diff --git a/src/AcDream.Core/Physics/PhysicsDataCache.cs b/src/AcDream.Core/Physics/PhysicsDataCache.cs
index f50a144..fbe8a2d 100644
--- a/src/AcDream.Core/Physics/PhysicsDataCache.cs
+++ b/src/AcDream.Core/Physics/PhysicsDataCache.cs
@@ -359,6 +359,25 @@ public sealed class PhysicsDataCache
     }
 
     public GfxObjPhysics? GetGfxObj(uint id) => _gfxObj.TryGetValue(id, out var p) ? p : null;
+
+    /// <summary>
+    /// Issue #101 (2026-05-25): retail-faithful phantom check for
+    /// GfxObj-only entity sources. Returns true when the entity's
+    /// <c>SourceGfxObjOrSetupId</c> is a GfxObj (high byte
+    /// <c>0x01</c>) AND has no cached <see cref="GfxObjPhysics"/> —
+    /// meaning the underlying GfxObj had <c>HasPhysics=False</c> or
+    /// a null <c>PhysicsBSP.Root</c>, so <see cref="CacheGfxObj"/>
+    /// short-circuited at the early-return on line 45/46. Retail's
+    /// <c>CPartArray::InitParts</c> emits NO collision shapes for
+    /// these — acdream's <c>mesh-aabb-fallback</c> synthesis at
+    /// <c>GameWindow.cs:6116</c> must do the same.
+    /// </summary>
+    public bool IsPhantomGfxObjSource(uint sourceId)
+    {
+        if ((sourceId & 0xFF000000u) != 0x01000000u) return false;
+        return GetGfxObj(sourceId)?.BSP?.Root is null;
+    }
+
     public SetupPhysics? GetSetup(uint id) => _setup.TryGetValue(id, out var p) ? p : null;
     public CellPhysics? GetCellStruct(uint id) => _cellStruct.TryGetValue(id, out var p) ? p : null;
     public int GfxObjCount => _gfxObj.Count;
diff --git a/tests/AcDream.Core.Tests/Physics/PhysicsDataCachePhantomSourceTests.cs b/tests/AcDream.Core.Tests/Physics/PhysicsDataCachePhantomSourceTests.cs
new file mode 100644
index 0000000..5312aaf
--- /dev/null
+++ b/tests/AcDream.Core.Tests/Physics/PhysicsDataCachePhantomSourceTests.cs
@@ -0,0 +1,74 @@
+using System.Collections.Generic;
+using AcDream.Core.Physics;
+using DatReaderWriter.Enums;
+using DatReaderWriter.Types;
+using Xunit;
+
+namespace AcDream.Core.Tests.Physics;
+
+/// <summary>
+/// Issue #101 (2026-05-25) — phantom-stair fix. Retail's
+/// <c>CPartArray::InitParts</c> emits collision shapes only from
+/// Setup-level <c>CylSpheres</c>/<c>Spheres</c> or per-Part
+/// <c>PhysicsBSP</c>. There is NO synthesis from visual mesh AABB.
+/// Acdream's <c>mesh-aabb-fallback</c> path at
+/// <c>GameWindow.cs:6116</c> previously fired for ANY entity that
+/// reached it with <c>entityBsp==0 &amp;&amp; entityCyl==0</c>,
+/// including GfxObj-only stabs whose GfxObj has
+/// <c>HasPhysics=False</c>. This produced the 10 phantom 0.80 m
+/// cylinders that block the Holtburg upper-floor staircase (see
+/// <c>docs/research/2026-05-25-a6-stairs-cyl-retail-investigation.md</c>).
+///
+/// <para>
+/// <see cref="PhysicsDataCache.IsPhantomGfxObjSource"/> captures the
+/// retail rule as a predicate: "the entity's source is a GfxObj
+/// (high byte 0x01) AND the cache has no <see cref="GfxObjPhysics"/>
+/// entry for it." When this returns true, the caller suppresses the
+/// fallback synthesis.
+/// </para>
+/// </summary>
+public class PhysicsDataCachePhantomSourceTests
+{
+    [Fact]
+    public void IsPhantomGfxObjSource_SetupHighByte_ReturnsFalse()
+    {
+        // Setup source (high byte 0x02) is never the GfxObj-phantom case.
+        // The existing isPhantomSetup check at GameWindow.cs:6090 handles
+        // the Setup-side phantom. This predicate is scoped to GfxObj only.
+        var cache = new PhysicsDataCache();
+        Assert.False(cache.IsPhantomGfxObjSource(0x020019FFu));   // door setup
+        Assert.False(cache.IsPhantomGfxObjSource(0x02000266u));   // some setup
+    }
+
+    [Fact]
+    public void IsPhantomGfxObjSource_GfxObjUncached_ReturnsTrue()
+    {
+        // GfxObj source (high byte 0x01) with NO cached GfxObjPhysics
+        // = the phantom case. The stair-step GfxObj 0x0100081A from
+        // issue #101's broken-stairs capture has HasPhysics=False and
+        // does not enter the cache. Acdream should treat it as phantom.
+        var cache = new PhysicsDataCache();
+        Assert.True(cache.IsPhantomGfxObjSource(0x0100081Au));
+    }
+
+    [Fact]
+    public void IsPhantomGfxObjSource_GfxObjCached_ReturnsFalse()
+    {
+        // GfxObj source (high byte 0x01) WITH a cached GfxObjPhysics
+        // (i.e. the GfxObj's HasPhysics flag was set and its PhysicsBSP
+        // root is non-null) is NOT phantom. The staircase BSP entity
+        // 0x40B50089 from issue #101's capture, backed by GfxObj
+        // 0x01000C16 with hasPhys=True, is this case.
+        var cache = new PhysicsDataCache();
+        var leaf = new PhysicsBSPNode { Type = BSPNodeType.Leaf };
+        var fakePhysics = new GfxObjPhysics
+        {
+            BSP             = new PhysicsBSPTree { Root = leaf },
+            PhysicsPolygons = new Dictionary<ushort, Polygon>(),
+            Vertices        = new VertexArray(),
+            Resolved        = new Dictionary<ushort, ResolvedPolygon>(),
+        };
+        cache.RegisterGfxObjForTest(0x01000C16u, fakePhysics);
+        Assert.False(cache.IsPhantomGfxObjSource(0x01000C16u));
+    }
+}