tdd

Write one failing test that describes desired behavior through a public interface, then write only enough code to make it pass. Repeat one slice at a time, and keep refactoring out of the loop until the review stage.

A seam is the public boundary where you observe behavior without reaching inside the code. Tests live at seams, never against internals, and all 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 for mockability.

Tests break on refactor when they are coupled to implementation details, such as mocking internal collaborators, testing private methods, or asserting on call counts. Behavior-focused tests through public interfaces survive internal changes.

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 such as a known literal or worked example.