# boxed/mutmut

Mutation testing system

Repository: https://github.com/boxed/mutmut
Canonical: https://ross.abutalabs.com/products/mutmut
Homepage: https://mutmut.readthedocs.io
Language: Python
License: BSD-3-Clause
License Family: permissive
Topics: testing, mutation-testing, python
Last push: 2026-08-17T18:29:43+00:00

## Health v2 (maintenance only)
Score: 76/100 (v2, computed 2026-09-03T02:20:16.233290+00:00)
- activity 98, release rhythm 35, longevity 100
- inputs: {"age_days": 3580, "days_push": 16, "days_rel": null, "gap_med": null, "n_releases_24m": 0}
- flags: no_releases
- formula: round(0.45*activity + 0.35*rhythm + 0.20*longevity); archived -> min(score, 10)

## Adoption (not part of the score)
Stars 1406, forks 168 (observed 2026-08-28T04:04:38.092421+00:00)

## What it is
mutmut is a mutation testing tool for Python that mutates source code to verify test suite quality. It offers an interactive terminal UI, incremental runs, parallel execution, and commands to apply or browse mutants.

## Use cases
- measure how effective my pytest suite is
- find gaps in test coverage with mutation testing
- run mutation testing on a Python project
- browse and apply surviving mutants to improve tests
- incrementally retest mutants after improving tests
- check which tests catch a specific code mutation

## When to choose
- you have a Python project with pytest tests and want to assess test quality
- you want an easy-to-use mutation tester with a terminal UI and resumable runs
- you need fast, parallel mutation testing that knows which tests to run per mutant

## When to avoid
- your project runs on Windows without WSL (fork is required)
- you need mutation testing for languages other than Python
- you need to mutate code outside of functions and prefer mutmut 2's execution model

## Facets
- artifact type: cli-tool
- maturity: active
- function: testing, cli, developer-tools
- domain: testing, developer-tools
- platform: python, cli
- tags: mutation-testing, pytest, test-quality, tui, code-mutation, python, linux, macos

## Member repositories
- boxed/mutmut (main) score 76

## Provenance
- Observed fields: from GitHub, fetched 2026-08-28T04:04:38.092421+00:00.
- Health v2: computed from the inputs above; adoption is never an input.
- Inferred fields (summary, facets, guidance): AI-extracted, prompt v1, taxonomy v1, on 2026-08-30T04:38:41.698636+00:00, confidence not recorded.
  - readme: https://github.com/boxed/mutmut (fetched 2026-08-28T04:04:38.092421+00:00, sha 211102764728)
  - registry_pypi: https://pypi.org/pypi/mutmut/json (fetched 2026-08-29T11:52:59.909364+00:00, sha bd2110c1f993)
- Data as of 2026-08-30T08:39:29.467469+00:00.