Muter

Free plan MacSelf-hosted

What Muter is

Muter is a free mutation testing utility for Swift test suites on macOS. It introduces code changes, called mutants, and reports how many tests catch them, with scores for the whole suite and individual files. It can run inside Xcode, from the command line or incrementally in CI, and supports Xcode projects and workspaces as well as Swift Package Manager. Reports are available as plain text, JSON, HTML or Xcode issue navigator results. Muter works on a copy of the codebase, leaving the original unchanged. It supports Swift projects tested with xcodebuild, including iOS, macOS, tvOS and watchOS apps, but the utility itself requires macOS 10.15 or later and does not support Objective-C. The project says it has no network access, collects no analytics and does not phone home. Mutation testing can take a long time; UI tests may add to that, and some mutations in @resultBuilder methods may fail to compile.

Who it suits

Muter suits Swift developers who want to assess test suites using Xcode or Swift Package Manager. Teams should account for potentially long runs and the macOS requirement.

What it does well

Reports mutation scores for suites and individual files.
Runs in Xcode, the command line and incrementally in CI.
Supports Xcode and Swift Package Manager tests.
Works on a complete code copy.

What to know first

Runs only on macOS 10.15 or later.
Does not support Objective-C.
Mutation testing can take a long time.
Some @resultBuilder mutations may fail to compile.

Limits

Muter is free, with no paid plans listed. The project recommends starting with unit tests because UI tests can take a long time; longer suites may need separate schemes or targets and enough run time.

Plans

Plan	Billed and limits	Price
Open-source Muter	No paid plans or pricing listed	Free

As github.com lists them, read 8 Oct 2026. The maker's own page has the final word.

Getting started

Open the Muter project website on GitHub.
Use a Mac running macOS 10.15 or later.
Install Muter through Homebrew, as documented by the project.
Configure it for an Xcode project or workspace, or a Swift Package Manager project.
Run it in Xcode, from the command line or incrementally in CI.

Verdict

Muter offers several ways to run Swift mutation tests and inspect results, with no network access according to its project. Its scope is limited to Swift on macOS, and longer test runs may need planning.

Questions

How much does Muter cost?

Muter is free and open source. No paid plans are listed.

Which languages and test setups does it support?

It supports Swift code with Xcode test targets and Swift Package Manager tests. Objective-C is not currently supported.

Which platforms can use Muter?

The utility runs on macOS 10.15 or later. It supports projects tested with xcodebuild, including iOS, macOS, tvOS and watchOS projects.

How can I run Muter?

It can run inside Xcode, from the command line or incrementally in CI.

What reports can Muter produce?

It can output plain text, JSON, HTML or Xcode issue navigator results.

Does Muter send code or analytics over the network?

Muter has no network access, collects no analytics and does not phone home.

Facts from Muter's own pages

Compatibility caveat: Muter documents that mutations in @resultBuilder methods may fail to compile, with the limitation noted as potentially absent when using Swift 5.9 or later. · github.com, 8 Oct 2026
Integrations: The README documents Xcode integration, Homebrew installation, and configuration support for Xcode projects and workspaces and Swift Package Manager. · github.com, 8 Oct 2026
Language limit: Muter supports Swift code but does not currently support Objective-C. · github.com, 8 Oct 2026
Mutation operators: The documented operators are RelationalOperatorReplacement, RemoveSideEffects, ChangeLogicalConnector, and SwapTernary. · github.com, 8 Oct 2026
Mutation score: Each run reports a mutation score based on the mutants killed versus the total mutants introduced, with scores for the whole suite and individual files. · github.com, 8 Oct 2026
Privacy: The README says Muter has no network access, collects no analytics, and never phones home. · github.com, 8 Oct 2026
Purpose: Muter is a mutation testing utility for assessing the quality of Swift test suites. · github.com, 8 Oct 2026
Reports: Muter can output reports as plain text, JSON, HTML, or Xcode issue navigator results. · github.com, 8 Oct 2026
Run modes: Muter can run within Xcode, from the command line, and incrementally in CI. · github.com, 8 Oct 2026
Runtime caveat: The README notes mutation testing can take a long time and recommends allocating enough time and considering separate schemes or targets for longer-running tests. · github.com, 8 Oct 2026
Source handling: Muter performs its work on a complete copy of the codebase so it does not leave mutations in the original code. · github.com, 8 Oct 2026
Supported app platforms: Muter says it supports projects tested with xcodebuild, including iOS, macOS, tvOS, and watchOS, and runs only on macOS 10.15 or higher. · github.com, 8 Oct 2026
Test suites: It supports any kind of test target or suite when the application code is Swift, though the README recommends starting with unit tests because UI tests can take a long time. · github.com, 8 Oct 2026

Where Muter stands on Freedom251

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