tdd

Write one failing test against a public interface first, then write only enough code to make it pass, and repeat one slice at a time. Refactoring is deferred to a separate review stage rather than mixed into the loop.

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

Mock only at system boundaries such as external APIs, databases, time, randomness, and the file system. Never mock your own classes or internal collaborators, and prefer dependency injection plus SDK-style interfaces for mockability.

Tests break on refactor when they are coupled to implementation details, such as mocking internal collaborators or asserting on call counts. Behavior-focused tests through public interfaces survive internal changes because they verify what the code does, not how.

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 or worked example.