diff --git a/CLAUDE.md b/CLAUDE.md
index 9d0a22e..0e95e7d 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -138,6 +138,10 @@ All shippable work commits **directly to `main`**; `main` equals what is deploye
 
 - Code quality: verify generated code for bugs/edge cases; add debug logging; **ask rather than
   guess** when you'd otherwise invent input/output.
+- **A health check issues no block I/O** — no `statfs`, no `getdents`, no read, write or `fsync`, **not
+  even behind a timeout**. Liveness is decided from `/proc` and kernel state. The full rule + the
+  measurement lives in `felhom.eu/CLAUDE.md` "Code quality rules"; it is repeated here because health
+  checks are written in THIS repo and that file does not load in an agent-only session. R-117 spike §6.3.
 - Update `REUSE.md` if you added/changed/deprecated a shared helper or pattern (same commit).
 - Testing doctrine (non-hollow tests, red-proofs, seams): use the `felhom-testing` skill.
 - **Logging**: the slog logger fans out to journald (configured level) + the always-DEBUG `applog.Ring`