Mutation Testing

This evaluates how well your tests actually catch bugs by intentionally breaking your code and checking if tests fail. The skill covers Stryker for JavaScript/TypeScript, PITest for Java, and mutmut for Python. It's most useful when you need confidence that critical business logic is truly tested, not just executed. The guides walk through setup and interpreting mutation scores, which measure the percentage of code changes your tests detect. Honest take: mutation testing is slow and you won't run it on everything, but it's excellent for finding gaps in tests that otherwise show 100% line coverage. The best practice advice here is solid, especially about targeting important code and aiming for 80% mutation score rather than chasing perfection.

skills.sh installs: 478
GitHub stars: 379
last updated: Aug 21, 2026

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Install

$ npx -y skills add aj-geddes/useful-ai-prompts --skill mutation-testing --agent claude-code

Installs into .claude/skills of the current project.

Files

Overview

Mutation testing assesses test suite quality by introducing small changes (mutations) to source code and verifying that tests fail. If tests don't catch a mutation, it indicates gaps in test coverage or test quality. This technique helps identify weak or ineffective tests.

When to Use

Evaluating test suite effectiveness
Finding untested code paths
Improving test quality metrics
Validating critical business logic is well-tested
Identifying redundant or weak tests
Measuring real test coverage beyond line coverage
Ensuring tests actually verify behavior

Quick Start

Minimal working example:

# Install Stryker
npm install --save-dev @stryker-mutator/core @stryker-mutator/jest-runner

# Initialize configuration
npx stryker init

# Run mutation testing
npx stryker run