Mutation Testing Quality Gate

Use Stryker Mutator to verify that tests actually catch breakage — not just that they pass. Often used after TDD (e.g. in Ralph’s quality gate).

Principle #10 (Agents drift, gates don't) — mutation testing closes the gap CRAP can't see: 100% coverage with weak assertions still loses a 95% mutation score. Pair the two. Suppressing a surviving mutant by .skip-ing the test is the most common gate-bypass — refuse it. See /principles.

What It Does

Stryker modifies your source code (mutates it) and checks if your tests catch the change. If a mutation survives (tests still pass), there's a gap in your test coverage.

Examples of mutations:

• > 0 → >= 0 (boundary condition)
• a + b → a - b (arithmetic operator)
• if (condition) → if (true) (removed conditional)
• return value → return "" (string literal)
• { ...body } → {} (removed block)

If your tests don't catch these, they're not testing real behavior.