diff --git a/src/AcDream.App/Rendering/GameWindow.cs b/src/AcDream.App/Rendering/GameWindow.cs
index c2aae70..7c53a03 100644
--- a/src/AcDream.App/Rendering/GameWindow.cs
+++ b/src/AcDream.App/Rendering/GameWindow.cs
@@ -111,7 +111,9 @@ public sealed class GameWindow : IDisposable
     // LandblockMesh.Build without re-deriving these each time.
     private float[]? _heightTable;
     private AcDream.Core.Terrain.TerrainBlendingContext? _blendCtx;
-    private Dictionary<uint, AcDream.Core.Terrain.SurfaceInfo>? _surfaceCache;
+    // Was: Dictionary<uint, SurfaceInfo>. ConcurrentDictionary so the off-thread
+    // mesh builder (A.5 T11+) can call LandblockMesh.Build without a lock.
+    private System.Collections.Concurrent.ConcurrentDictionary<uint, AcDream.Core.Terrain.SurfaceInfo>? _surfaceCache;
 
     // Phase A.1 Task 8: worker thread pre-builds EnvCell room-mesh sub-meshes
     // (CPU only) and stores them here. ApplyLoadedTerrain (render thread) drains
@@ -1465,7 +1467,7 @@ public sealed class GameWindow : IDisposable
             RoadAlphaRCodes:   terrainAtlas.RoadAlphaRCodes);
 
         _heightTable = heightTable;
-        _surfaceCache = new Dictionary<uint, AcDream.Core.Terrain.SurfaceInfo>();
+        _surfaceCache = new System.Collections.Concurrent.ConcurrentDictionary<uint, AcDream.Core.Terrain.SurfaceInfo>();
 
         // (Bindless detection moved above — must precede TerrainAtlas.Build /
         // TerrainModernRenderer ctor so they can consume BindlessSupport.)
diff --git a/src/AcDream.Core/Terrain/LandblockMesh.cs b/src/AcDream.Core/Terrain/LandblockMesh.cs
index 573acf5..81e6724 100644
--- a/src/AcDream.Core/Terrain/LandblockMesh.cs
+++ b/src/AcDream.Core/Terrain/LandblockMesh.cs
@@ -46,7 +46,7 @@ public static class LandblockMesh
         uint landblockY,
         float[] heightTable,
         TerrainBlendingContext ctx,
-        Dictionary<uint, SurfaceInfo> surfaceCache)
+        System.Collections.Generic.IDictionary<uint, SurfaceInfo> surfaceCache)
     {
         ArgumentNullException.ThrowIfNull(block);
         ArgumentNullException.ThrowIfNull(heightTable);
@@ -105,6 +105,10 @@ public static class LandblockMesh
                 uint palCode = TerrainBlending.GetPalCode(
                     rBL, rBR, rTR, rTL, ttBL, ttBR, ttTR, ttTL);
 
+                // Lookup-or-build pattern. Not atomic under concurrent access
+                // (TryGetValue then assign), but BuildSurface is deterministic —
+                // two workers building the same palCode produce equal SurfaceInfo,
+                // last-write-wins is benign.
                 if (!surfaceCache.TryGetValue(palCode, out var surf))
                 {
                     surf = TerrainBlending.BuildSurface(palCode, ctx);