diff --git a/CLAUDE.md b/CLAUDE.md
index e0fd293cd..a31fcf386 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -60,6 +60,7 @@ Operational knowledge lives in `.okf/` (`build/`, `content/`, `design/`, `workfl
 
 - **TDD**: RED → GREEN → REFACTOR. Doctrine: `docs/20-29-testing-qa/` + `docs/incidents/25.0x`.
 - **Behavior-focused tests ONLY** — reject implementation/existence/config tests. Don't hardcode tunables (`q=90`, exact sizes); assert the shape (`q=\d+`, has `<picture>`). A test that breaks on a knob change is testing config — relax it.
+- **Fault-inject BOTH directions, or the gate is half-tested (BLOCKING, extended 2026-08-22).** Plant the defect and watch it FAIL, then plant a legitimate lookalike and watch it PASS. The second half was missing and cost four false positives in one day: a marker flagged our own real `jt_site` case study as fabrication, an AI-tell rule fired on "anyone on **the team** could remember", a phrase marker flagged a quoted exhibit inside a ```diff fence as our own claim, and a single-line `<img>` grep reported a working diagram as missing because Hugo wraps the tag. Each time the instrument's verdict was trusted and correct work was nearly changed. **When a gate accuses something, reproduce the accusation by hand before believing it.**
 - **A new test isn't done until you've BROKEN THE CODE and watched it fail (BLOCKING).** Green proves nothing on its own — a test that passes because it asserts nothing passes forever. So: inject the exact defect the test exists to catch, quote the failure message in your handback, then revert and re-run green. Both halves are the evidence. Where a gate can't discriminate yet, say so in the test's own comment rather than pretending. Measured 2026-08-22 (`docs/20-29-testing-qa/20.11`): fault-injecting 8 realistic defects caught only 3 — the testimonial gate passed a *vanished* blockquote (`""` is a substring of everything), the rendered ratchet carried 3 hits of slack that swallowed a planted banned phrase, and the link job excluded 90% of links while reporting green for a year.
 - **Framework**: Minitest (`test/system/`, `test/unit/`). Test runner matrix: the header **Test** line — don't restate it.
 - **Visual regression + gates**: full matrix, tolerances, baseline procedure → `.okf/build/test-gates.md` (canonical). Tolerance 0.0 for refactoring, ≤0.03 for new features.