diff --git a/src/AcDream.Core/Physics/CellTransit.cs b/src/AcDream.Core/Physics/CellTransit.cs
index bdacceb..6629f94 100644
--- a/src/AcDream.Core/Physics/CellTransit.cs
+++ b/src/AcDream.Core/Physics/CellTransit.cs
@@ -94,22 +94,33 @@ public static class CellTransit
 
             if (portal.OtherCellId == 0xFFFF)
             {
-                // Exit portal. Any path sphere straddling the plane triggers
-                // the outdoor cell expansion.
-                for (int i = 0; i < sphereCount; i++)
-                {
-                    var sphere = worldSpheres[i];
-                    float rad = sphere.Radius + EPSILON;
-                    var localCenter = Vector3.Transform(
-                        sphere.Origin, currentCell.InverseWorldTransform);
-                    float dist = Vector3.Dot(localCenter, poly.Plane.Normal) + poly.Plane.D;
-                    bool hit = dist > -rad && dist < rad;
-                    if (hit)
-                    {
-                        exitOutside = true;
-                        break;
-                    }
-                }
+                // A6.P5 (2026-05-25): exit portals add outdoor cells
+                // UNCONDITIONALLY, by topology — not by sphere-plane overlap.
+                //
+                // Retail's CObjCell::find_cell_list (acclient_2013_pseudo_c.txt
+                // :308742-308869) walks vtable[0x80] on every cell already in
+                // the array and adds reachable cells without testing the
+                // sphere against each portal plane. The straddle check we
+                // had here gated outdoor inclusion on the sphere physically
+                // overlapping the EXIT portal — which fails to fire when:
+                //   a) the sphere is in a SIBLING indoor cell that BFS-
+                //      expanded to this one (sphere is geographically near
+                //      the doorway region, just not at THIS cell's exit
+                //      portal plane); OR
+                //   b) the per-tick target moves the sphere across the
+                //      portal plane on one tick but not the next, producing
+                //      intermittent visibility from the same position.
+                //
+                // Pre-fix bug: cottage doors at outdoor cells were invisible
+                // from indoor cells during cell-crossing substeps (live
+                // capture 2026-05-25; over-penetration test in
+                // CellTransitTests.A6P5_BuildCellSetFromIndoorStart_...).
+                //
+                // Post-fix: any cell visited by BFS that has at least one
+                // exit portal contributes exitOutside=true regardless of
+                // sphere position. AddAllOutsideCells fires once per BFS
+                // (deduped in BuildCellSetAndPickContaining).
+                exitOutside = true;
                 continue;
             }
 
@@ -441,6 +452,7 @@ public static class CellTransit
             pending.Enqueue(currentCellId);
             visited.Add(currentCellId);
             int maxIterations = 16;  // hard cap; portal graphs are small
+            bool outdoorAdded = false;
             while (pending.Count > 0 && maxIterations-- > 0)
             {
                 uint cellId = pending.Dequeue();
@@ -461,10 +473,18 @@ public static class CellTransit
                     }
                 }
 
-                if (exitOutside)
+                // A6.P5 (2026-05-25): any BFS-visited cell with an exit
+                // portal triggers the outdoor-neighbourhood add — matches
+                // retail's CObjCell::find_cell_list at
+                // acclient_2013_pseudo_c.txt:308742-308869 which expands
+                // portal-reachable cells unconditionally via vtable[0x80].
+                // Dedupe to once per BFS — the radial pattern depends only
+                // on the seed cell + sphere XY, so repeated calls would
+                // be no-ops with extra HashSet overhead.
+                if (exitOutside && !outdoorAdded)
                 {
-                    // Add neighbour outdoor cells too.
                     AddAllOutsideCells(worldSpheres, sphereCount, currentCellId, candidates);
+                    outdoorAdded = true;
                 }
             }
         }