TDD Workflow

Disciplined test-first development with the Red-Green-Refactor cycle. Tests shape API design; fearless refactoring becomes possible.

Context

You are guiding the engineer through TDD. Your role is to help them:

• Write clear, focused tests that describe desired behavior before implementation
• Implement minimal code that passes the test (no over-engineering)
• Refactor with confidence, knowing tests will catch mistakes
• Keep cycles short (5-15 minutes) to maintain momentum
• Use test failures as design feedback

TDD is not just about testing; it's about using tests as a design tool to improve code clarity and maintainability.

Domain Context

Based on Kent Beck, Bob Martin, and J.B. Rainsberger:

• Red Phase: Write test describing desired behavior; it fails because feature doesn't exist. This failure is essential—it proves the test actually tests something.
• Green Phase: Write minimal code to pass the test. "Minimal" means: just enough logic; no premature optimization; no extra features.
• Refactor Phase: Improve code without changing behavior. Extract methods, improve names, reduce duplication. Tests protect against regressions.
• Design Feedback: If tests are hard to write, the API is hard to use. If mocking is complex, coupling is too tight. Tests reveal design problems early.
• Regression Safety: Previous tests continue passing, so refactoring is fearless. This enables continuous improvement.
• Cycle Length: 5-15 minutes per cycle. Longer cycles indicate the test is too complex; break it down.

When to Use This Skill

• Starting a new feature where behavior is well-defined
• Fixing a bug (write test for the bug first, then implement fix)
• Adding to a codebase already using TDD (maintain consistency)
• When you want to confidently refactor without fear of regressions
• When API design is uncertain (tests force clarity)

Prerequisites

Before starting TDD, confirm:

• Clear Requirements: Understand what "done" looks like for this feature
• Test Framework: Know how to write tests in your language (unittest, pytest, Jest, xUnit)
• Development Environment: Can run tests locally in <5 seconds
• Existing Tests: Review existing tests to match style and patterns
• Integration Plan: Where will this code plug into the system?

Instructions (Detailed)

1. Understand the Feature

Read requirements carefully. Ask clarifying questions:

• What is the input? What is the expected output?
• What are edge cases or error conditions?
• How does this interact with existing code?
• What business value does this provide?

Write down the acceptance criteria. These become test cases.

2. Write the First Failing Test (Red)

Write a single test that describes one piece of desired behavior. Make it fail.

Example (Python):

def test_user_email_validation_rejects_missing_at_symbol():
    # Arrange
    validator = EmailValidator()
    # Act
    is_valid = validator.is_valid("userexample.com")
    # Assert
    assert is_valid is False

Rules for this phase:

• One assertion per test (or tightly related assertions)
• Clear name: test_<subject>_<condition>_<expected_outcome>
• Test behavior, not implementation: Don't test private methods; test observable behavior
• Arrange-Act-Assert: Set up state, call code, verify outcome
• Make it fail first: Run the test; confirm it fails with the error you expect

3. Implement Minimal Code (Green)

Write the simplest code that passes the test. No optimization, no extra features.

Example (Python):

class EmailValidator:
    def is_valid(self, email: str) -> bool:
        return "@" in email

This is deliberately naive. It passes the test. Now the test can guide the next iteration.

Rules for this phase:

• No premature optimization: Avoid performance tweaks; optimize when profiling shows a problem
• No extra features: Implement only what the test requires
• Copy-paste is acceptable: Duplication is fixed in Refactor phase
• Comment if unclear: If you needed to think hard, add a comment for future readers

4. Verify the Test Passes (Green)

Run the test. It should pass.

python -m pytest test_email_validator.py::test_user_email_validation_rejects_missing_at_symbol -v

If it fails, debug the implementation, not the test. The test defines correctness.

5. Check Existing Tests Still Pass

Run the full test suite. No regressions.

python -m pytest tests/

If an existing test breaks, you've changed behavior unintentionally. Fix the implementation.

6. Refactor (Refactor)

Now improve the code: better names, extract methods, reduce duplication. Tests protect you.

Example: The naive implementation passes one test. Write the next test for a positive case:

def test_user_email_validation_accepts_valid_email():
    validator = EmailValidator()
    assert validator.is_valid("user@example.com") is True

This test fails (your naive implementation doesn't validate the domain). Now refactor:

class EmailValidator:
    def is_valid(self, email: str) -> bool:
        if not email or "@" not in email:
            return False
        local, domain = email.rsplit("@", 1)
        if not local or not domain or "." not in domain:
            return False
        return True

Run all tests. Both pass. The implementation evolved because tests revealed missing behavior.

7. Repeat: Write Next Test

Start again at step 2. Each cycle improves the implementation and test coverage.

Example test sequence for email validator:

1. Test missing @ → minimal implementation
2. Test valid email → refactor to handle domain
3. Test spaces in email → refactor validation
4. Test multiple @ symbols → refactor parsing
5. Test empty local part → refactor
6. Eventually: regex or use a library

Each test adds one behavioral requirement. The implementation grows steadily, staying simple.

8. Watch for Long Cycles

If a cycle takes >15 minutes:

• Break test into smaller pieces (test one behavior per test)
• Implementation might need a helper function (implement with next test)
• You might be over-engineering; step back and check requirements

Output Format

When using this skill, deliver:

1. Test Code: Failing test demonstrating desired behavior (Red)
2. Implementation: Minimal code passing the test (Green)
3. Refactored Code: Improved implementation if duplication emerged (Refactor)
4. Test Suite Status: All tests passing, no regressions
5. Next Test: What behavior should the next test verify?

Example output structure:

## Red: Write Test
[test code showing failure]

## Green: Implement
[minimal implementation]

## Verify
- Test passes: ✓
- Suite passes: ✓
- No regressions: ✓

## Refactor (if needed)
[improved code with better names/structure]

## Next Cycle
The next test should verify [specific behavior], because [reason].

Worked Example

Feature: User password strength validator

Requirement: Passwords must be 8+ characters, contain uppercase, lowercase, digit, and special character.

Cycle 1: Test minimum length

Red: Test

def test_password_rejects_too_short():
    validator = PasswordValidator()
    assert validator.is_strong("Pass1!") is False  # 6 chars, needs 8

Green: Implement

class PasswordValidator:
    def is_strong(self, password: str) -> bool:
        return len(password) >= 8

Verify: Test passes. Suite passes.

Cycle 2: Test requires uppercase

Red: Test

def test_password_rejects_missing_uppercase():
    validator = PasswordValidator()
    assert validator.is_strong("password1!") is False  # no uppercase

Green: Implement

class PasswordValidator:
    def is_strong(self, password: str) -> bool:
        if len(password) < 8:
            return False
        return any(c.isupper() for c in password)

Verify: Both tests pass.

Cycle 3: Test requires lowercase

Red: Test

def test_password_rejects_missing_lowercase():
    validator = PasswordValidator()
    assert validator.is_strong("PASSWORD1!") is False  # no lowercase

Green: Implement

class PasswordValidator:
    def is_strong(self, password: str) -> bool:
        if len(password) < 8:
            return False
        if not any(c.isupper() for c in password):
            return False
        return any(c.islower() for c in password)

Verify: All three tests pass.

Cycle 4: Test requires digit

Red: Test

def test_password_rejects_missing_digit():
    validator = PasswordValidator()
    assert validator.is_strong("Password!") is False  # no digit

Green: Implement

class PasswordValidator:
    def is_strong(self, password: str) -> bool:
        if len(password) < 8:
            return False
        if not any(c.isupper() for c in password):
            return False
        if not any(c.islower() for c in password):
            return False
        return any(c.isdigit() for c in password)

Verify: All four tests pass.

Cycle 5: Test requires special character

Red: Test

def test_password_rejects_missing_special_char():
    validator = PasswordValidator()
    assert validator.is_strong("Password1") is False  # no special char

Green: Implement

class PasswordValidator:
    SPECIAL_CHARS = "!@#$%^&*()-_=+[]{}|;:',.<>?/`~"

    def is_strong(self, password: str) -> bool:
        if len(password) < 8:
            return False
        if not any(c.isupper() for c in password):
            return False
        if not any(c.islower() for c in password):
            return False
        if not any(c.isdigit() for c in password):
            return False
        return any(c in self.SPECIAL_CHARS for c in password)

Verify: All five tests pass.

Cycle 6: Refactor

Notice repetition in validation checks. Extract helper:

class PasswordValidator:
    SPECIAL_CHARS = "!@#$%^&*()-_=+[]{}|;:',.<>?/`~"

    def is_strong(self, password: str) -> bool:
        return (
            self._has_min_length(password, 8)
            and self._has_uppercase(password)
            and self._has_lowercase(password)
            and self._has_digit(password)
            and self._has_special_char(password)
        )

    def _has_min_length(self, password: str, length: int) -> bool:
        return len(password) >= length

    def _has_uppercase(self, password: str) -> bool:
        return any(c.isupper() for c in password)

    def _has_lowercase(self, password: str) -> bool:
        return any(c.islower() for c in password)

    def _has_digit(self, password: str) -> bool:
        return any(c.isdigit() for c in password)

    def _has_special_char(self, password: str) -> bool:
        return any(c in self.SPECIAL_CHARS for c in password)

Verify: All tests still pass. Refactoring complete.

Cycle 7: Test happy path

Red: Test

def test_password_accepts_valid_strong_password():
    validator = PasswordValidator()
    assert validator.is_strong("MyPass1!") is True

Green: Already implemented; test passes immediately.

Decision Framework

When writing tests, use these decisions:

• If unclear how feature should behave: Write a test capturing your understanding. Discuss with product owner.
• If implementation is taking >15 min: Break cycle into smaller test. Implementation is too complex for one test.
• If test is hard to write: Consider design. Is the API easy to use? Does it have too many dependencies?
• If test requires complex mocking: Implementation has tight coupling. Consider refactoring.
• If test passes without new implementation: Test is redundant; skip it or combine with another test.
• If adding feature breaks existing tests: Don't change tests; fix implementation to satisfy both old and new requirements.

Anti-Patterns (Expanded)

1. Skipping Red Phase

Mistake: Write code, then write tests afterward.

Why LLMs make this: Implementing feels like progress; tests feel like overhead. Code-first pressure is high.

Guard: Before writing any implementation, write a test that currently fails. Run it; confirm failure message is clear.

Example:

• Bad: Write EmailValidator.is_valid() implementation, then write test
• Good: Write test that fails, then implement is_valid()

---

2. Writing Tests After Code

Mistake: Implement feature; then write tests to verify it works.

Why LLMs make this: Tests are supposed to document behavior; code already documents behavior.

Guard: Tests written first reveal design problems that post-hoc tests miss. Make it a workflow rule.

Example:

• Bad: Implement UserService.create_user(), then write test
• Good: Write test of UserService.create_user() first; implement after

---

3. Over-Engineering in Green Phase

Mistake: Implement "production-quality" code on first pass (optimization, generalization, error handling).

Why LLMs make this: Training emphasizes complete, robust implementations.

Guard: If implementation doesn't feel deliberately naive, you're over-engineering. Write simplest code that passes test; refactor later.

Example:

• Bad: Implement regex email validation on first test; refactor later
• Good: Implement "@" in email on first test; evolve with each test

---

4. Not Running Tests Frequently

Mistake: Write multiple tests without running; run them all at once.

Why LLMs make this: Batching feels efficient.

Guard: Run tests after each Red-Green-Refactor cycle. Rapid feedback (minutes, not hours) reveals problems immediately.

Example:

• Bad: Write 5 tests, implement 5 features, run tests once
• Good: Red-Green-Refactor each test individually; run after each cycle

---

5. Refactoring Without Running Tests

Mistake: Refactor code, then run tests later; hope nothing broke.

Why LLMs make this: Large refactors feel productive.

Guard: Before refactoring, tests must pass. After each small refactoring step (rename, extract, move), run tests. If tests fail, revert and try smaller step.

Example:

• Bad: Refactor entire validation class; run tests once at end
• Good: Rename one variable, run tests; extract one method, run tests; repeat

---

6. Unclear Test Names

Mistake: Write tests with names like test_email() or test_password_1().

Why LLMs make this: Generating descriptive names takes extra effort.

Guard: Test name should describe the condition and expected outcome. Read the test name aloud; does it explain what's being tested?

Example:

• Bad: test_email(), test_password_validation()
• Good: test_email_validation_rejects_missing_at_symbol(), test_password_validator_requires_min_8_chars()

---

Quality Checklist

Before considering a test complete, verify:

• Test name clearly describes behavior tested — Read aloud; is it understandable?
• Test currently fails (Red phase) — Run it; confirm it fails as expected
• Minimal implementation passes test (Green phase) — Implementation should feel naive, not complete
• All existing tests still pass — No regressions introduced
• Test uses Arrange-Act-Assert — Setup, call code, verify result
• One logical assertion per test — Test one behavior; separate tests for edge cases
• No test interdependencies — Tests can run in any order, independently
• Refactored code is cleaner — After Refactor phase, code should be easier to read
• Next test is identified — What behavior should the next cycle verify?
• Cycle time was <15 minutes — If longer, next test should be smaller

Further Reading

• Kent Beck, Test-Driven Development: By Example (2002) — Foundational; essential reading. Walks through real TDD examples with money and xUnit.
• Robert C. Martin, Clean Code (2008), Chapter 9 — Focuses on unit test quality: readability, independence, cleanliness.
• J.B. Rainsberger, Integrated Tests Are a Scam (2010) — Why isolated unit tests + TDD beats integration test-first approaches.
• Roy Osherove, The Art of Software Testing (2nd ed., 2011) — TDD strategy, test coverage, test maintainability.
• Steve Freeman & Nat Pryce, Growing Object-Oriented Software, Guided by Tests (2009) — TDD in practice on real projects; mockist testing approach.
• Sandro Mancuso, The Software Craftsman (2014) — Professionalism, TDD, and continuous improvement discipline.