The Pitfalls of Test Coverage: Introducing Mutation Testing with Stryker and Cosmic Ray

The article addresses the limitations of traditional code coverage metrics, which can be misleading when tests lack proper assertions. It introduces mutation testing using Stryker (for TypeScript/Vitest) and Cosmic Ray (for Python/Pytest) to verify whether tests actually catch logic errors. A real-world example with a VideoSplitter module demonstrates how 95%+ line coverage still missed critical boundary conditions, which mutation testing exposed through surviving mutants.