tddlisted

# TDD Red → green, one slice at a time. This skill is the reference that makes the loop produce tests worth keeping — most of it applies on every cycle, so consult it before and during the loop, not after. ## Seams — agree before writing A **seam** is the public boundary where behavior is observable: an exported function, an HTTP route, a CLI invocation. Tests live at seams; they never reach into internals. Before the first test, write down the seams under test and confirm them with the user — which boundaries get tests and which stay untested is a decision, and settling it up front is what keeps effort on critical paths instead of every edge case. No test at an unagreed seam. ## The loop 1. **Red.** One failing test at an agreed seam, for the next smallest real behavior. Run it; watch it fail for the right reason. 2. **Green.** The least code that passes — nothing speculative. Run it. 3. **Repeat.** The next test responds to what the last cycle taught. Work in vertical slices, never layers: writing all tests then all implementation verifies *imagined* behavior and locks in structure before understanding. 4. **Refactor outside the loop.** Cleanup happens at a checkpoint after a green cycle or in a review pass — not interleaved guesswork inside red-green. ## Test quality rules - **Expected values from an outside source of truth** — a known literal, a worked example, the spec. Never recompute the expected value the same way the code does; that test can only agree with i