# mbj/mutant

Mutation testing for Ruby. AI writes your code. AI writes your tests. But who tests the tests?

Repository: https://github.com/mbj/mutant
Canonical: https://ross.abutalabs.com/products/mutant
Language: Ruby
License: NOASSERTION
License Family: other
Topics: mutation-testing, ruby, coverage, rspec, minitest, code-review, code-quality-analyzer, mutant, static-analysis, dynamic-analysis
Last push: 2026-08-24T03:51:55+00:00

## Health v2 (maintenance only)
Score: 93/100 (v2, computed 2026-09-03T02:20:16.233290+00:00)
- activity 99, release rhythm 80, longevity 100
- inputs: {"age_days": 5154, "days_push": 9, "days_rel": 135, "gap_med": 13.0, "n_releases_24m": 5}
- flags: no_license
- formula: round(0.45*activity + 0.35*rhythm + 0.20*longevity); archived -> min(score, 10)

## Adoption (not part of the score)
Stars 2186, forks 163 (observed 2026-08-28T04:06:23.700160+00:00)

## What it is
Mutant is a mutation testing tool for Ruby that systematically mutates your code and verifies your test suite catches each change. It integrates with RSpec and Minitest and reports surviving mutations as gaps in test coverage or opportunities for code simplification.

## Use cases
- measure the effectiveness of my ruby test suite
- find tests that pass but don't actually verify behavior
- check test coverage beyond line coverage with mutation testing
- verify ai-generated tests are meaningful
- identify weak or shallow tests in a ruby codebase
- run mutation testing with rspec or minitest

## When to choose
- you write Ruby and want rigorous verification that tests actually assert behavior, not just execute code
- you want to review AI-generated code and tests for real coverage
- you need per-mutation actionable feedback: add a test or accept a simplification

## When to avoid
- your project is not written in Ruby
- your test suite is too slow, since mutation testing reruns tests many times
- you only need simple line or branch coverage metrics

## Facets
- artifact type: library
- maturity: active
- function: testing, code-review, developer-tools
- domain: testing, developer-tools
- platform: ruby, cli, cross-platform
- tags: mutation-testing, rspec, minitest, test-quality, code-quality, static-analysis, dynamic-analysis, ruby

## Member repositories
- mbj/mutant (main) score 93

## Provenance
- Observed fields: from GitHub, fetched 2026-08-28T04:06:23.700160+00:00.
- Health v2: computed from the inputs above; adoption is never an input.
- Inferred fields (summary, facets, guidance): AI-extracted, prompt v1, taxonomy v1, on 2026-08-30T02:47:54.910295+00:00, confidence not recorded.
  - readme: https://github.com/mbj/mutant (fetched 2026-08-28T04:06:23.700160+00:00, sha 6d0a5b8fba8b)
- Data as of 2026-08-30T08:39:29.467469+00:00.