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Mutant

github.com

From $20.83/mo Free plan macOSLinux
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Spec score 8.2 #5 of 25 in Mutation Testing Tools

What it is

Mutant is Ruby mutation-testing software that changes code in controlled ways to check whether tests catch the changes. It alters arithmetic, logical and bitwise operators, removes statements, and changes return values. It works with RSpec, Minitest and Test::Unit, and supports incremental runs for CI. Supported Ruby versions are 3.2, 3.3, 3.4 and 4.0 for runtime, syntax and mutations. Rails support is verified for versions 7.2, 8.0 and 8.1, with parallel-worker database isolation for PostgreSQL and SQLite. Mutant runs on developers’ machines or in CI, and includes parallel execution and reports of surviving mutants. It cannot generate mutations for methods defined inside closures, including methods created with module_eval, class_eval, define_method or define_singleton_method. The open-source plan is free for public repositories with the full feature set. Commercial monthly pricing is $30 a month per developer; yearly pricing is $250 a year per developer. CI use requires no additional subscriptions.

Spec sheet

Purpose
Mutant is mutation testing for Ruby that systematically modifies code and verifies that tests catch each change.Read 1 Oct 2026 · github.com
Headquarters
Naxxar, MaltaRead 28 Sep 2026 · github.com
CI workflow
Mutant supports incremental mode for running mutation testing in CI.Read 1 Oct 2026 · github.com
Commercial licensing
Commercial subscriptions are required per developer using Mutant on commercial code, while CI execution requires no additional subscriptions.Read 1 Oct 2026 · github.com
Creator
Mutant was created and is developed by Markus Schirp and is cited as subject of IEEE-published research.Read 1 Oct 2026 · github.com
Data sharing
Customer information is never shared or sold to anyone.Read 1 Oct 2026 · github.com
Execution model
Mutant runs exclusively on developers’ machines or in CI.Read 1 Oct 2026 · github.com
Mutation operators
Mutant mutates arithmetic, logical and bitwise operators, removes statements, and modifies return values.Read 1 Oct 2026 · github.com
Rails integration
Mutant supports Rails applications with parallel-worker database isolation for PostgreSQL and SQLite.Read 1 Oct 2026 · github.com
Rails versions
The Rails integration is verified for Rails 7.2, 8.0, and 8.1.Read 1 Oct 2026 · github.com
Ruby support
Mutant supports Ruby versions 3.2, 3.3, 3.4, and 4.0 for runtime, syntax, and mutations.Read 1 Oct 2026 · github.com
Source-code privacy
Schirp DSO LTD states that it does not get access to customers’ source code, customer data, or other sensitive material.Read 1 Oct 2026 · github.com
Subject limitation
Mutant cannot emit mutations for methods defined within closures such as module_eval, class_eval, define_method, or define_singleton_method.Read 1 Oct 2026 · github.com
Support channels
Mutant provides Discord, GitHub Issues, and release announcements as communication channels.Read 1 Oct 2026 · github.com
Test integrations
Mutant integrates with RSpec, Minitest, and Test::Unit.Read 1 Oct 2026 · github.com

Plans and prices

Open source Free public repositories · full feature set Read 1 Oct 2026 · github.com
Commercial monthly $30 a month per developer any number of repositories · CI use requires no additional subscriptions Read 1 Oct 2026 · github.com
Commercial yearly $250 a year per developer any number of repositories · CI use requires no additional subscriptions Read 1 Oct 2026 · github.com
Enterprise Priced on request volume subscriptions · custom plans Read 1 Oct 2026 · github.com

Every Mutant plan and price →

Strong points and limits

Strong points

  • Integrates with RSpec, Minitest and Test::Unit
  • Incremental mode supports CI runs
  • Parallel execution and surviving-mutant reports
  • Free plan covers public repositories with full features

Limits

  • Cannot mutate methods defined within closures
  • Commercial use requires per-developer subscriptions

Who it suits

Mutant suits Ruby developers using supported test frameworks who want to assess whether tests detect code changes. Commercial users should account for per-developer subscriptions; CI execution does not require an additional subscription.

Verdict

Mutant specifies its supported Ruby versions, test integrations and Rails coverage, as well as a notable limitation. Public repositories can use the full feature set free; commercial use is priced per developer.

Where Specifiction places it

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