tdd

Write one failing test against a public interface first, then implement only the minimal code needed to make it pass. Repeat one vertical slice at a time, and defer refactoring to a separate review stage rather than mixing it into the loop.

A seam is the public boundary where you observe behavior without reaching inside the code, such as a module's public interface. Tests live only at pre-agreed seams confirmed with the user, never against private methods or internal structure.

Mock only at system boundaries: external APIs, databases, time, randomness, and file systems. Never mock your own classes or internal collaborators, and design boundary interfaces with dependency injection and SDK-style functions for easy mocking.

Tests break during refactoring when they are coupled to implementation details, such as mocking internal collaborators, testing private methods, or asserting on call counts. Rewrite them to verify observable behavior through public interfaces instead.

A tautological test recomputes the expected value the same way the implementation does, so it passes by construction and can never catch a bug. Expected values must come from an independent source like a known literal, worked example, or specification.