Agent skill
mutation-testing
Use PROACTIVELY when checking if tests catch real bugs, assessing test suite quality, finding weak tests, or measuring mutation score. Validates test effectiveness beyond coverage metrics by introducing code mutations. Supports Stryker (JS/TS), PIT (Java), mutmut (Python). Not for projects without existing test suites.
Install this agent skill to your Project
npx add-skill https://github.com/cskiro/claudex/tree/main/plugins/mutation-testing/skills/mutation-testing
SKILL.md
Mutation Testing
Overview
This skill sets up mutation testing to evaluate test suite quality by introducing deliberate code changes (mutants) and verifying tests catch them. A high mutation score indicates tests are effective at catching real bugs.
Version: 0.1.0 Status: Initial Release
What This Skill Does
Mutation Testing Workflow:
- Analyze codebase to identify mutation targets
- Generate mutants (deliberate code changes)
- Run test suite against each mutant
- Report mutation score and surviving mutants
- Recommend test improvements for weak spots
When to Use This Skill
Use when:
- Coverage is high but confidence in tests is low
- Validating test suite effectiveness
- Finding tests that pass but don't catch real bugs
- Preparing for production release
- Improving test quality beyond coverage metrics
Don't use if:
- No existing test suite
- Test coverage below 50% (improve coverage first)
- CI pipeline can't afford extra runtime
Trigger Phrases
- "Set up mutation testing"
- "Are my tests catching bugs?"
- "Check test effectiveness" / "Test quality analysis"
- "Run mutation analysis" / "Mutation score"
- "Find weak tests"
Supported Frameworks
| Language | Framework | Command |
|---|---|---|
| JavaScript/TypeScript | Stryker | npx stryker run |
| Java | PIT | mvn org.pitest:pitest-maven:mutationCoverage |
| Python | mutmut | mutmut run |
Mutation Score
| Score | Interpretation |
|---|---|
| 90%+ | Excellent - tests catch most bugs |
| 75-89% | Good - some gaps to address |
| 50-74% | Fair - significant testing gaps |
| <50% | Poor - tests need major improvement |
Common Mutations
| Type | Example | Tests Should Catch |
|---|---|---|
| Arithmetic | + → - |
Math logic errors |
| Conditional | > → >= |
Boundary conditions |
| Boolean | true → false |
Logic inversions |
| Return | return x → return null |
Null handling |
| Remove call | validate() → removed |
Missing validations |
Installation Summary
For JavaScript/TypeScript (Stryker):
npm install --save-dev @stryker-mutator/core
npx stryker init
For Python (mutmut):
pip install mutmut
mutmut run
Quick Start
- Ensure test suite exists and passes
- Install mutation testing framework
- Configure mutation targets (critical code paths)
- Run mutation analysis
- Review surviving mutants
- Add/improve tests for uncaught mutations
Performance Considerations
- Mutation testing is compute-intensive
- Start with critical modules only
- Use incremental mode for CI
- Cache results between runs
- Consider parallel execution
Success Criteria
- Mutation testing framework installed
- Configuration targets critical code paths
- Initial mutation run completes
- Mutation score reported
- Action plan for surviving mutants
Additional Resources
- Workflow guide:
workflow/mutation-analysis.md - Configuration reference:
reference/stryker-config.md - Example report:
examples/mutation-report.md
Version History
- 0.1.0 - Initial release with Stryker, PIT, mutmut support
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