# Mutation Testing

> Run mutmut to measure test effectiveness by mutating code

- Skill: `loopyluci/mutation-testing` (Agent Skill)
- Install (CLI): `npx skillmds@latest add loopyluci/mutation-testing`
- Raw SKILL.md: https://api.skillmd.com/api/skills/loopyluci/mutation-testing/raw
- Safety review: pending (external: skill-scanner PASS, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: LoopyLuci (https://skillmd.com/u/loopyluci)
- Updated: 2026-09-21
- Page: https://skillmd.com/skills/loopyluci/mutation-testing

---


# Mutation Testing

## Install
```bash
pip install mutmut
```

## Run
```bash
mutmut run --paths-to-mutate src/
mutmut results
```

## Interpreting Results
- Killed: test caught the mutation (good)
- Survived: test missed it (bad)
- Timeout: mutation caused infinite loop

## Aim For
- >80% mutation score
- Focus on survived mutants first
- Add tests for untested branches