/tdd-mastery

name: tdd-mastery
description: Test-Driven Development Iron Law. Write the test first. Watch it fail. Write minimal code to pass. No exceptions.

<domain_overview>

🧪 TDD MASTERY: THE IRON LAW

> **Philosophy:** If you didn't watch the test fail, you don't know if it tests the right thing. TDD is not optional—it's the foundation of trustworthy code. **TEST-FIRST INTEGRITY MANDATE (CRITICAL):** Never write production code before a test exists and has been seen failing. AI-generated code often attempts to write implementation and tests simultaneously or implementation first. You MUST strictly adhere to the Red-Green-Refactor cycle. Any code submitted without a preceding failing test or that generates tests after the implementation must be rejected as "Legacy Code on Arrival".

---

🚨 THE IRON LAW

NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST

Write code before the test? **Delete it. Start over.**

**No exceptions:**

• Don't keep it as "reference"
• Don't "adapt" it while writing tests
• Don't look at it
• Delete means delete

Implement fresh from tests. Period. </domain_overview> <core_workflow>

🔴 RED-GREEN-REFACTOR CYCLE

Phase 1: RED - Write Failing Test

Write one minimal test showing what should happen.

**Good Example:**

test('retries failed operations 3 times', async () => {
  let attempts = 0;
  const operation = () => {
    attempts++;
    if (attempts < 3) throw new Error('fail');
    return 'success';
  };

  const result = await retryOperation(operation);

  expect(result).toBe('success');
  expect(attempts).toBe(3);
});

*Clear name, tests real behavior, one thing*

**Bad Example:**

test('retry works', async () => {
  const mock = jest.fn()
    .mockRejectedValueOnce(new Error())
    .mockResolvedValueOnce('success');
  await retryOperation(mock);
  expect(mock).toHaveBeenCalledTimes(2);
});

*Vague name, tests mock not code*

**Requirements:**

• One behavior per test
• Clear, descriptive name
• Real code (mocks only if unavoidable)

Phase 2: VERIFY RED - Watch It Fail

**MANDATORY. Never skip.**

npm test path/to/test.test.ts
# or
pytest tests/path/test.py::test_name -v

Confirm:

• Test fails (not errors)
• Failure message is expected
• Fails because feature missing (not typos)

**Test passes?** You're testing existing behavior. Fix test.

**Test errors?** Fix error, re-run until it fails correctly.

Phase 3: GREEN - Minimal Code

Write **simplest code** to pass the test.

**Good:**

async function retryOperation<T>(fn: () => Promise<T>): Promise<T> {
  for (let i = 0; i < 3; i++) {
    try {
      return await fn();
    } catch (e) {
      if (i === 2) throw e;
    }
  }
  throw new Error('unreachable');
}

*Just enough to pass*

**Bad:**

async function retryOperation<T>(
  fn: () => Promise<T>,
  options?: {
    maxRetries?: number;
    backoff?: 'linear' | 'exponential';
    onRetry?: (attempt: number) => void;
  }
): Promise<T> {
  // YAGNI - You Aren't Gonna Need It
}

*Over-engineered*

Don't add features, refactor other code, or "improve" beyond the test.

Phase 4: VERIFY GREEN - Watch It Pass

**MANDATORY.**

npm test path/to/test.test.ts

Confirm:

• Test passes
• Other tests still pass
• Output pristine (no errors, warnings)

**Test fails?** Fix code, not test.

**Other tests fail?** Fix now.

Phase 5: REFACTOR - Clean Up

After green only:

• Remove duplication
• Improve names
• Extract helpers

Keep tests green. Don't add behavior.

Phase 6: COMMIT

git add tests/path/test.ts src/path/file.ts
git commit -m "feat: add specific feature with tests"

**Repeat** for next behavior.

--- </core_workflow>

<quality_standards>

📋 GOOD TEST QUALITIES

| Quality | Good | Bad | |---------|------|-----| | **Minimal** | One thing. "and" in name? Split it. | `test('validates email and domain and whitespace')` | | **Clear** | Name describes behavior | `test('test1')` | | **Shows intent** | Demonstrates desired API | Obscures what code should do | | **Real behavior** | Tests actual code | Tests mock behavior |

---

🚫 COMMON RATIONALIZATIONS (ALL INVALID)

| Excuse | Reality | |--------|---------| | "Too simple to test" | Simple code breaks. Test takes 30 seconds. | | "I'll test after" | Tests passing immediately prove nothing. | | "Already manually tested" | Ad-hoc ≠ systematic. No record, can't re-run. | | "Deleting X hours is wasteful" | Sunk cost fallacy. Keeping unverified code is debt. | | "Keep as reference" | You'll adapt it. That's testing after. Delete means delete. | | "Need to explore first" | Fine. Throw away exploration, start with TDD. | | "Test hard = skip test" | Hard to test = hard to use. Simplify design. | | "TDD will slow me down" | TDD faster than debugging. Pragmatic = test-first. | | "Existing code has no tests" | You're improving it. Add tests for existing code. |

---

🚨 RED FLAGS - STOP AND START OVER

If you catch yourself:

• Writing code before test
• Test passes immediately
• Can't explain why test failed
• Tests added "later"
• "Just this once"
• "I already manually tested it"
• "Keep as reference"
• "TDD is dogmatic, I'm being pragmatic"

**ALL of these mean: Delete code. Start over with TDD.**

--- </quality_standards>

<bug_fix_protocol>

🐛 BUG FIX WORKFLOW

Bug found? Write failing test reproducing it. Follow TDD cycle.

**Example:**

Bug: Empty email accepted

RED:
test('rejects empty email', async () => {
  const result = await submitForm({ email: '' });
  expect(result.error).toBe('Email required');
});

VERIFY RED:
$ npm test
FAIL: expected 'Email required', got undefined

GREEN:
function submitForm(data: FormData) {
  if (!data.email?.trim()) {
    return { error: 'Email required' };
  }
  // ...
}

VERIFY GREEN:
$ npm test
PASS

**Never fix bugs without a test.**

--- </bug_fix_protocol>

<integration_and_tooling>

🔗 RALPH WIGGUM INTEGRATION

When Ralph Wiggum is act