mutation-testing

# 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:**

1. **Analyze** codebase to identify mutation targets
2. **Generate** mutants (deliberate code changes)
3. **Run** test suite against each mutant
4. **Report** mutation score and surviving mutants
5. **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`