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).

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.

Setup (one-time per project)