test-mutation — Coverage proves the test ran; mutants prove it would notice

Degree of freedom: MIXED — scope and triage [HIGH freedom]; first run and CI floor [LOW freedom — measure, then ratchet]. Never start with the whole repo.

Install and run the gate coverage cannot close. Coverage proves the test ran the code; mutation testing proves the test would notice if the code were wrong. The tool makes hundreds of small deliberate bugs (mutants) — flip > to >=, delete a statement, replace a return — and reruns the tests. Survivors mark code the tests execute but verify nothing.

This is the specific counter to agent-generated suites: an agent can trivially produce 90% coverage with no meaningful assertions, and every coverage ratchet will wave them through. Mutants do not care about coverage theater.

This skill vs neighbors