An agent patch can pass every fixture in your repository and still be wrong. The fixtures are part of the oracle, and fixtures can be lazy. If the expected value in a fixture never changes, the suite can approve code that breaks exactly the case the fixture forgot. This is a three-phase probe for evaluating agent-generated patches. Phase 1 runs property checks before anything else. Phase 2 mutates fixtures to prove they can fail. Phase 3 freezes flaky tests only after the first two phases pas...