Mutation Testing

This validates your tests by deliberately breaking your code to see if the tests catch it. Install Stryker for TypeScript/JavaScript (works with Vitest or bun test) or mutmut for Python, and it'll introduce small mutations like flipping operators or changing conditionals. Tests that still pass are weak. You're aiming for 80%+ of mutants killed. The Bun runner is nice if you're using native bun test, though it runs sequentially for accurate coverage mapping. The workflow is straightforward: get decent coverage first, run mutation testing, check the HTML report to see which mutants survived, then tighten up those assertions. It's slower than regular testing but finds gaps that coverage percentages miss.