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Automated code reviews via mutation testing - semantic code coverage.


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What is Mutant?

An automated code review tool, with a side effect of producing semantic code coverage metrics.

Think of mutant as an expert developer that simplifies your code while making sure that all tests pass.

That developer never has a bad day and is always ready to jump on your PR.

Each reported simplification signifies either:

A) A piece of code that does more than the tests ask for. You can probably use the simplified version of the code. OR:

B) If you have a reason to not take the simplified version as it violates a requirement: There was no test that proves the extra requirement. Likely you are missing an important test for that requirement.

On extensive mutant use A) happens more often than B), which leads to overall less code enter your repository at higher confidence for both the author and the reviewer.

BTW: Mutant is a mutation testing tool, which is a form of code coverage. But each reported uncovered mutation is actually a call to action, just like a flag in a code review would be.

Getting started:

  • Start with reading the nomenclature. No way around that one, sorry.
  • Then select and setup your integration, also make sure you can reproduce the examples in the integration specific documentation.
  • Use mutant during code reviews and on CI in incremental mode.
  • Do not merge code with new alive mutations. If you really must bypass: Add the subjects with open problems to the ignored subjects.

Operating Systems

Mutant is supported and tested under Linux and Mac OS X.

Ruby Versions

Mutant supports multiple ruby versions at different levels:

  • Runtime, indicates mutant can execute on a specific Ruby Version / implementation.
  • Syntax, depends on Runtime support, and indicates syntax new to that Ruby version can be used.
  • Mutations, depends on Syntax support, and indicates syntax new to that Ruby version is being analysed.

Supported indicates if a specific Ruby version / Implementation is actively supported. Which means:

  • New releases will only be done if all tests pass on supported Ruby versions / implementations.
  • New features will be available.
Implementation Version Runtime Syntax Mutations Supported
cRUBY/MRI 3.1 ✔️ ✔️ ✔️ ✔️
cRUBY/MRI 3.2 ✔️ ✔️ ✔️ ✔️
cRUBY/MRI 3.3 ✔️ ✔️ ✔️ ✔️
jruby TBD 📧 📧 📧 📧
mruby TBD 📧 📧 📧 📧
cRUBY/MRI < 3.1


  • ✔️ Supported.
  • ⚠️ Experimental Support.
  • 🔜 Active work in progress.
  • 📧 Planned, please contact me on interest.
  • ⛔ Not being planned, or considered, still contact me on interest.


Mutant is commercial software, with a free usage option for opensource projects. Opensource projects have to be on a public repository.

Commercial projects have to pay a monthly or annual subscription fee.

Opensource usage

Usage is free and does not require a signup. But it requires the code is under an opensource license and public. Specify --usage opensource on the CLI or usage: opensource in the config file.

Commercial usage

Commercial use requires payment via a subscription and requires a signup. See pricing for available plans.

After payment specify --usage commercial on the CLI or usage: commercial in the config file.


Mutant is free for opensource use!

For commercial use mutants pricing is subscription based.

Currency Duration Cost Payment Methods
USD 1 month 90$ Credit Card
USD 1 year 900$ Credit Card, ACH transfer
EUR 1 month 90€ Credit Card, SEPA Direct Debit
EUR 1 year 900€ Credit Card, SEPA Direct Debit, SEPA Transfer

Costs are per developer using mutant on any number of repositories.

Volume subscriptions with custom plans are available on request.

Should you want to procure a commercial mutant subscription please mail me to start the payment process.

Please include the following information:

  • Your business invoice address.
  • A payment email address, if different from your email address.
  • Only for the EU: A valid VAT-ID is required, no sales to private customers to avoid the horrors cross border VAT / MOSS. VAT for EU customers outside of Malta will use reverse charging.

Also feel free to ask any other question I forgot to proactively answer here.

Also checkout the commercial FAQ.



Try the following:


Mutant, as published in the opensource version, would not exist without the help of contributors spending lots of their private time.

Additionally, the following features where sponsored by organizations:

  • The mutant-minitest integration was sponsored by Arkency
  • Mutant's initial concurrency support was sponsored by an undisclosed company that does currently not wish to be listed here.


Contents of this repository are maintained by:

Schirp DSO LTD
Director: Markus Schirp
Vat-ID: MT24186727
Registration: C80467

Office address:
2, Carob Lane,
Sir Harry Luke Street
Naxxar NXR 2209,

Registred Address
Phoenix Business Centre,
The Penthouse,
Old Railway Track,
Santa Venera SVR9022,