Mutation Testing Skill | Agent Skills

Mutation Testing

Use PIT mutation testing to identify tests that pass but don't verify behavior correctly.

When to use this skill

Improving test quality beyond line coverage metrics
Finding weak test assertions that don't catch bugs
Validating critical business logic has strong tests
Setting up mutation testing in CI pipelines
Analyzing which tests need strengthening
When asked to "improve test quality with mutation testing"

Skill Contents

Available Resources

📚 references/ - Detailed documentation

Quick Start

1. Check readiness

bash java/scripts/check-pitest-readiness.sh

2. Apply PIT configuration

Copy java/templates/pitest.gradle to gradle/pitest.gradle and apply in build.gradle.

3. Run mutation testing

# QUICK level - fast feedback
./gradlew pitest -Ppitest.mutators=DEFAULTS

# STANDARD level - recommended
./gradlew pitest -Ppitest.mutators=STRONGER

# COMPREHENSIVE level - thorough
./gradlew pitest -Ppitest.mutators=ALL

4. Analyze results

open build/reports/pitest/index.html

Focus on SURVIVED mutants - these indicate weak tests.

5. Improve tests

Add specific assertions that kill the surviving mutants.

Key Concepts

Mutation Score vs Line Coverage

Line coverage: Does your test execute the code?
Mutation score: Does your test actually verify the code works correctly?

High line coverage with low mutation score = tests run code but don't verify results.

Intensity Levels

| Level | Mutators | Use Case | Time | |-------|----------|----------|------| | QUICK | DEFAULTS (~11) | Initial analysis, CI | 1x | | STANDARD | STRONGER (~14) | Regular improvement | 1.5x | | COMPREHENSIVE | ALL (~30+) | Critical logic | 3-5x |

Mutation Statuses

| Status | Meaning | Action | |--------|---------|--------| | Killed | Test detected mutation | Good | | Survived | Test missed mutation | Improve test | | No Coverage | No test covers code | Write test | | Timed Out | Infinite loop | Usually OK |

References

| Reference | Description | |-----------|-------------| | references/intensity-levels.md | Detailed mutator descriptions | | references/improving-tests.md | Patterns for killing mutants | | references/configuration.md | PIT configuration options | | references/troubleshooting.md | Common issues and solutions |

Related Rules

java-testing-guidelines - Spock/JUnit patterns
java-jacoco-coverage - Line coverage configuration

Related Skills

| Skill | Purpose | |-------|---------| | java-testing | Test configuration | | java-coverage | JaCoCo coverage |

Agent Skills: Mutation Testing

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