mutation-testing

Mutation testing evaluates test suite quality by introducing small program changes, called mutants, and observing whether existing tests fail to kill them, revealing behavioral gaps where code coverage exists but actual verification is missing.

To run mutation testing in CI for large codebases, apply diff-aware execution and curated operator subsets using tools like PIT or Stryker, enabling selective and incremental mutation runs that scale practically without testing unchanged code.

High code coverage only measures code reach, whereas mutation testing provides the verification signal by introducing mutants to check if tests actually fail, distinguishing between executed code and properly verified behavior.

Survived mutants are handled by generating an actionable list of test gaps to fix, while equivalent mutants, which produce identical behavior, must be identified and excluded from the mutation score computation to ensure accurate test suite verification metrics.

Yes, you can use PIT or Stryker for incremental mutation testing by applying diff-aware strategies that selectively target only the changed code in your CI run, keeping execution practical for large codebases while verifying new behavior.