diff --git a/CLAUDE.md b/CLAUDE.md
index bc477e1..bea14fd 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -181,6 +181,46 @@ and the deferral compounds across families. Skipping a tidy-iter
 requires an explicit JOURNAL entry naming the reason (e.g. a
 sibling family is blocking, or the user has asked to defer).
 
+### Performance regressions
+
+`bench/check.py` is the second non-optional element of the
+tidy-iter, alongside the architect drift report. Run it at every
+family close. The exit code is the gate:
+
+- **Exit 0 (green).** All metrics within their per-metric
+  tolerance vs. `bench/baseline.json`. Tidy-iter can close.
+- **Exit 1 (red).** At least one metric regressed beyond
+  tolerance. Treat the regression like an architect-drift item:
+  fix (revert the offending iter, refactor the hot path, diagnose
+  with `ailang-bencher` or `ailang-debugger`), or ratify
+  (`bench/check.py --update-baseline` together with a JOURNAL
+  entry naming the iter that intentionally moved the metric and
+  why).
+- **Exit 2 (parser misalignment).** The bench-output format
+  changed under check.py's parsers. Update parsers or baseline
+  schema before re-running. Never claim a regression on exit-2.
+
+Skipping the bench-check at a family boundary requires the same
+explicit JOURNAL entry as skipping the architect drift review:
+name the reason, name the sibling family that's blocking, name
+when it'll be re-run.
+
+A bug-fix iter that adds a new bench fixture (e.g. proving the
+fix's invariant under load) bumps `bench/baseline.json` as part
+of the same commit. The new fixture's numbers are baseline-zero
+on first capture; they become regression-relevant from the next
+tidy-iter onward.
+
+Two non-goals to keep clear: (1) per-metric tolerances are tuned
+for run-to-run noise on a quiet developer machine, NOT the
+language's correctness bar — Decision-10 thresholds (rc/bump ≤
+1.3× / p99/median ≤ 5×) live in DESIGN.md and are evaluated
+against absolute numbers, separate from the regression check.
+(2) Improvements (metric beat its baseline by more than tolerance)
+do NOT auto-update the baseline; they are surfaced in the report
+so the orchestrator can ratify them via `--update-baseline` with
+a paired JOURNAL entry.
+
 ### Roles of JOURNAL.md and DESIGN.md
 
 - **DESIGN.md** is the canonical specification. It describes what