tdd

Focus on integration-style tests that verify behavior through public interfaces rather than internal implementation details. This approach ensures your test suite remains reliable and validates what the system does without failing unnecessarily when you refactor code.

The red-green-refactor loop is a test-driven development workflow where you write a failing tracer-bullet test first, implement minimal code to pass it, then refactor only after achieving green. This vertical slice process repeats one test at a time to build features incrementally.

Mock only system boundaries like external APIs and databases, not internal collaborators. Mocking internal components creates brittle tests coupled to implementation details, while mocking external boundaries isolates your tests from unpredictable outside systems and maintains refactoring confidence.

Yes, test-driven development applies to both building new features and fixing bugs. Writing tracer-bullet tests one at a time using the red-green-refactor loop provides integration-style coverage and predictable confidence whether you are adding functionality or resolving defects.

Tests break during refactoring when they verify implementation details rather than public behavior. By testing through public interfaces and mocking only external boundaries, your tests describe what the system does and remain stable regardless of how internal code is restructured.

Use a vertical slice workflow with tracer bullets, writing one test at a time and implementing minimal code to reach green before refactoring. This avoids horizontal slicing crap tests by focusing each test on system behavior through public interfaces rather than isolated internal functions.