tdd

name	tdd
description	Trigger: writing new production code, RED-GREEN-REFACTOR, failing test first. Enforces the Three Laws of TDD before any implementation.

Trigger Conditions

Load this skill before writing ANY production code change — new feature, bug fix, or refactor. If there is no failing test yet, write one first; do not skip straight to implementation.

Practice: The Three Laws (PRD §2.1)

1. You may not write production code unless it is to make a failing test pass.
2. You may not write more of a test than is sufficient to fail (compilation/import failures count as a valid failure).
3. You may not write more production code than is sufficient to pass the currently failing test.

Cycle: RED → GREEN → REFACTOR, always in that order, never skipped:

• RED — write the smallest failing test that names the next behavior. It MUST fail for the right reason (missing behavior, not a typo).
• GREEN — write the minimum production code to make that one test pass. Resist adding anything the test doesn't demand yet.
• REFACTOR — with the test green, clean up duplication and naming, in production code and test code alike. Re-run the test after every refactor step; it must stay green.

Sad-path scenarios (invalid input, boundary conditions, partial failures, contract violations) are first-class RED cases — not an afterthought once the happy path passes.

Worked Example

Adding input validation to a Withdraw(amount int) error method that must reject a negative amount:

1. RED: write TestWithdraw_RejectsNegativeAmount asserting Withdraw(-10) returns a non-nil error and the balance is unchanged. Run it — it fails because Withdraw currently accepts any amount.
2. GREEN: add the minimal guard — if amount < 0 { return ErrNegativeAmount } — nothing else. Run the test again — it passes.
3. REFACTOR: extract the guard into a small validateAmount helper if a second validation rule is about to land, rename ErrNegativeAmount for clarity if needed. Re-run the full test suite — still green.

Only after this cycle completes does the next behavior (e.g. rejecting an amount above the available balance) get its own RED step.