ctx-tdd

/ctx-tdd -- Test-Driven Development

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.

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"? That's rationalization. See the table below.

---

The Iron Law

NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST

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

• 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.

```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 assertion focus
</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 the mock not the code

Requirements:

• One behavior per test
• Clear name describing that behavior
• Real code paths (mocks only when unavoidable)

Verify RED -- Watch It Fail

MANDATORY. Never skip.

Run the test. Confirm:

• Test fails (not errors from typos/syntax)
• Failure message matches your expectation
• Fails because the feature is missing, not because setup is broken

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

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

GREEN -- Minimal Code

Write the simplest code that makes the test pass.

```typescript async function retryOperation(fn: () => Promise): Promise { 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 ```typescript async function retryOperation( fn: () => Promise, options?: { maxRetries?: number; backoff?: 'linear' | 'exponential'; onRetry?: (attempt: number) => void; } ): Promise { /* YAGNI */ } ``` Over-engineered beyond what the test demands

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

Verify GREEN -- Watch It Pass

MANDATORY.

Run the test. Confirm:

• New test passes
• All existing tests still pass
• Output is clean (no warnings, no errors)

Test fails? Fix production code, not the test.

Other tests break? Fix those now before moving on.

REFACTOR -- Clean Up (After Green Only)

• Remove duplication
• Improve names
• Extract helpers

Keep tests green throughout. Don't add behavior during refactor.

Repeat

Next failing test for the next behavior.

---

Common Rationalizations

Excuse	Reality
"Too simple to test"	Simple code breaks. The test takes 30 seconds.
"I'll test after"	Tests passing immediately prove nothing.
"Tests after achieve the same goals"	Tests-after = "what does this do?" Tests-first = "what should this do?"
"Already manually tested"	Ad-hoc is not 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, then start with TDD.
"Test hard = design unclear"	Listen to the test. Hard to test = hard to use.
"TDD will slow me down"	TDD is faster than debugging. "Pragmatic" shortcuts = debugging in production.
"This is different because..."	It's not.

---

Red Flags -- STOP and Start Over

Any of these mean: delete the code and restart with TDD.

• Production code written before a test
• Test passes on first run (you're testing existing behavior)
• Can't explain why the test failed
• Tests added "later"
• Rationalizing "just this once"
• "Keep as reference" or "adapt existing code"

---

Bug Fix Workflow

Bug found? Write a failing test that reproduces it first. Then follow the cycle.

RED

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

Verify RED -- test fails, expected 'Email required', got undefined

GREEN

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

Verify GREEN -- all tests pass

REFACTOR -- extract validation if needed

Never fix bugs without a failing test first.

---

When Stuck

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

---

Verification Checklist

Before claiming work is complete:

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

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

---

Anti-Patterns

When adding mocks or test utilities, read ${CLAUDE_SKILL_DIR}/references/testing-anti-patterns.md to avoid common pitfalls. The short version: mocks are tools to isolate, not things to test.

---

Gotchas

• "Too simple to need TDD": Simple is where unexamined assumptions cause the most wasted work. The cycle can be fast, but you must follow it.
• Test runner discovery: Before writing the first test, check the project for an existing test runner (package.json scripts, Makefile targets, pytest.ini). Match the existing pattern.
• Mocking creep: If mock setup exceeds test logic, you're testing mocks, not code. Read the anti-patterns reference.
• Refactor drift: Refactoring is for cleaning up, not adding behavior. If you catch yourself adding a feature during refactor, stop -- write a new failing test first.

---

Skill Files

• SKILL.md -- this file (process, Iron Law, cycle, rationalizations)
• ${CLAUDE_SKILL_DIR}/references/testing-anti-patterns.md -- 5 anti-patterns with gate functions and Good/Bad examples