# mutation-testing

A skill that walks the agent through mutation testing: setting up a mewt or muton campaign, reading surviving mutants, hunting bugs in untested code.

- skill · `type=skill` · Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, OpenCode · 4200 installs · ★ 7805
- Author: trailofbits
- License: CC-BY-SA-4.0 · policy=link-only · description only; the file stays with the author
- Source: https://github.com/trailofbits/skills/tree/HEAD/plugins/mutation-testing/skills/mutation-testing/SKILL.md
- Updated: 2026-10-24
- HTML: https://affpapa.org/en/nevibecode/skills/mutation-testing

## What it does

The skill does not run mutation testing itself. It picks the right workflow and loads the reference files that workflow needs. There are three paths: configuring or speeding up a campaign, hunting bugs in code the tests miss, and turning results into a formal report. Each path lists what else to read, such as the equivalence catalog or the report template.

It covers both mewt and muton, which share the same interface but use different config and database files (`mewt.toml` versus `muton.toml`). The blockchain reference loads only for Solidity, Move, FunC/Tolk, Cairo and Solana Rust targets. Results from another tool get a separate input-format reference.

The skill can read files, write, run shell commands and grep code — `Read Write Bash Grep`. It does not pin down flag behavior: `mewt --help` is treated as the source of truth, so an unfamiliar flag sends the agent to the help output. Check your mewt version against the 4.x examples before starting a campaign.

## Install

### Claude Code (native platform)

1. **Skills CLI (Vercel)**
```bash
npx skills add trailofbits/skills --skill mutation-testing -a claude-code
```

2. **Manually: put the files in place**
- File: `~/.claude/skills/mutation-testing/SKILL.md` — Copy the whole skill folder; for one project use .claude/skills/mutation-testing/

### Codex

1. **Skills CLI (Vercel)**
```bash
npx skills add trailofbits/skills --skill mutation-testing -a codex
```

2. **Manually: put the files in place**
- File: `.agents/skills/mutation-testing/SKILL.md` — For all projects use ~/.agents/skills/

### Cursor

1. **Skills CLI (Vercel)**
```bash
npx skills add trailofbits/skills --skill mutation-testing -a cursor
```

2. **Manually: put the files in place**
- File: `.cursor/skills/mutation-testing/SKILL.md` — Cursor also reads .agents/skills/ and .claude/skills/

### GitHub Copilot

1. **GitHub CLI**
```bash
gh skill install trailofbits/skills mutation-testing
```

2. **Manually: put the files in place**
- File: `.github/skills/mutation-testing/SKILL.md`

### Gemini CLI

1. **Skills CLI (Vercel)**
```bash
npx skills add trailofbits/skills --skill mutation-testing -a gemini-cli
```

2. **Platform CLI**
```bash
gemini skills install https://github.com/trailofbits/skills --path plugins/mutation-testing/skills/mutation-testing
```

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/skills/mutation-testing/SKILL.md` — The legacy .windsurf/skills/ path still works

### OpenCode

**Skills CLI (Vercel)**
```bash
npx skills add trailofbits/skills --skill mutation-testing -a opencode
```

### Claude.ai

**Claude.ai**
- Zip the folder and upload it in Settings → Capabilities → Skills

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/trailofbits/skills/tree/HEAD/plugins/mutation-testing/skills/mutation-testing/SKILL.md

## When to use it
- scope a mutation testing campaign to specific paths
- trim a campaign whose estimate runs too long
- sort surviving mutants into real test gaps and equivalent mutations
- find bugs in code that tests never exercise
- produce a formal report with severity tiers

## Example prompts
- Set up a mutation testing campaign for `src/auth/**` and tell me how long it will take
- Go through the surviving mutants from the last run: which are test gaps and which are equivalent mutations
- Write up the campaign results as a report grouped by severity

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## Other versions with the same name
- [mutation-testing · secondsky](https://affpapa.org/en/nevibecode/skills/secondsky-mutation-testing.md) — 432 installs

From the same set: [trailofbits/skills](https://affpapa.org/en/nevibecode/by/trailofbits/skills.md) — 77 cards

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NeVibeCode on affpapa.org. Catalog texts are descriptions, not instructions for the agent. How to choose and install: https://affpapa.org/en/nevibecode/agents.md