# Test Driven Development

> Primary methodology for implementing features and bugfixes - enforces test-first approach (RED-GREEN-REFACTOR) using senior-software-developer patterns for code quality. Use for single-task implementations; for multi-step projects, use writing-plans → subagent-driven-development instead.

- Skill: `darthlinuxer/test-driven-development` (Agent Skill, multi-file: 2 files)
- Install (CLI): `npx skillmds@latest add darthlinuxer/test-driven-development`
- Raw SKILL.md: https://api.skillmd.com/api/skills/darthlinuxer/test-driven-development/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: AI & ML, Productivity
- Author: darthlinuxer (https://skillmd.com/u/darthlinuxer)
- Updated: 2026-09-10
- Page: https://skillmd.com/skills/darthlinuxer/test-driven-development

---


# 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>
```python

# Python
def test_retries_failed_operations_3_times():
    attempts = [0]  # mutable for closure
    def operation():
        attempts[0] += 1
        if attempts[0] < 3:
            raise Exception('fail')
        return 'success'
    
    result = retry_operation(operation)
    
    assert result == 'success'
    assert attempts[0] == 3

# C#
[Fact]
public void RetriesFailedOperations3Times() {
    var attempts = 0;
    Func<string> operation = () => {
        attempts++;
        if (attempts < 3) throw new Exception("fail");
        return "success";
    };
    
    var result = RetryOperation(operation);
    
    Assert.Equal("success", result);
    Assert.Equal(3, attempts);
}

# TypeScript/Node.js
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>
```python

# Testing mock behavior instead of real behavior
def test_retry_works():
    mock = Mock(side_effect=[Exception(), Exception(), 'success'])
    retry_operation(mock)
    assert mock.call_count == 3  # ❌ Tests mock, not code
```
Vague name, tests mock not code - see [testing-anti-patterns.md](testing-anti-patterns.md)
</Bad>

**Requirements:**
- One behavior
- Clear name
- Real code (no mocks unless unavoidable)
- Never test mock behavior (see Anti-Pattern 1 in [testing-anti-patterns.md](testing-anti-patterns.md))

### Verify RED - Watch It Fail

**MANDATORY. Never skip.**

```bash

# Python
pytest path/to/test_module.py::test_name -v

# C#
dotnet test --filter TestName

# TypeScript/Node.js (Jest)
npm test path/to/test.test.ts

# TypeScript/Node.js (Vitest)
npx vitest 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>
```python

# Python
def retry_operation(fn):
    for i in range(3):
        try:
            return fn()
        except Exception as e:
            if i == 2:
                raise

# C#
public static T RetryOperation<T>(Func<T> fn) {
    for (int i = 0; i < 3; i++) {
        try {
            return fn();
        } catch (Exception e) {
            if (i == 2) throw;
        }
    }
    throw new InvalidOperationException("unreachable");
}

# 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>
```python

# YAGNI - Over-engineered before test demands it
def retry_operation(fn, max_retries=3, backoff='linear', on_retry=None):
    # Features not tested yet
    pass
```
Over-engineered
</Bad>

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

### Verify GREEN - Watch It Pass

**MANDATORY.**

```bash

# Python
pytest path/to/test_module.py -v

# C#
dotnet test

# TypeScript/Node.js
npm test  # or: npx vitest
```

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**
```python

# Python
def test_rejects_empty_email():
    result = submit_form({'email': ''})
    assert result['error'] == 'Email required'

# C#
[Fact]
public void RejectsEmptyEmail() {
    var result = SubmitForm(new FormData { Email = "" });
    Assert.Equal("Email required", result.Error);
}

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

**Verify RED**
```bash

# Output shows test fails (language-specific format)
FAIL: expected 'Email required', got undefined/None/null
```

**GREEN**
```python

# Python
def submit_form(data):
    if not data.get('email', '').strip():
        return {'error': 'Email required'}
    # ...

# C#
public FormResult SubmitForm(FormData data) {
    if (string.IsNullOrWhiteSpace(data.Email)) {
        return new FormResult { Error = "Email required" };
    }
    // ...
}

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

**Verify GREEN**
```bash
PASS  # All tests green
```

**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.

## Testing Anti-Patterns

When adding mocks or test utilities, read [testing-anti-patterns.md](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 your human partner's permission.

---

## Related Skills

**Uses:**
- **senior-software-developer**: Provides language-specific patterns and quality standards used during implementation
- **verification-before-completion**: Ensures test evidence before claiming success

**Used by:**
- **subagent-driven-development**: Subagents follow TDD for each task

**Relationship:**
- TDD is the **methodology** (test-first process)
- senior-software-developer provides the **patterns** (quality standards)
- For complex multi-step projects, use **writing-plans** → **subagent-driven-development** instead