Testing
Mutation Testing
Use this skill whenever mutation testing is relevant. Triggers: "mutation testing", "mutation score", "mutant", "mutants", "PIT", "pitest", "cargo-mutants", "Stryker", "stryker-mutator", "mutmut", "mutation operators", "killed mutant", "survived mutant", "equivalent mutant", "test quality", "test effectiveness", "test the tests", "mutation analysis", "are my tests good enough", "is my coverage meaningful", "do my tests actually catch bugs". Also trigger when verifying test thoroughness, setting up CI quality gates for test effectiveness, or discussing why 100% code coverage is insufficient. Covers theory, all major frameworks, result interpretation, and the critical insight that survived mutants almost always mean "improve your tests" not "fix your code". Always consult before doing mutation testing work — it prevents the most common agent mistake of treating survived mutants like regular test failures.