diff --git a/CLAUDE.md b/CLAUDE.md
index 265929a..81e1f27 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -33,6 +33,13 @@ git push CFDManager notes
 `git clean -xfd` deletes the working copy, since it is untracked — restore it with
 the first command above.
 
+Drift is guarded automatically: a `PostToolUse` hook
+(`.claude/hooks/notes_guard.py`, wired in `.claude/settings.local.json`) compares
+this file against `notes` after every edit and demands a described commit when the
+two diverge — necessary because an untracked file never shows up in `git status`.
+Both live outside git, excluded through `.git/info/exclude` rather than
+`.gitignore`, so no branch has to carry them.
+
 The consequence to keep in mind: **this file describes the whole project, but no
 single branch contains all of it.** Before assuming a directory exists, check
 `git branch --show-current` against the map below — `rust/` and