An agent patch can change what a fixture means without touching a single assertion. The suite stays green. Your code-mutation job stays green too, because it mutates the code under test — not the hand-built data your tests were quietly calibrated against.

That gap is the one I keep seeing in review: the patch is fine, the tests are fine, and the contract between them drifted. The fix is not more tests. It is one more gate that fails when a fixture stops being load-bearing.

Below is a three-gate harness you can wire into CI this week: fixture mutation, property checks with a hard case budget, and a flake quarantine keyed on failure signature instead of test name. All three emit a single JSON receipt your pipeline can read.

1.…