Side by side

mutator vs mutmut

Both stand on mutation testing tools. Which one you can start free with, which one lets you read the code, and what each costs once you pay.

mutator

Free plan Linux

mutator offers mutation testing for R packages, including CI support and controls for test execution. Developers using the generic installed-tests fallback should account for its full-suite run instead of coverage-guided selection.

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mutmut

Free plan LinuxMacSelf-hosted+1

mutmut provides an incremental mutation-testing workflow with pytest, mutant browsing, and optional coverage or type-checking filters. Its fork-support requirement and Windows WSL constraint are important setup considerations.

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The facts

	mutator	mutmut
Free plan	Yes	Yes
Open source	Not published	Not published
Runs on	Linux	Linux, Mac, Self-hosted, Windows
Lowest paid plan	Not published	Not published
Plans published	None	None
Maker	Not published	Not published
Website	pdonatbouillud.com	mutmut.readthedocs.io
On mutation testing tools	#17 of 25	#13 of 25

mutator: what it does well

Reports a mutation score and surviving mutants
Supports testthat and tinytest
Runs tests in parallel
Includes a reusable GitHub Actions workflow
Users can cap mutants and override timeouts

What to know first

Generic installed-tests fallback runs the full suite
Coverage-guided selection is unavailable for the generic fallback
Equivalent-mutant detection uses an OpenAI-compatible API

mutmut: what it does well

Remembers completed work for incremental runs.
Selects tests to run for each mutant.
Interactive terminal interface for browsing mutants.
Can use coverage.py, mypy, or pyrefly filters.
Exports CI statistics and mutation score badge JSON.

What to know first

Requires Python 3.10 or later.
Windows users must run it inside WSL.
pyright and ty are not supported for mutant filtering.
Version 2 uses a different model outside functions.

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