Mutant

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Score7.1

Rank#2 of 25

From$20.83/mo

Free planYes

Runs onLinux, macOS

Summary

Mutant is mutation testing software for Ruby. It makes systematic code changes, such as altering arithmetic, logical, or bitwise operators, removing statements, and changing return values, then checks whether tests catch them. It integrates with RSpec, Minitest, and Test::Unit, and supports incremental mode for CI workflows. Mutant supports Ruby 3.2, 3.3, 3.4, and 4.0 for runtime, syntax, and mutations. Its Rails integration is verified for Rails 7.2, 8.0, and 8.1, with parallel-worker database isolation for PostgreSQL and SQLite. One limitation is that it cannot emit mutations for methods defined within closures, including methods created with module_eval, class_eval, define_method, or define_singleton_method. Mutant runs on developers' machines or in CI. The open-source plan is free for public repositories with the full feature set. Commercial subscriptions cost 30.00 USD per month or 250.00 USD per year per developer; CI use requires no additional subscription.

Who it is for

Mutant suits Ruby developers who want to check whether their tests detect code changes, including teams using the listed test frameworks or supported Rails versions. Its open-source plan is for public repositories; commercial use requires a per-developer subscription.

What is good

Integrates with RSpec, Minitest, and Test::Unit.
Supports incremental mode for CI.
Supports Ruby 3.2, 3.3, 3.4, and 4.0.
CI use requires no additional commercial subscription.
Open-source plan includes the full feature set for public repositories.

What to know first

Cannot emit mutations for methods defined within closures.
Commercial subscriptions are required per developer using Mutant on commercial code.
Commercial monthly plan costs 30.00 USD per month per developer.

Verdict

Mutant offers Ruby mutation testing with integrations for three test frameworks and incremental CI support. Check the closure-related limitation and commercial licensing terms before adopting it for commercial code.

Mutant plans and pricing

All plans

Open source Free public repositories · full feature set github.com · 1 Oct 2026

Commercial monthly $30/mo per developer any number of repositories · CI use requires no additional subscriptions github.com · 1 Oct 2026

Commercial yearly $250/yr per developer any number of repositories · CI use requires no additional subscriptions github.com · 1 Oct 2026

Enterprise Not published volume subscriptions · custom plans github.com · 1 Oct 2026

Compared on mutation testing tools

Free plan: Yesgithub.com
Paid from: $30/mogithub.com
Supported languages: Rubygithub.com
Test frameworks: RSpec, Minitest, Test::Unitgithub.com
Incremental analysis: Yesgithub.com
Parallel execution: Yesgithub.com
Surviving mutant reports: Yesgithub.com
Mutation operators: Arithmetic, logical, bitwise, statement removal, return-value modification, orderinggithub.com

Facts

Purpose: Mutant is mutation testing for Ruby that systematically modifies code and verifies that tests catch each change.github.com · 1 Oct 2026
Test integrations: Mutant integrates with RSpec, Minitest, and Test::Unit.github.com · 1 Oct 2026
CI workflow: Mutant supports incremental mode for running mutation testing in CI.github.com · 1 Oct 2026
Ruby support: Mutant supports Ruby versions 3.2, 3.3, 3.4, and 4.0 for runtime, syntax, and mutations.github.com · 1 Oct 2026
Rails integration: Mutant supports Rails applications with parallel-worker database isolation for PostgreSQL and SQLite.github.com · 1 Oct 2026
Rails versions: The Rails integration is verified for Rails 7.2, 8.0, and 8.1.github.com · 1 Oct 2026
Subject limitation: Mutant cannot emit mutations for methods defined within closures such as module_eval, class_eval, define_method, or define_singleton_method.github.com · 1 Oct 2026
Execution model: Mutant runs exclusively on developers’ machines or in CI.github.com · 1 Oct 2026
Source-code privacy: Schirp DSO LTD states that it does not get access to customers’ source code, customer data, or other sensitive material.github.com · 1 Oct 2026
Data sharing: Customer information is never shared or sold to anyone.github.com · 1 Oct 2026
Support channels: Mutant provides Discord, GitHub Issues, and release announcements as communication channels.github.com · 1 Oct 2026
Commercial licensing: Commercial subscriptions are required per developer using Mutant on commercial code, while CI execution requires no additional subscriptions.github.com · 1 Oct 2026
Creator: Mutant was created and is developed by Markus Schirp and is cited as subject of IEEE-published research.github.com · 1 Oct 2026