tdd

Write one failing test against a public interface first, then implement only enough code to make it pass, and repeat per vertical slice. Confirm the seams under test before writing anything, and keep refactoring out of the red-green cycle.

A seam is the public boundary where you observe behavior without reaching inside the code. Tests live at seams, never against internals, and the seams under test should be agreed with the user before any test is written.

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

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

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