Write focused, strategic tests for core user flows with behavior-based testing, clear naming, and proper mocking of external dependencies. Use this skill when writing unit tests, integration tests, or end-to-end tests. When working on files in tests/, __tests__/, spec/, or *.test.ts, *.spec.ts files. When deciding what to test during feature development. When naming test cases descriptively. When mocking databases, APIs, or external services. When focusing on testing behavior rather than implementation details.