Mutation Testing

This evaluates how well your tests actually catch bugs by intentionally breaking your code and checking if tests fail. The skill covers Stryker for JavaScript/TypeScript, PITest for Java, and mutmut for Python. It's most useful when you need confidence that critical business logic is truly tested, not just executed. The guides walk through setup and interpreting mutation scores, which measure the percentage of code changes your tests detect. Honest take: mutation testing is slow and you won't run it on everything, but it's excellent for finding gaps in tests that otherwise show 100% line coverage. The best practice advice here is solid, especially about targeting important code and aiming for 80% mutation score rather than chasing perfection.