diff --git a/CLAUDE.md b/CLAUDE.md
index b4afb2c..086758d 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -554,3 +554,5 @@ field being typed in and reset the scroll position.
 ## Commit rules
 One changeset = one commit, single-line imperative message, **no AI attribution
 of any kind** (global rule). `git push origin main` pushes to both remotes.
+
+- Server code must not import `../frontend/...` directly: the Docker image copies frontend/ to `server/public/`. Use the existsSync fallback pattern (see piano.js / direct-relay.js).