Which programming languages are supported by this skill?

The skill currently supports JavaScript and TypeScript (via Stryker), Java (via PIT), and Python (via mutmut).

What is the difference between code coverage and a mutation score?

Code coverage measures which lines of code were executed during a test, while a mutation score measures if the tests actually noticed when the logic on those lines was intentionally changed or broken.

Is mutation testing slow to run?

Yes, mutation testing is compute-intensive because it runs your test suite multiple times. This skill includes guidance on targeting specific modules and using incremental modes to optimize performance.

When should I start using mutation testing?

It is best used when you already have an existing test suite with at least 50% code coverage and want to ensure your tests are actually effective at catching bugs.

Mutation Testing

Name: Mutation Testing
Author: cskiro

bycskiro

•

Security & Testing

Validates test suite effectiveness by introducing deliberate code mutations to identify logic gaps that traditional coverage metrics miss.

The Mutation Testing skill enhances Claude's ability to evaluate the true quality of your test suites by introducing 'mutants'—intentional, small logic errors—into your codebase. While standard code coverage only measures which lines are executed, this skill identifies if your tests actually detect changes in behavior, providing a mutation score that reflects real-world reliability. It supports industry-standard tools like Stryker for JavaScript, PIT for Java, and mutmut for Python, helping developers move beyond superficial metrics to create robust, bug-resistant applications.

Key Features

01Guidance on interpreting mutation results to improve overall test architecture

02Analysis of mutation scores to identify weak or ineffective test cases

03Identification of surviving mutants with specific improvement recommendations

04Automated setup and configuration for Stryker, PIT, and mutmut frameworks

052 GitHub stars

06Targeted testing for critical code paths to balance performance and depth

Use Cases

01Auditing and upgrading legacy test suites to ensure modern quality standards

02Verifying that a high-coverage test suite actually catches logic regressions

03Improving testing for critical business logic before a major production release

What are Skills?·How to Install

Install with 🐟 Skill.Fish

npx skillfish add cskiro/claudex mutation-testing

For use in Claude.ai and ChatGPT

Download Skill