Mutation Testing Expert

What it does

This skill provides expert knowledge for performing mutation testing to ensure that your test suite actually catches bugs. Instead of relying on simple code coverage, it introduces small, intentional changes (mutants) to the source code to see if existing tests fail. If a test passes despite a mutation, it identifies a 'survived mutant,' signaling a weak test that needs strengthening.

When to use it

Use this skill when you want to improve the quality of your test suite, verify if your assertions are sufficiently strict, or establish a mutation score benchmark for critical business logic.

What's included

• Instructions: Detailed guides for setting up and running Stryker for TypeScript/JavaScript (including Vitest and Bun native runners) and mutmut for Python.
• Best Practices: Guidance on mutation score targets, identifying common weak test patterns (e.g., insufficient assertions), and boundary condition testing.
• Workflows: Step-by-step processes for running tests, analyzing reports, and iteratively improving test quality.

Compatible agents

Primarily designed for Claude Code CLI, but applicable to any agent with shell access (e.g., Codex, Gemini CLI) capable of running Node.js and Python environments.