diff --git a/src/AcDream.App/Rendering/GameWindow.cs b/src/AcDream.App/Rendering/GameWindow.cs
index 6227452..e148d74 100644
--- a/src/AcDream.App/Rendering/GameWindow.cs
+++ b/src/AcDream.App/Rendering/GameWindow.cs
@@ -6100,6 +6100,17 @@ public sealed class GameWindow : IDisposable
                 }
             }
 
+            // Issue #101 (2026-05-25): retail-faithful phantom check for
+            // GfxObj-only entity sources. Retail's CPartArray::InitParts
+            // emits NO collision shapes when the source GfxObj has
+            // HasPhysics=False / null PhysicsBSP.Root. Our mesh-aabb-fallback
+            // below previously synthesized a clamped [0.30, 1.50]m cylinder
+            // from the visual mesh AABB — that's the source of the 10
+            // phantom 0.80m cyls that block the Holtburg upper-floor
+            // staircase (issue #101). Suppress synthesis for these.
+            // Spec: docs/research/2026-05-25-a6-stairs-cyl-retail-investigation.md.
+            bool isPhantomGfxObj = _physicsDataCache.IsPhantomGfxObjSource(entity.SourceGfxObjOrSetupId);
+
             // VISUAL mesh-bounds collision: for SCENERY entities (IDs with
             // 0x80000000 bit set, indicating procedurally-placed scenery),
             // ALWAYS compute a cylinder from the world-space mesh AABB.
@@ -6114,6 +6125,7 @@ public sealed class GameWindow : IDisposable
             // L-fix3: skip entirely when the Setup is phantom — retail
             // decorative meshes have no collision data on purpose.
             if (!isPhantomSetup
+                && !isPhantomGfxObj
                 && !_isLandblockStab
                 && (_isOutdoorMesh || (entityBsp == 0 && entityCyl == 0))
                 && entity.MeshRefs.Count > 0)
diff --git a/src/AcDream.Core/Physics/PhysicsDataCache.cs b/src/AcDream.Core/Physics/PhysicsDataCache.cs
index fbe8a2d..c7b65a5 100644
--- a/src/AcDream.Core/Physics/PhysicsDataCache.cs
+++ b/src/AcDream.Core/Physics/PhysicsDataCache.cs
@@ -370,7 +370,7 @@ public sealed class PhysicsDataCache
     /// 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.
+    /// <c>GameWindow.cs:6127</c> must do the same.
     /// </summary>
     public bool IsPhantomGfxObjSource(uint sourceId)
     {