Tdd

/tdd - Test-Driven Development Workflow

Strict TDD workflow: tests first, then implementation.

When to Use

• "Implement X using TDD"
• "Build this feature test-first"
• "Write tests for X then implement"
• Any feature where test coverage is critical
• Bug fixes that need regression tests

TDD Philosophy

Overview

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.

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 - Write Failing Test

Write one minimal test showing what should happen.

Good:

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:

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

Vague name, tests mock not code.

Requirements:

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

Verify RED - Watch It Fail

MANDATORY. Never skip.

npm test path/to/test.test.ts
# or
pytest path/to/test_file.py

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.

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 - over-engineered
}

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.

Common Rationalizations

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"
• "Keep as reference" or "adapt existing code"

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

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

Workflow Execution

Workflow Overview

┌────────────┐    ┌──────────┐    ┌──────────┐    ┌───────────┐
│   plan-    │───▶│ validator  │───▶│  implementer  │───▶│ validator  │
│   agent    │    │          │    │          │    │           │
└────────────┘    └──────────┘    └──────────┘    └───────────┘
   Design          Write           Implement        Verify
   approach        failing         minimal          all tests
                   tests           code             pass

Agent Sequence

Core Principle

NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST

Each agent follows the TDD contract:

• validator writes tests that MUST fail initially
• implementer writes MINIMAL code to make tests pass
• validator confirms the full suite passes

Execution

Phase 1: Plan Test Cases

Task(
  subagent_type="plan-agent",
  prompt="""
  Design TDD approach for: [FEATURE_NAME]

  Define:
  1. What behaviors need to be tested
  2. Edge cases to cover
  3. Expected test structure

  DO NOT write any implementation code.
  Output: Test plan document
  """
)

Phase 2: Write Failing Tests (RED)

Task(
  subagent_type="validator",
  prompt="""
  Write failing tests for: [FEATURE_NAME]

  Test plan: [from phase 1]

  Requirements:
  - Write tests FIRST
  - Run tests to confirm they FAIL
  - Tests must fail because feature is missing (not syntax errors)
  - Create clear test names describing expected behavior

  DO NOT write any implementation code.
  """
)

Phase 3: Implement (GREEN)

Task(
  subagent_type="implementer",
  prompt="""
  Implement MINIMAL code to pass tests: [FEATURE_NAME]

  Tests location: [test file path]

  Requirements:
  - Write ONLY enough code to make tests pass
  - No additional features beyond what tests require
  - No "improvements" or "enhancements"
  - Run tests after each change

  Follow Red-Green-Refactor strictly.
  """
)

Phase 4: Validate

Task(
  subagent_type="validator",
  prompt="""
  Validate TDD implementation: [FEATURE_NAME]

  - Run full test suite
  - Verify all new tests pass
  - Verify no existing tests broke
  - Check test coverage if available
  """
)

TDD Rules Enforced

1. validator cannot write implementation code
2. implementer cannot add untested features
3. Tests must fail before implementation
4. Tests must pass after implementation

Example

User: /tdd Add email validation to the signup form

Claude: Starting /tdd workflow for email validation...

Phase 1: Planning test cases...
[Spawns plan-agent]
Test plan:
- Valid email formats
- Invalid email formats
- Empty email rejection
- Edge cases (unicode, long emails)

Phase 2: Writing failing tests (RED)...
[Spawns validator]
✅ 8 tests written, all failing as expected

Phase 3: Implementing minimal code (GREEN)...
[Spawns implementer]
✅ All 8 tests now passing

Phase 4: Validating...
[Spawns validator]
✅ 247 tests passing (8 new), 0 failing

TDD workflow complete!

Refactor Phase (Optional)

After GREEN, you can add a refactor phase:

Task(
  subagent_type="implementer",
  prompt="""
  Refactor: [FEATURE_NAME]

  - Clean up code while keeping tests green
  - Remove duplication
  - Improve naming
  - Extract helpers if needed

  DO NOT add new behavior. Keep all tests passing.
  """
)

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