tunneleven/c4flow:tdd

/c4flow:tdd — Test-Driven Development

Phase: 3: Implementation (sub-agent phase within c4flow:code)

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

Core principle: If you didn't watch the test fail, you don't know if it tests the right thing.

Violating the letter of the rules is violating the spirit of the rules.

How TDD Fits C4Flow

TDD is embedded as a sub-agent phase in c4flow:code. Each task goes through a full RED-GREEN-REFACTOR cycle with a mandatory RED gate pause — the coordinator pauses after RED is confirmed and shows you the test + failure output before any implementation begins.

BEADS → CODE_LOOP (per task: TDD sub-agent with RED gate → verify → review → PR → merge)

RED gate format (from c4flow:code):

── RED STATE CONFIRMED ──────────────────────────────────
Task: <title> (<id>)
Test: <file>
Failure: <output>
Does this test correctly capture the requirement? [yes / adjust]
─────────────────────────────────────────────────────────

• Trivial test detection: if a test passes immediately (before implementation), it's flagged as invalid
• Adjust loop: if you say "adjust", the sub-agent revises and confirms failure again before proceeding
• See skills/code/SKILL.md for full coordinator instructions

When to Use

Always:

• New features
• Bug fixes
• Refactoring
• Behavior changes

Exceptions (ask the user):

• Throwaway prototypes
• Generated code
• Configuration files

Thinking "skip TDD just this once"? Stop. That's rationalization.

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.

Red-Green-Refactor

  ┌──────────┐     ┌──────────┐     ┌──────────┐
  │   RED    │────▶│  GREEN   │────▶│ REFACTOR │
  │  Write   │     │ Minimal  │     │ Clean up │
  │ failing  │     │  code    │     │          │
  │  test    │     │ to pass  │     │          │
  └──────────┘     └──────────┘     └────┬─────┘
       ▲                                  │
       └──────────────────────────────────┘
                    Next test

RED - Write Failing Test

Write one minimal test showing what should happen.

<Good> ```typescript 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
</Good>

<Bad>
```typescript
test('retry works', async () => {
  const mock = jest.fn()
    .mockRejectedValueOnce(new Error())
    .mockRejectedValueOnce(new Error())
    .mockResolvedValueOnce('success');
  await retryOperation(mock);
  expect(mock).toHaveBeenCalledTimes(3);
});

Vague name, tests mock not code </Bad>

Requirements:

• One behavior
• Clear name
• Real code (no mocks unless unavoidable)

Verify RED - Watch It Fail

MANDATORY. Never skip.

# Use whatever test runner the project has
npm test path/to/test.test.ts
# or: pytest path/to/test.py
# or: go test ./path/to/...

Confirm:

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

Test passes? Two possibilities — distinguish them carefully:

1. Test is trivial — it passes because it tests nothing meaningful. Rewrite it.
2. Code already exists — the feature was already implemented. Verify this, then write a test for an unimplemented edge case or extend the feature.

TRIVIAL TEST examples — these are invalid, rewrite them:

// ❌ Trivial: tests language semantics, not your code
test('adds numbers', () => {
  expect(1 + 1).toBe(2);
});

// ❌ Trivial: tests a getter that was already there before this task
test('returns name', () => {
  const user = new User({ name: 'Alice' });
  expect(user.name).toBe('Alice');  // getter existed before this task
});

// ❌ Trivial: undefined === undefined, not a behavior test
test('uninitialized cache returns nothing', () => {
  const cache = new Cache();
  expect(cache.get('key')).toBeUndefined();  // passes before any implementation
});

Valid RED that happens to look like it passes — check first if the implementation already exists:

// This test passes? Check if retryOperation() exists in the codebase.
// If it doesn't exist → the test runner found a syntax error or name conflict, not a pass.
// If it does exist → prior work already shipped this. Write a test for something NOT yet done.
test('retries failed operations 3 times', async () => {
  ...
});

When rewriting a trivial test, ask: "What behavior would break if I deleted the implementation?" Write that test instead.

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

GREEN - Minimal Code

Write simplest code to pass the test.

<Good> ```typescript 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 </Good> <Bad> ```typescript async function retryOperation<T>( fn: () => Promise<T>, options?: { maxRetries?: number; backoff?: 'linear' | 'exponential'; onRetry?: (attempt: number) => void; } ): Promise<T> { // YAGNI } ``` Over-engineered </Bad>

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

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.

REFACTOR - Clean Up

After green only:

• Remove duplication
• Improve names
• Extract helpers

Keep tests green. Don't add behavior.

Repeat

Next failing test for next feature.

Good Tests

| 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 |

Why Order Matters

"I'll write tests after to verify it works"

Tests written after code pass immediately. Passing immediately proves nothing:

• Might test wrong thing
• Might test implementation, not behavior
• Might miss edge cases you forgot
• You never saw it catch the bug

Test-first forces you to see the test fail, proving it actually tests something.

"I already manually tested all the edge cases"

Manual testing is ad-hoc. You think you tested everything but:

• No record of what you tested
• Can't re-run when code changes
• Easy to forget cases under pressure
• "It worked when I tried it" != comprehensive

Automated tests are systematic. They run the same way every time.

"Deleting X hours of work is wasteful"

Sunk cost fallacy. The time is already gone. Your choice now:

• Delete and rewrite with TDD (X more hours, high confidence)
• Keep it and add tests after (30 min, low confidence, likely bugs)

The "waste" is keeping code you can't trust. Working code without real tests is technical debt.

"TDD is dogmatic, being pragmatic means adapting"

TDD IS pragmatic:

• Finds bugs before commit (faster than debugging after)
• Prevents regressions (tests catch breaks immediately)
• Documents behavior (tests show how to use code)
• Enables refactoring (change freely, tests catch breaks)

"Pragmatic" shortcuts = debugging in production = slower.

"Tests after achieve the same goals - it's spirit not ritual"

No. Tests-after answer "What does this do?" Tests-first answer "What should this do?"

Tests-after are biased by your implementation. You test what you built, not what's required. You verify remembered edge cases, not discovered ones.

Tests-first force edge case discovery before implementing. Tests-after verify you remembered everything (you didn't).

30 minutes of tests after != TDD. You get coverage, lose proof tests work.

Common Rationalizations

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

Red Flags - STOP and Start Over

• Code before test
• Test after implementation
• Test passes immediately
• Can't explain why test failed
• Tests added "later"
• Rationalizing "just this once"
• "I already manually tested it"
• "Tests after achieve the same purpose"
• "It's about spirit not ritual"
• "Keep as reference" or "adapt existing code"
• "Already spent X hours, deleting is wasteful"
• "TDD is dogmatic, I'm being pragmatic"
• "This is different because..."

All of these mean: Delete code. Start over with TDD.

Example: Bug Fix

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

REFACTOR Extract validation for multiple fields if needed.

Verification Checklist

Before marking work complete:

• [ ] Every new function/method has a test
• [ ] Watched each test fail before implementing
• [ ] Each test failed for expected reason (feature missing, not typo)
• [ ] Wrote minimal code to pass each test
• [ ] All tests pass
• [ ] Output pristine (no errors, warnings)
• [ ] Tests use real code (mocks only if unavoidable)
• [ ] Edge cases and errors covered

Can't check all boxes? You skipped TDD. Start over.

When Stuck

| Problem | Solution | |---------|----------| | Don't know how to test | Write wished-for API. Write assertion first. Ask the user. | | Test too complicated | Design too complicated. Simplify interface. | | Must mock everything | Code too coupled. Use dependency injection. | | Test setup huge | Extract helpers. Still complex? Simplify design. |

Debugging Integration

Bug found? Write failing test reproducing it. Follow TDD cycle. Test proves fix and prevents regression.

Never fix bugs without a test.

Testing Anti-Patterns

When adding mocks or test utilities, read skills/tdd/testing-anti-patterns.md to avoid common pitfalls:

• Testing mock behavior instead of real behavior
• Adding test-only methods to production classes
• Mocking without understanding dependencies

Final Rule

Production code -> test exists and failed first
Otherwise -> not TDD

No exceptions without the user's permission.