Mutation Score - CI/CD Glossary Definition

A mutation score is the percentage of injected code mutations (deliberate bugs) that a test suite detects, measuring how good the tests are at catching faults rather than just how much code they execute.

How mutation testing works

A tool like Stryker or PIT flips operators (> to >=), removes statements, and changes constants, then reruns the tests. A "killed" mutant means a test failed and caught it; a "survived" mutant means a gap.

Why it beats coverage

Code coverage shows lines were run; mutation score shows they were actually asserted on. High coverage with a low mutation score reveals tests that execute code but verify nothing.

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