dev-tddlisted

# Dev TDD Strict red-green-refactor. No exceptions. ## The Law 1. **Red first.** Write a failing test before writing any implementation code. 2. **Green minimal.** Write the minimum code to make the test pass — nothing more. 3. **Refactor clean.** Clean up with all tests green. Do not add features during refactor. Repeat. One cycle at a time. No skipping steps. ## Hard Rules - **No implementation without a failing test.** If you write code before a test, stop. Delete the code. Write the test first. - **No horizontal slicing.** Do not build "the data layer" then "the business layer" then "the UI layer". Build one complete vertical slice (input → output) at a time, tested end-to-end. - **No green shortcuts.** Do not hard-code return values to pass tests (unless as a deliberate step in triangulation — but you must write another test that forces real logic within the same cycle). - **Test must fail for the right reason.** Before writing implementation, confirm the test fails with the expected failure message — not a compile error, not a wrong error. ## Vertical Slice Pattern Each TDD cycle targets one user-visible behaviour: ``` TEST: "given {input}, {system} produces {output}" IMPL: minimal code to satisfy the test REFACTOR: clean without changing behaviour ``` Examples of vertical slices: - "Given a valid email, signup returns a user ID" - "Given an invalid email, signup returns a 400 with a message" - NOT: "Build the User model" (horizontal) ## Running tests vs. run