Codacy
Static analysis and quality gates for engineering teams.
Mutant
Automated Ruby code reviews that simplify code and ensure test safety
Mutant runs mutation testing during pull‑requests and CI, suggesting simplifications that keep all tests green while highlighting missing test coverage.
Overview
Overview
Mutant is a mutation‑testing driven code review assistant for Ruby projects. It analyses each change, generates mutations, and reports simplifications that preserve existing test outcomes, effectively surfacing over‑engineered code and uncovered requirements.
Capabilities
During pull‑request reviews or incremental CI runs, Mutant flags alive mutations as actionable items, offering developers concise suggestions to reduce code size without breaking tests. It integrates with popular test frameworks such as RSpec and Minitest, supports Linux and macOS, and runs on Ruby 3.2‑3.4 (cRUBY/MRI). The tool also produces semantic code‑coverage metrics that go beyond line‑level statistics, helping teams identify missing tests for implicit requirements.
Deployment & Licensing
Open‑source projects can use Mutant for free by declaring --usage opensource. Commercial teams require a subscription per developer, with monthly or annual plans available in USD and EUR. Configuration is performed via CLI flags or a config file, and the tool can be added to CI pipelines in incremental mode to prevent merging code with new alive mutations.
Highlights
Mutation‑testing based code review with automatic simplification suggestions
Semantic coverage metrics that expose missing test requirements
Incremental CI mode that blocks merges with new alive mutations
Native integrations for RSpec and Minitest on Linux/macOS
Pros
- Catches over‑engineered code while keeping tests green
- Improves test suite quality by highlighting uncovered semantics
- Fits naturally into existing Ruby CI pipelines
- Supports latest Ruby 3.2‑3.4 releases
Considerations
- Limited to Ruby; not applicable to other languages
- Commercial use requires a paid subscription
- Requires familiarity with Mutant's nomenclature
- Experimental support for JRuby and mruby only
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Fit guide
Great for
- Open‑source Ruby libraries seeking automated code quality checks
- Teams using RSpec or Minitest with CI pipelines
- Developers who want semantic coverage beyond line metrics
- Projects aiming to reduce technical debt through safe refactoring
Not ideal when
- Non‑Ruby codebases
- Small scripts without a test suite
- Organizations unwilling to pay for commercial licensing
- Environments that require Windows support
How teams use it
Pull‑request review
Automatically suggests code simplifications that keep tests passing, reducing technical debt before merge.
CI regression guard
Fails builds when new alive mutations appear, ensuring every change is fully covered by tests.
Test suite audit
Identifies uncovered requirements, prompting addition of missing tests to improve semantic coverage.
Legacy code cleanup
Reveals dead or over‑specified code sections, guiding safe refactoring of older modules.
Tech snapshot
Ruby100%
Roff1%
Shell1%
Frequently asked questions
How does Mutant differ from traditional mutation testing?
Mutant treats each uncovered mutation as a code‑review action, providing concrete simplification suggestions rather than just a coverage score.
Which Ruby versions are officially supported?
Supported runtimes include cRUBY/MRI 3.2, 3.3, and 3.4 with full syntax and mutation analysis.
Is there a free tier for open‑source projects?
Yes, open‑source usage is free when the repository is public and the `--usage opensource` flag is set.
How can Mutant be integrated into a CI pipeline?
Add Mutant in incremental mode to the CI script; the build will fail if alive mutations remain after the run.
What licensing applies for commercial use?
Commercial projects need a paid subscription per developer, with monthly or annual plans available in USD and EUR.
Project at a glance
Active- Stars
- 2,062
- Watchers
- 2,062
- Forks
- 157
Repo age13 years old
Last commit2 days ago
Primary languageRuby
Last synced 4 hours ago