# Mutation Testing

> Evaluate test suite quality by introducing code mutations and verifying tests catch them. Use for mutation testing, test quality, mutant detection, Stryker, PITest, and test effectiveness analysis.

- Skill: `aj-geddes-useful-ai-prompts/mutation-testing` (Agent Skill, multi-file: 7 files)
- Install (CLI): `npx skillmds@latest add aj-geddes-useful-ai-prompts/mutation-testing`
- Raw SKILL.md: https://api.skillmd.com/api/skills/aj-geddes-useful-ai-prompts/mutation-testing/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: aj-geddes (https://skillmd.com/u/aj-geddes-useful-ai-prompts)
- Updated: 2026-09-09
- Page: https://skillmd.com/skills/aj-geddes-useful-ai-prompts/mutation-testing

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# Mutation Testing

## Table of Contents

- [Overview](#overview)
- [When to Use](#when-to-use)
- [Quick Start](#quick-start)
- [Reference Guides](#reference-guides)
- [Best Practices](#best-practices)

## Overview

Mutation testing assesses test suite quality by introducing small changes (mutations) to source code and verifying that tests fail. If tests don't catch a mutation, it indicates gaps in test coverage or test quality. This technique helps identify weak or ineffective tests.

## When to Use

- Evaluating test suite effectiveness
- Finding untested code paths
- Improving test quality metrics
- Validating critical business logic is well-tested
- Identifying redundant or weak tests
- Measuring real test coverage beyond line coverage
- Ensuring tests actually verify behavior

## Quick Start

Minimal working example:

```bash
# Install Stryker
npm install --save-dev @stryker-mutator/core @stryker-mutator/jest-runner

# Initialize configuration
npx stryker init

# Run mutation testing
npx stryker run
```

## Reference Guides

Detailed implementations in the `references/` directory:

| Guide | Contents |
|---|---|
| [Stryker for JavaScript/TypeScript](references/stryker-for-javascripttypescript.md) | Stryker for JavaScript/TypeScript |
| [PITest for Java](references/pitest-for-java.md) | PITest for Java |
| [mutmut for Python](references/mutmut-for-python.md) | mutmut for Python |
| [Mutation Testing Reports](references/mutation-testing-reports.md) | Mutation Testing Reports |

## Best Practices

### ✅ DO

- Target critical business logic for mutation testing
- Aim for 80%+ mutation score on important code
- Review survived mutants to improve tests
- Mark equivalent mutants to exclude them
- Use mutation testing in CI for critical modules
- Test boundary conditions thoroughly
- Verify actual behavior, not just code execution

### ❌ DON'T

- Expect 100% mutation score everywhere
- Run mutation testing on all code (too slow)
- Ignore equivalent mutants
- Test getters/setters with mutations
- Run mutations on generated code
- Skip mutation testing on complex logic
- Focus only on line coverage