diff --git a/src/AcDream.Core/Physics/PhysicsEngine.cs b/src/AcDream.Core/Physics/PhysicsEngine.cs
index 984d5a8..29cdf9f 100644
--- a/src/AcDream.Core/Physics/PhysicsEngine.cs
+++ b/src/AcDream.Core/Physics/PhysicsEngine.cs
@@ -107,13 +107,20 @@ public sealed class PhysicsEngine
         float targetZ;
         uint targetCellId;
 
-        bool currentlyIndoor = cellId >= 0x0100;
+        // Only the low 16 bits of cellId carry the cell index. Outdoor
+        // cells are 0x0001–0x0040; indoor (EnvCell) cells are 0x0100+.
+        // The full 32-bit cellId includes the landblock prefix in the
+        // high 16 bits (e.g., 0xA9B40001), so we MUST mask before
+        // comparing. Without the mask, every cell looks "indoor" because
+        // 0xA9B40001 >= 0x0100 → the engine always takes the "stay
+        // indoors" path and snaps Z to an EnvCell floor 28m below.
+        bool currentlyIndoor = (cellId & 0xFFFFu) >= 0x0100;
 
         if (currentlyIndoor && bestCellZ is not null)
         {
             // Stay indoors on the best cell's floor.
             targetZ = bestCellZ.Value;
-            targetCellId = bestCell!.CellId;
+            targetCellId = bestCell!.CellId & 0xFFFFu;
         }
         else if (currentlyIndoor && bestCellZ is null)
         {
@@ -126,7 +133,7 @@ public sealed class PhysicsEngine
         {
             // Walked into an indoor cell from outdoor — transition to indoor.
             targetZ = bestCellZ.Value;
-            targetCellId = bestCell!.CellId;
+            targetCellId = bestCell!.CellId & 0xFFFFu;
         }
         else
         {