Side by side

cargo-mutants vs Cosmic Ray

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.

cargo-mutants

Free plan Linux

cargo-mutants offers Rust-focused mutation analysis with configuration, reporting, and CI guidance. Its value depends on dependable tests, and projects must build for the host platform using Cargo.

Visit cargo-mutants

Cosmic Ray provides a configurable way to assess Python tests through mutation runs, with serial and distributed execution options. Teams should account for potentially long runs and protect their working code before execution.

Visit Cosmic Ray

The facts

	cargo-mutants	Cosmic Ray
Free plan	Yes	Yes
Open source	Not published	Not published
Runs on	Linux	Linux, Self-hosted
Lowest paid plan	Not published	Not published
Plans published	None	None
Maker	Not published	Not published
Website	mutants.rs	cosmic-ray.readthedocs.io
On mutation testing tools	#12 of 25	#8 of 25

cargo-mutants: what it does well

Finds code changes that tests may fail to catch.
Supports incremental analysis and parallel execution.
Can report surviving mutants and serve as a quality gate.
Configuration can be stored in `.cargo/mutants.toml`.
Documents CI workflows, including GitHub Actions.

What to know first

Cross-compilation is not supported.
Build systems other than Cargo are unsupported.
Useful results depend on reliable, non-flaky tests.
Provided as-is with no warranty.

Cosmic Ray: what it does well

Free to use under the MIT license.
Supports unittest, pytest, and nose.
HTTP distribution can run work across local or remote workers.
Reports are available in summary and HTML formats.
Mutation filters can skip selected changes.

What to know first

Mutation runs can take a long time.
Mutations are applied directly to code on disk.
Configuration options are not fully documented.
Tests sharing production modules may be mutated.

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