Skip to content
SPECIFICTION
Get listed

Muter

github.com

Free plan macOSSelf-hosted
Visit github.com
Spec score 7.5 #14 of 25 in Mutation Testing Tools

What it is

Muter is a free, open-source mutation testing utility for Swift test suites on macOS. It changes code in a complete copy of the codebase, leaving the original untouched, then reports mutation scores for the full suite and individual files. Runs can happen in Xcode, from the command line or incrementally in continuous integration. Configuration covers Xcode projects, workspaces and Swift Package Manager. Reports can be plain text, JSON, HTML or Xcode issue navigator results, including surviving mutants. Documented operators replace relational operators, remove side effects, change logical connectors and swap ternary expressions. Muter supports projects tested with xcodebuild, including iOS, macOS, tvOS and watchOS applications, but the utility runs only on macOS 10.15 or newer. It supports Swift, not Objective-C. Any test target or suite can be used with Swift application code, although UI tests may take a long time. The project states that it has no network access and collects no analytics.

Spec sheet

Purpose
Muter is a mutation testing utility for assessing the quality of Swift test suites.Read 8 Oct 2026 · github.com
Integrations
The README documents Xcode integration, Homebrew installation, and configuration support for Xcode projects and workspaces and Swift Package Manager.Read 8 Oct 2026 · github.com
Privacy
The README says Muter has no network access, collects no analytics, and never phones home.Read 8 Oct 2026 · github.com
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.Read 8 Oct 2026 · github.com
Language limit
Muter supports Swift code but does not currently support Objective-C.Read 8 Oct 2026 · github.com
Mutation operators
The documented operators are RelationalOperatorReplacement, RemoveSideEffects, ChangeLogicalConnector, and SwapTernary.Read 8 Oct 2026 · github.com
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.Read 8 Oct 2026 · github.com
Reports
Muter can output reports as plain text, JSON, HTML, or Xcode issue navigator results.Read 8 Oct 2026 · github.com
Run modes
Muter can run within Xcode, from the command line, and incrementally in CI.Read 8 Oct 2026 · github.com
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.Read 8 Oct 2026 · github.com
Source handling
Muter performs its work on a complete copy of the codebase so it does not leave mutations in the original code.Read 8 Oct 2026 · github.com
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.Read 8 Oct 2026 · github.com
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.Read 8 Oct 2026 · github.com

Plans and prices

Open-source Muter Free No paid plans or pricing listed Read 8 Oct 2026 · github.com

Every Muter plan and price →

Strong points and limits

Strong points

  • Free and open source
  • Runs in Xcode, command line and CI
  • Supports JSON, HTML and Xcode reports
  • Leaves the original code unchanged
  • Has no network access or analytics

Limits

  • Supports Swift but not Objective-C
  • Runs only on macOS 10.15 or newer
  • Mutation testing can take a long time
  • Some result-builder mutations may fail to compile

Who it suits

It suits Swift teams using Xcode or Swift Package Manager who want mutation scores and surviving-mutant reports. It is intended for macOS 10.15-or-newer environments.

Verdict

Muter gives Swift projects several execution modes, mutation operators and report formats without a paid plan. Its macOS-only runtime, Swift scope and potentially lengthy runs define where it fits.

Where Specifiction places it

Alternatives to MuterAll alternatives →