test-qualitylisted

# Test quality A test earns its place by failing: one that cannot name the production change that would break it pins nothing. ## The pre-write gate Before writing a test, answer in one sentence: **what production change should make this fail?** No answer means there is no test to write yet — the behavior is not pinned down; go read or draft the contract. An answer as vague as "if the code is wrong" means the assertion is not specific enough to be worth making. ## The mutation check For a test that already exists, break the production code on purpose and confirm the test goes red. Five mutations, cheapest first: | Mutation | Applied to | | --- | --- | | wrong constant | a returned or compared literal — off by one, or negated | | wrong branch | flip a condition, or swap the arms | | missing side effect | delete the write, emit, or call the test claims to check | | empty return | return the empty value instead of the real one | | missing validation | delete a guard, then feed it the input that guard rejects | A mutation the suite survives is a hole: fix the test, then revert the mutation. Apply this to the specific code the test names — a whole-file mutation run is a different and much slower tool. **On a defect, the defect is the mutation.** The test goes red against the unfixed production code, and red again when the fix is reverted. That pair is what "red observed" means; a suite inherited green earns the same check before anyone trusts it. Coverage counts lines exe