tdd

Behavior-focused integration testing validates software through public interfaces rather than internal implementation details, ensuring tests survive refactoring. This approach confirms actual user-facing functionality instead of coupling tests to specific code structures that change during updates.

Vertical tracer bullet workflow enforces one-test-one-implementation cycles during test-driven development. Instead of bulk test writing, you write a single test for observed behavior, implement minimal code to pass it, then iterate to the next edge case.

Tests break during refactoring because of implementation-coupled test design, where tests validate internal code structure instead of public interface behavior. Writing integration-style tests through public interfaces prevents these failures by avoiding horizontal test writing and speculative feature implementation.

Apply red-green-refactor cycles by writing a failing integration test for a specific user-facing behavior, implementing the minimal code required to pass it, and then refactoring the codebase. This vertical slice approach ensures each test validates real, observed functionality before moving forward.

Avoid horizontal test writing when building new features or fixing bugs, because it validates imagined behavior instead of actual user-facing functionality. Horizontal slicing leads to brittle, implementation-coupled tests that fail to verify real software requirements and break during refactoring.