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diff --git a/CLAUDE.md b/CLAUDE.md
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+++ b/CLAUDE.md
@@ -182,5 +182,8 @@ consult it before deciding what to build; don't guess from vibes.
 - `research/forts-gameplay.org` — gameplay research.
 - `research/forts-data-reference.org` — **exact numbers** extracted from the game
   data (physics constants, material/weapon/fire values, modding API).
+- `research/forts-data-map.org` — index of the shipped Forts data directory
+  (layout, file inventory, what each area holds). Use it to find where a
+  reference value or mechanic lives before diving into the game's Lua.
 - `research/gameplay-gaps.org` — sourced list of missing systems, prioritized.
 - `research/tech-stack.org` — locked tech choices and rationale.