TDD enforces the Iron Law -- no production code without a failing test first -- through a strict RED-GREEN-REFACTOR loop: detect the stack and test framework, write one failing test for the next behavior, verify it fails for the *expected* reason (not a syntax error or missing fixture), write the simplest code that passes it, verify the full suite is green, refactor while staying green, then repeat for the next behavior.

It never skips a VERIFY step -- a test that passes on first run proves nothing and must be investigated, not accepted.

TDD Skill

Write the test first. Watch it fail. Write minimal code to pass.

The Iron Law

No production code without a failing test first.

Every line of production code must be justified by a test that failed without it. No exceptions. No shortcuts. No "I'll test after."

Agent Workflow

1. DETECT  -> Identify stack, test framework, existing test patterns
2. RED     -> Write ONE failing test for the next behavior
3. VERIFY  -> Run test, confirm it fails for the EXPECTED reason
   IF the test fails for a DIFFERENT reason (syntax error, wrong import, missing fixture) → fix the test itself and re-run. Do NOT proceed to GREEN until the failure matches the intended behavior.
4. GREEN   -> Write the SIMPLEST code that makes the test pass
5. VERIFY  -> Run tests, confirm ALL pass (new + existing)
6. REFACTOR -> Clean up while keeping all tests green
7. REPEAT  -> Next behavior = next failing test

CRITICAL: Never skip VERIFY steps. A test that passes on first run proves nothing.

RED-GREEN-REFACTOR Cycle

See references/red-green-refactor.md for the detailed cycle with rules and verification steps.

Reference Guide

Topic	Reference
Full RED-GREEN-REFACTOR cycle	red-green-refactor.md
Common mistakes and red flags	anti-patterns.md
Per-stack test commands	stack-commands.md

Quick Reference: Test Commands

Stack	Run Tests	Watch Mode
React/Next.js	`npx vitest run`	`npx vitest`
Laravel	`php artisan test`	`php artisan test --watch`
Swift	`swift test`	-
Generic TS	`bunx vitest run`	`bunx vitest`
Go	`go test ./...`	-
Rust	`cargo test`	`cargo watch -x test`

Forbidden Behaviors

Never write production code before a failing test
Never skip the VERIFY RED step
Never accept a test that passes on first run without investigation
Never mock what you can test directly
Never write more than one test at a time in RED phase
Never add features beyond what the current test requires