llms.txt@docs git:20261013.052e74cA

Immutable. This exact content never changes and is served at /api/v1/blob/1a08a773f4c59d24.

# Mutineer

> Clean-room mutation testing for Ruby. Mutineer mutates your source one change at a
> time, runs your test suite (Minitest or RSpec) against each mutant, and reports the
> mutants your tests failed to catch — the gaps where green coverage is a lie. Prism +
> stdlib only, zero runtime dependencies (Ruby ≥ 3.4), with a versioned JSON contract
> built for CI gates and AI coding agents.

Line coverage tells you which code *ran* under test; it says nothing about whether your
tests would *notice if that code broke*. Mutation testing closes that gap — exactly where
AI-generated code and tests are weakest (tests that execute and pass but assert nothing).

Key facts:
- **Clean-room**: written from scratch, with no code from Mutant or any other mutation-testing tool. MIT for every use. How it differs from Mutant: [README](https://github.com/davidteren/mutineer#mutineer-and-mutant).
- **Zero runtime dependencies** — Prism + stdlib only. No `eval`. Test-framework changes stay in disposable child processes.
- **One mutation per mutant**, validity-checked by re-parsing; 20 mutation operators.
- **Fork-isolated, parallel** execution (Linux + macOS); coverage-guided per-line test selection.
- **Programmatic**: versioned JSON contract, structured exit codes (0 pass / 1 below threshold, incomplete run, baseline regression or runtime error / 2 misinvocation), `--since` diff-scoped runs, `--threshold` hard gate, `--baseline` delta gating.
- **Rails**: `--daemon` boots the app once and forks per mutant with per-worker DB isolation (SQLite test databases) so `--jobs N` is parallel-safe.
- **Production readiness**: [Is it production-ready?](https://github.com/davidteren/mutineer#is-it-production-ready) has the support matrix. The docs home page shows the same section.
- Distributed as a RubyGem and a GitHub Action (composite `action.yml`).

## Docs

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- [Overview, operators, and usage](https://davidteren.github.io/mutineer/): what Mutineer is, the mutation operators, install, and CLI flags.
- [For AI agents & CI](https://davidteren.github.io/mutineer/agentic-coding.html): the agent inner loop, CI regression gating, the GitHub Action, and equivalent-mutant suppression.
- [Equivalent skips](https://davidteren.github.io/mutineer/equivalent-skips.html): built-in chain-link names and endless ranges Mutineer does not mutate, and why each is equivalent.
- [Rails rollout](https://davidteren.github.io/mutineer/rails.html): four stages from one Rails model to a CI gate, plus GitHub Actions and GitLab CI recipes.
- [JSON report schema](https://davidteren.github.io/mutineer/json-schema.html): the versioned JSON contract — `summary`, `survivors[]`, `no_coverage[]`, `uncapturable[]`, `unplaceable[]`, `ran_at_load[]`, `no_verdict[]`, `ignored[]`, `per_source[]`, and the `baseline` and `matrix` blocks.
- [Ruby API (YARD)](https://davidteren.github.io/mutineer/api/): class reference generated from this checkout — tracks the shipped gem.
- [Sample HTML report](https://davidteren.github.io/mutineer/sample-report.html): example report data displayed in the website theme; the CLI’s self-contained HTML output has a different presentation.
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## Source & packages

- [GitHub repository](https://github.com/davidteren/mutineer): source, issues, `AGENTS.md`, and `CHANGELOG.md`.
- [RubyGems package](https://rubygems.org/gems/mutineer): `gem install mutineer`.
- [GitHub Marketplace action](https://github.com/marketplace/actions/mutineer-ruby): drop-in PR mutation gate.

## Optional

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- [Full docs as one file](https://davidteren.github.io/mutineer/llms-full.txt): README + agent/CI guide + JSON schema concatenated for LLM ingestion.
- [Agent skill](https://davidteren.github.io/mutineer/skill.md): compact skill card — install, the agent loop, exit codes, and links.
- [Landing page (Markdown)](https://davidteren.github.io/mutineer/index.md): homepage / CLI essentials twin of the HTML landing page.
- [Agent & CI guide (Markdown)](https://davidteren.github.io/mutineer/agentic-coding.md): Markdown twin of the agent inner-loop and CI recipes.
- [Equivalent skips (Markdown)](https://davidteren.github.io/mutineer/equivalent-skips.md): Markdown twin of the built-in equivalent-skip list.
- [Rails rollout (Markdown)](https://davidteren.github.io/mutineer/rails.md): Markdown twin of the Rails rollout guide.
- [JSON schema (Markdown)](https://davidteren.github.io/mutineer/json-schema.md): Markdown twin of the JSON report schema reference.
- [Agent access policy](https://davidteren.github.io/mutineer/agents.txt): read-only access policy and pointers at `llms.txt` / `sitemap.xml`.
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