mutineer

name	mutineer
description	Clean-room mutation testing for Ruby (Prism + stdlib, zero deps). Mutates source one change at a time, runs the suite against each mutant, and reports the mutants tests fail to catch — with a versioned JSON contract and CI/agent gating.

Mutineer

Mutation testing for Ruby, built for CI gates and AI coding agents. Line coverage tells you which code ran; mutation testing tells you whether your tests would notice if it broke.

Install

gem install mutineer

Run

mutineer run <source...> --test <test...> [options]

Example — mutate lib/calculator.rb, gate CI below 90%:

mutineer run lib/calculator.rb --test test/calculator_test.rb --threshold 90

Agent loop

1. Edit code + tests on a branch.
2. Run diff-scoped, as JSON:

   mutineer run app/ --since origin/main --format json --output .mutineer/run.json
   

3. Parse survivors[] — each carries a diff and stable id. For each, write/strengthen a test that fails under that change.
4. Re-run. Stop when summary.survived == 0 or summary.score >= target.

Exit codes

• 0 — pass (score ≥ threshold, no baseline regression)
• 1 — tests too weak, or a regression was introduced
• 2 — misinvocation (bad flag/path), not a test-quality signal

References

• Docs: https://davidteren.github.io/mutineer/
• Agent & CI guide: https://davidteren.github.io/mutineer/agentic-coding.html
• JSON schema: https://davidteren.github.io/mutineer/json-schema.html
• Source: https://github.com/davidteren/mutineer