Mutation Testing

Mutation testing evaluates test quality by introducing small, deliberate bugs (mutations) into code and checking if tests catch them. This provides a behavioral measure of test effectiveness beyond simple coverage metrics.

Core Concept

Mutation testing workflow:

1. Generate mutations (small code changes)
2. Run test suite against each mutation
3. Classify results:
   • Killed: Test fails (good - test caught the bug)
   • Survived: Test passes (bad - test missed the bug)
   • Timeout: Test hangs or exceeds time limit
4. Calculate mutation score: killed / (total - timeouts)
5. For survived mutants, generate targeted tests

Why mutation testing matters: Tests can achieve 100% line coverage while missing critical bugs. Mutation testing reveals if tests actually verify behavior or just execute code.

When to Use Mutation Testing