diff --git a/src/AcDream.Core/Physics/CellTransit.cs b/src/AcDream.Core/Physics/CellTransit.cs
index bd62333..c7d9cf0 100644
--- a/src/AcDream.Core/Physics/CellTransit.cs
+++ b/src/AcDream.Core/Physics/CellTransit.cs
@@ -155,25 +155,19 @@ public static class CellTransit
     /// <c>BuildingObj::find_building_transit_cells</c> +
     /// <c>EnvCell::check_building_transit</c>. For each portal of the
     /// outdoor building, look up the destination interior cell and test
-    /// whether the sphere center is inside it via
-    /// <see cref="BSPQuery.PointInsideCellBsp"/>. If so, add the interior
-    /// cell to <paramref name="candidates"/>.
+    /// whether the sphere overlaps it via
+    /// <see cref="BSPQuery.SphereIntersectsCellBsp"/>. If so, add the
+    /// interior cell to <paramref name="candidates"/>.
     ///
     /// <para>
-    /// <b>Retail divergence:</b> retail's <c>check_building_transit</c>
-    /// uses <c>CCellStruct::sphere_intersects_cell</c> (radius-aware
-    /// BSP-vs-sphere test) which fires the moment ANY part of the sphere
-    /// overlaps the destination cell. Our port uses
-    /// <see cref="BSPQuery.PointInsideCellBsp"/> (radius-less, tests only
-    /// the sphere CENTER). Practical effect: entry into a building fires
-    /// when the player's foot-sphere center crosses the destination cell
-    /// boundary — roughly <paramref name="sphereRadius"/> (~0.48m) DEEPER
-    /// into the doorway than retail. If visual verification at the cottage
-    /// door shows a noticeable "late entry" effect (player visually inside
-    /// the building before walls switch from outdoor-stab to indoor-cell),
-    /// port <c>sphere_intersects_cell</c> in a follow-up.
-    /// <paramref name="sphereRadius"/> is plumbed through for that future
-    /// upgrade; currently unused.
+    /// Issue #89 closed (2026-05-20): uses retail's radius-aware
+    /// <c>CCellStruct::sphere_intersects_cell</c>
+    /// (<c>acclient_2013_pseudo_c.txt:317666</c>) ported as
+    /// <see cref="BSPQuery.SphereIntersectsCellBsp"/>. Promotes CellId to
+    /// the interior cell the moment ANY part of the foot-sphere crosses
+    /// the cell boundary — matches retail entry timing exactly and
+    /// closes the login-inside-inn classification race where the player
+    /// would briefly be classified outdoor and walk through walls.
     /// </para>
     /// </summary>
     public static void CheckBuildingTransit(
@@ -198,13 +192,22 @@ public static class CellTransit
             }
 
             // Sphere center in the OTHER cell's local space.
+            // Issue #89 closed (2026-05-20): use radius-aware sphere-overlap
+            // (matches retail's CCellStruct::sphere_intersects_cell at
+            // acclient_2013_pseudo_c.txt:317666) instead of point-only. This
+            // promotes the player's CellId to the interior cell the moment
+            // ANY part of the foot-sphere crosses the cell boundary — the
+            // entry-side counterpart to issue #90's sticky-stay fix. Without
+            // it, login-inside-the-inn keeps the player classified outdoor
+            // until they walk further in (sphere center crosses), letting
+            // them run through exterior walls on the way out.
             var localCenter = Vector3.Transform(worldSphereCenter, otherCell.InverseWorldTransform);
-            bool inside = BSPQuery.PointInsideCellBsp(otherCell.CellBSP.Root, localCenter);
+            bool inside = BSPQuery.SphereIntersectsCellBsp(otherCell.CellBSP.Root, localCenter, sphereRadius);
 
             if (PhysicsDiagnostics.ProbeIndoorBspEnabled)
             {
                 Console.WriteLine(System.FormattableString.Invariant(
-                    $"[check-bldg] portal->0x{portal.OtherCellId:X8} wpos=({worldSphereCenter.X:F3},{worldSphereCenter.Y:F3},{worldSphereCenter.Z:F3}) lpos=({localCenter.X:F3},{localCenter.Y:F3},{localCenter.Z:F3}) inside={inside}"));
+                    $"[check-bldg] portal->0x{portal.OtherCellId:X8} wpos=({worldSphereCenter.X:F3},{worldSphereCenter.Y:F3},{worldSphereCenter.Z:F3}) lpos=({localCenter.X:F3},{localCenter.Y:F3},{localCenter.Z:F3}) r={sphereRadius:F3} inside={inside}"));
             }
 
             if (inside)