tdd-fail-first

name	tdd-fail-first
description	Enforced RED-GREEN-REFACTOR test-driven development. Use when implementing any behavior change that can be expressed as a test — new features, bug fixes, edge cases. The test MUST fail first; a test that passes on first run proves nothing.
source	Jesse Vincent (obra) — Superpowers enforced TDD
am_pattern	B2, B4
triggers	["tdd","test-driven","fail first","write the test first","red-green"]

You are implementing with fail-first TDD. The non-negotiable rule: you must watch the test fail before you make it pass. A test written after the code, or a test that passes immediately, is not evidence — it may be testing nothing.

The loop

1. RED — write the smallest test that expresses the next increment of behavior. Run it. Confirm it fails, and fails for the right reason (the assertion, not an import error or typo). Paste the failure output into your working notes. If it passes, the test is wrong or the behavior already exists — stop and figure out which.
2. GREEN — write the minimum code to make that test pass. Resist implementing ahead of the tests. Run the test; confirm green. Run the whole suite; confirm nothing else broke.
3. REFACTOR — with green as the safety net, clean up: remove duplication introduced in step 2, improve names. Re-run the suite after each refactor.
4. Repeat until the acceptance criteria from the plan are covered.

Bug fixes

For a bug, the RED step is a reproduction test: a test that fails on the current code because of the bug, and encodes the expected correct behavior. Never fix a bug without one — a fix without a failing repro test can't prove it fixed anything, and the bug can silently return.

Rules

• One failing test at a time. Don't write ten red tests and then code.
• Test behavior through public interfaces, not implementation details — a refactor that preserves behavior should not break tests.
• Never weaken, skip, or delete a failing test to get to green. If a test seems wrong, say so explicitly and get agreement before changing it.
• If the runner isn't obvious, find it in AGENTS.md (Commands section) — never guess.