        => Assert.True(
+            BitConverter.SingleToInt32Bits(expected)
+            == BitConverter.SingleToInt32Bits(actual),
+            $"{context}: expected 0x{BitConverter.SingleToInt32Bits(expected):X8}, "
+            + $"actual 0x{BitConverter.SingleToInt32Bits(actual):X8}");
+
+    private readonly record struct ReplayFrame(
+        ResolveResult Result,
+        TransientStateFlags TransientState,
+        bool ContactPlaneValid,
+        Plane ContactPlane,
+        uint ContactPlaneCellId,
+        bool ContactPlaneIsWater,
+        bool WalkablePolygonValid,
+        Plane WalkablePlane,
+        Vector3 WalkableUp,
+        Vector3 SlidingNormal);
+
     private static string SolutionRoot()
     {
         string? directory = AppContext.BaseDirectory;