Implement one failing behaviour at a time — read the spec, write the test, write the smallest production change that turns it green. Use when coding from a spec, starting a TDD slice, or when a task is about to sprawl.
--- name: implement-a-behavior-slice category: coding description: >- Implement one failing behaviour at a time — read the spec, write the test, write the smallest production change that turns it green. Use when coding from a spec, starting a TDD slice, or when a task is about to sprawl. --- # Implement a Behaviour Slice The failure mode is starting in the middle of the system and "just adding the feature." An hour later you have touched four modules, none of the original scenarios is green, and you cannot say which change did what. A slice is the opposite: one scenario, one failing test, the smallest production change that makes that test pass, then stop. The next scenario is a new slice. Sprawl is what happens when you skip the stop. ## Before any production code Read three things, in this order, and do not start until you have: 1. **The operator task** — what a human asked for, in their words. 2. **The inbound spec** — the scenarios, the contract, the Then that will tell you it worked. 3. **The current project state** — what already exists. Open the files you are about to touch. Run the tests. If they are not green, you do not have a baseline, and you must not start a slice on a red suite. If the spec is missing a Then, you are not implementing. Send it back. Coding from a vibe produces a vibe, and reviews cannot check a vibe.
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