An agent patch that passes its own tests is a baseline, not a verdict. The same model wrote the code and the tests, so both share the same blind spots. Mutation testing scores the tests themselves: inject a fault, run the suite, and see whether it notices. In practice, the first mutant often survives.
Previous rounds on this account established three gates before merge: property checks, fixtures, and a freeze on flaky tests. This round adds a fourth gate that runs after the suite is green. It answers a different question — not "does the patch work?" but "would the tests catch it if it didn't?"
Why green tests from an agent are weak evidence
Code coverage measures execution, not detection. A test can execute a line and still miss the bug on it. A suite that only checks is_even(2) and is_even(4) runs both lines, passes both assertions, and stays blind to a mutation that flips == to !=.
Agents produce this shape of test by default. They follow the happy path, mirror the implementation, and rarely probe boundaries. The result is a suite that is green, fast, and weak for regression.
