Mutation testing
Breaks the code on purpose to find out whether the suite notices — the only honest measure of test quality.
Installation
$ npx skills add trailofbits/skillsThe CLI's unit is the repository, so this installs everything in trailofbits/skills, not mutation-testing alone. It writes into whichever agent directories it finds — Claude Code, Codex, Cursor, Copilot, Windsurf, Zed.
Summary
Breaks the code on purpose to find out whether the suite notices — the only honest measure of test quality.
- Reach for it when coverage is high, confidence is not, and you suspect the tests assert very little.
- Sits in Test — proving the change does what it claims — and finding the assertions that were never written.
- Stack-agnostic — nothing in it assumes a particular language or framework.
- Published by Trail of Bits. Well regarded, but not the vendor of the thing it covers.
- Installing pulls the whole trailofbits/skills repository: this catalogue lists 12 other entries from it, and the repo may hold more.
Where it sits in delivery
TestProving the change does what it claims — and finding the assertions that were never written.Everything in Test →
The skill itself
The full text lives with its publisher and changes when they change it. Read it there rather than here — a mirrored copy of somebody else's procedure goes stale silently, and this page would have no way of telling you.
Related
More in Test
- PlaywrightAuthoring Playwright tests, shipped in the Playwright repository itself — locators, waiting, fixtures, traces.
microsoft/playwright - TDD (Osmani)The same discipline in the SDLC set, so it composes with the planning and review skills beside it.
addyosmani/agent-skills - Test-driven developmentThe strict red-green-refactor loop, written so an agent cannot quietly write the test after the code.
obra/superpowers - Verification before completionForces the agent to actually run the thing before it claims to be finished.
obra/superpowers - Code testing agentA multi-agent pipeline that researches, plans and then generates a working test suite — language-agnostic despite the publisher.Agent
dotnet/skills - Property-based testingTests the invariant across generated inputs instead of the three examples you thought of.
trailofbits/skills