diff --git a/CLAUDE.md b/CLAUDE.md
index 2dc8f45..2e8df1d 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -159,6 +159,24 @@ these, ideally all four:
   atlases, shader patterns. Most useful for "how do I do this GL thing
   with Silk.NET on net10 idiomatically?" Less useful for protocol or
   character appearance (dat editor, not game client).
+- **`references/WorldBuilder-ACME-Edition/`** — **Fork of WorldBuilder
+  with decompiled AC client source code.** THE most authoritative
+  reference for any terrain/rendering algorithm. Key files:
+    - `WorldBuilder.Tests/ClientReference.cs` — faithful C# port of
+      decompiled `CLandBlockStruct.cpp` with exact offsets. Ground truth
+      for split direction, palCode, vertex height, vertex position.
+      **Check this BEFORE implementing any terrain algorithm.**
+    - `WorldBuilder.Tests/TerrainConformanceTests.cs` — exhaustive
+      sweeps (4M+ cells) proving ACME matches the retail client.
+    - `StaticObjectManager.cs` — complete GfxObj+Setup rendering with
+      CreaturePalette assembly (the ObjDesc pipeline).
+    - `EnvCellManager.cs` — dungeon cell rendering with portal
+      visibility traversal and collision mesh caching.
+    - `TerrainGeometryGenerator.cs` — `GetHeight()` and `GetNormal()`
+      that match the mesh index buffer exactly.
+  Already proved its value: solved the triangle-boundary Z bug that
+  resisted 5 other fix attempts. Use as the oracle for ALL dat
+  interpretation algorithms.
 - **`references/Chorizite.ACProtocol/`** — clean-room C# protocol
   library generated from a protocol XML description. Useful sanity check
   on field order, packed-dword conventions, type-prefix handling. The