tdd
The agent codes better with a tight feedback loop than with a long specification. A failing test is the tightest loop there is: it states the target, and the target either goes green or it does not.
The loop
- Red. Write one failing test that states the next slice of behavior. Run it. Confirm it fails for the right reason - a test that passes before you write the code is testing nothing. If it errors instead of failing on the assertion, fix the test setup first.
- Green. Write the least code that makes the test pass. Not the general solution, not the abstraction - the minimal move. Run the test. Green.
- Refactor. Now clean up with the test as a net: remove duplication, name things, simplify. Re-run after every edit. Behavior is frozen; only the shape changes.
- Repeat for the next slice. Small slices. The rate of feedback is the speed limit - never take on a step too big to hold a single test.
What makes a test worth writing
- Tests behavior, not implementation. Assert on the observable result, not on which private method got called. A test that breaks on every refactor is a liability.
- One reason to fail. Each test pins one behavior. When it goes red you should know what broke without reading the body.
- Real seams, minimal mocks. Mock at the system boundary (network, clock, filesystem), not internal collaborators. A test that mocks the thing under test asserts nothing.
- No tautologies. A test that restates the implementation (
expect(x).toBe(x)after settingx) verifies nothing. Assert the value the behavior should produce.
When to reach for it
Reach for TDD when the behavior is specifiable and a test can pin it: business logic, parsers, state machines, bug fixes (write the failing case first, then fix). Skip it for pure exploration, throwaway spikes, and layout-only UI where a test asserts nothing a human would not eyeball.
Output contract
Show each red-green transition, not just the final green. If a test is hard to write, say what the difficulty reveals about the design - untestable code is usually badly-seamed code, and that is a finding, not an obstacle.
