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+---
+name: test-driven-development
+description: Use when implementing any feature or bugfix, before writing implementation code - write the test first, watch it fail, write minimal code to pass; ensures tests actually verify behavior by requiring failure first
+---
+
+# Test-Driven Development (TDD)
+
+## 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.**
+
+## When to Use
+
+**Always:**
+- New features
+- Bug fixes
+- Refactoring
+- Behavior changes
+
+**Exceptions (ask your human partner):**
+- 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
+
+```dot
+digraph tdd_cycle {
+    rankdir=LR;
+    red [label="RED\nWrite failing test", shape=box, style=filled, fillcolor="#ffcccc"];
+    verify_red [label="Verify fails\ncorrectly", shape=diamond];
+    green [label="GREEN\nMinimal code", shape=box, style=filled, fillcolor="#ccffcc"];
+    verify_green [label="Verify passes\nAll green", shape=diamond];
+    refactor [label="REFACTOR\nClean up", shape=box, style=filled, fillcolor="#ccccff"];
+    next [label="Next", shape=ellipse];
+
+    red -> verify_red;
+    verify_red -> green [label="yes"];
+    verify_red -> red [label="wrong\nfailure"];
+    green -> verify_green;
+    verify_green -> refactor [label="yes"];
+    verify_green -> green [label="no"];
+    refactor -> verify_green [label="stay\ngreen"];
+    verify_green -> next;
+    next -> red;
+}
+```
+
+### 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.**
+
+```bash
+npm test path/to/test.test.ts
+```
+
+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>
+```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.**
+
+```bash
+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**
+```typescript
+test('rejects empty email', async () => {
+  const result = await submitForm({ email: '' });
+  expect(result.error).toBe('Email required');
+});
+```
+
+**Verify RED**
+```bash
+$ npm test
+FAIL: expected 'Email required', got undefined
+```
+
+**GREEN**
+```typescript
+function submitForm(data: FormData) {
+  if (!data.email?.trim()) {
+    return { error: 'Email required' };
+  }
+  // ...
+}
+```
+
+**Verify GREEN**
+```bash
+$ 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 your human partner. |
+| 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.
+
+## Final Rule
+
+```
+Production code → test exists and failed first
+Otherwise → not TDD
+```
+
+No exceptions without your human partner's permission.