tdd

Avoid tautological tests in test-driven development by enforcing vertical slicing and behavior-driven patterns, ensuring each test validates a single verifiable behavior through a tracer bullet approach rather than mirroring internal implementation logic.

Structure unit tests for observable behavior by adhering to public interface design and dependency injection principles, which decouples tests from internal implementation and focuses validation on verifiable system outputs.

Use mocking in integration testing strictly at system boundaries, and avoid mocking internal collaborators in unit testing to prevent coupling tests to internal implementation details and maintain reliable specifications.

Plan a public interface using the red-green-refactor loop by defining a deep module with a small surface area, writing a failing test for a single behavior, and implementing the minimal code required to pass.

Test-driven development supports refactoring existing code by applying behavior-driven testing patterns to establish verifiable specifications before making changes, ensuring maintainability and robust code quality.

Horizontal slicing causes problems in test-driven development because it prevents each test from corresponding to a single verifiable behavior, leading to tautological tests and tight coupling to implementation details.