majiayu000 Implement minimal code to make failing tests pass in TDD green phase:
[Extended thinking: This tool uses the test-automator agent to implement the minimal code necessary to make tests pass. It focuses on simplicity, avoiding over-engineering while ensuring all tests become green.]
Implementation Process
Use Task tool with subagent_type="unit-testing::test-automator" to implement minimal passing code.
Prompt: "Implement MINIMAL code to make these failing tests pass: $ARGUMENTS. Follow TDD green phase principles:
Pre-Implementation Analysis
- Review all failing tests and their error messages
- Identify the simplest path to make tests pass
- Map test requirements to minimal implementation needs
- Avoid premature optimization or over-engineering
- Focus only on making tests green, not perfect code
Implementation Strategy
- Fake It: Return hard-coded values when appropriate
- Obvious Implementation: When solution is trivial and clear
- Triangulation: Generalize only when multiple tests require it
- Start with the simplest test and work incrementally
- One test at a time - don't try to pass all at once
Code Structure Guidelines
- Write the minimal code that could possibly work
- Avoid adding functionality not required by tests
- Use simple data structures initially
- Defer architectural decisions until refactor phase
- Keep methods/functions small and focused
- Don't add error handling unless tests require it
Language-Specific Patterns
- JavaScript/TypeScript: Simple functions, avoid classes initially
- Python: Functions before classes, simple returns
- Java: Minimal class structure, no patterns yet
- C#: Basic implementations, no interfaces yet
- Go: Simple functions, defer goroutines/channels
- Ruby: Procedural before object-oriented when possible
Progressive Implementation
- Make first test pass with simplest possible code
- Run tests after each change to verify progress
- Add just enough code for next failing test
- Resist urge to implement beyond test requirements
- Keep track of technical debt for refactor phase
- Document assumptions and shortcuts taken
Common Green Phase Techniques
- Hard-coded returns for initial tests
- Simple if/else for limited test cases
- Basic loops only when iteration tests require
- Minimal data structures (arrays before complex objects)
- In-memory storage before database integration
- Synchronous before asynchronous implementation
Success Criteria ✓ All tests pass (green) ✓ No extra functionality beyond test requirements ✓ Code is readable even if not optimal ✓ No broken existing functionality ✓ Implementation time is minimized ✓ Clear path to refactoring identified
Anti-Patterns to Avoid
- Gold plating or adding unrequested features
- Implementing design patterns prematurely
- Complex abstractions without test justification
- Performance optimizations without metrics
- Adding tests during green phase
- Refactoring during implementation
- Ignoring test failures to move forward
Implementation Metrics
- Time to green: Track implementation duration
- Lines of code: Measure implementation size
- Cyclomatic complexity: Keep it low initially
- Test pass rate: Must reach 100%
- Code coverage: Verify all paths tested
Validation Steps
- Run all tests and confirm they pass
- Verify no regression in existing tests
- Check that implementation is truly minimal
- Document any technical debt created
- Prepare notes for refactoring phase
Output should include:
- Complete implementation code
- Test execution results showing all green
- List of shortcuts taken for later refactoring
- Implementation time metrics
- Technical debt documentation
- Readiness assessment for refactor phase"
Post-Implementation Checks
After implementation:
- Run full test suite to confirm all tests pass
- Verify no existing tests were broken
- Document areas needing refactoring
- Check implementation is truly minimal
- Record implementation time for metrics
Recovery Process
If tests still fail:
- Review test requirements carefully
- Check for misunderstood assertions
- Add minimal code to address specific failures
- Avoid the temptation to rewrite from scratch
- Consider if tests themselves need adjustment
Integration Points
- Follows from tdd-red.md test creation
- Prepares for tdd-refactor.md improvements
- Updates test coverage metrics
- Triggers CI/CD pipeline verification
- Documents technical debt for tracking
Best Practices
- Embrace "good enough" for this phase
- Speed over perfection (perfection comes in refactor)
- Make it work, then make it right, then make it fast
- Trust that refactoring phase will improve code
- Keep changes small and incremental
- Celebrate reaching green state!
Complete Implementation Examples
Example 1: Minimal → Production-Ready (User Service)
Test Requirements:
describe('UserService', () => {
it('should create a new user', async () => {
const user = await userService.create({ email: 'test@example.com', name: 'Test' });
expect(user.id).toBeDefined();
expect(user.email).toBe('test@example.com');
});
it('should find user by email', async () => {
await userService.create({ email: 'test@example.com', name: 'Test' });
const user = await userService.findByEmail('test@example.com');
expect(user).toBeDefined();
});
});
Stage 1: Fake It (Minimal)
class UserService {
create(data: { email: string; name: string }) {
return { id: '123', email: data.email, name: data.name };
}
findByEmail(email: string) {
return { id: '123', email: email, name: 'Test' };
}
}
Tests pass. Implementation is obviously fake but validates test structure.
Stage 2: Simple Real Implementation
class UserService {
private users: Map<string, User> = new Map();
private nextId = 1;
create(data: { email: string; name: string }) {
const user = { id: String(this.nextId++), ...data };
this.users.set(user.email, user);
return user;
}
findByEmail(email: string) {
return this.users.get(email) || null;
}
}
In-memory storage. Tests pass. Good enough for green phase.
Stage 3: Production-Ready (Refactor Phase)
class UserService {
constructor(private db: Database) {}
async create(data: { email: string; name: string }) {
const existing = await this.db.query('SELECT * FROM users WHERE email = ?', [data.email]);
if (existing) throw new Error('User exists');
const id = await this.db.insert('users', data);
return { id, ...data };
}
async findByEmail(email: string) {
return this.db.queryOne('SELECT * FROM users WHERE email = ?', [email]);
}
}
Database integration, error handling, validation - saved for refactor phase.
Example 2: API-First Implementation (Express)
Test Requirements:
describe('POST /api/tasks', () => {
it('should create task and return 201', async () => {
const res = await request(app)
.post('/api/tasks')
.send({ title: 'Test Task' });
expect(res.status).toBe(201);
expect(res.body.id).toBeDefined();
expect(res.body.title).toBe('Test Task');
});
});
Stage 1: Hardcoded Response
app.post('/api/tasks', (req, res) => {
res.status(201).json({ id: '1', title: req.body.title });
});
Tests pass immediately. No logic needed yet.
Stage 2: Simple Logic
let tasks = [];
let nextId = 1;
app.post('/api/tasks', (req, res) => {
const task = { id: String(nextId++), title: req.body.title };
tasks.push(task);
res.status(201).json(task);
});
Minimal state management. Ready for more tests.
Stage 3: Layered Architecture (Refactor)
// Controller
app.post('/api/tasks', async (req, res) => {
try {
const task = await taskService.create(req.body);
res.status(201).json(task);
} catch (error) {
res.status(400).json({ error: error.message });
}
});
// Service layer
class TaskService {
constructor(private repository: TaskRepository) {}
async create(data: CreateTaskDto): Promise<Task> {
this.validate(data);
return this.repository.save(data);
}
}
Proper separation of concerns added during refactor phase.
Example 3: Database Integration (Django)
Test Requirements:
def test_product_creation():
product = Product.objects.create(name="Widget", price=9.99)
assert product.id is not None
assert product.name == "Widget"
def test_product_price_validation():
with pytest.raises(ValidationError):
Product.objects.create(name="Widget", price=-1)
Stage 1: Model Only
class Product(models.Model):
name = models.CharField(max_length=200)
price = models.DecimalField(max_digits=10, decimal_places=2)
First test passes. Second test fails - validation not implemented.
Stage 2: Add Validation
class Product(models.Model):
name = models.CharField(max_length=200)
price = models.DecimalField(max_digits=10, decimal_places=2)
def clean(self):
if self.price < 0:
raise ValidationError("Price cannot be negative")
def save(self, *args, **kwargs):
self.clean()
super().save(*args, **kwargs)
All tests pass. Minimal validation logic added.
Stage 3: Rich Domain Model (Refactor)
class Product(models.Model):
name = models.CharField(max_length=200)
price = models.DecimalField(max_digits=10, decimal_places=2)
category = models.ForeignKey(Category,
created_at = models.DateTimeField(auto_now_add=True)
updated_at = models.DateTimeField(auto_now=True)
class Meta:
indexes = [models.Index(fields=['category', '-created_at'])]
def clean(self):
if self.price < 0:
raise ValidationError("Price cannot be negative")
if self.price > 10000:
raise ValidationError("Price exceeds maximum")
def apply_discount(self, percentage: float) -> Decimal:
return self.price * (1 - percentage / 100)
Additional features, indexes, business logic added when needed.
Example 4: React Component Implementation
Test Requirements:
describe('UserProfile', () => {
it('should display user name', () => {
render(<UserProfile user={{ name: 'John', email: 'john@test.com' }} />);
expect(screen.getByText('John')).toBeInTheDocument();
});
it('should display email', () => {
render(<UserProfile user={{ name: 'John', email: 'john@test.com' }} />);
expect(screen.getByText('john@test.com')).toBeInTheDocument();
});
});
Stage 1: Minimal JSX
interface UserProfileProps {
user: { name: string; email: string };
}
const UserProfile: React.FC<UserProfileProps> = ({ user }) => (
<div>
<div>{user.name}</div>
<div>{user.email}</div>
</div>
);
Tests pass. No styling, no structure.
Stage 2: Basic Structure
const UserProfile: React.FC<UserProfileProps> = ({ user }) => (
<div className="user-profile">
<h2>{user.name}</h2>
<p>{user.email}</p>
</div>
);
Added semantic HTML, className for styling hook.
Stage 3: Production Component (Refactor)
const UserProfile: React.FC<UserProfileProps> = ({ user }) => {
const [isEditing, setIsEditing] = useState(false);
return (
<div className="user-profile" role="article" aria-label="User profile">
<header>
<h2>{user.name}</h2>
<button => setIsEditing(true)} aria-label="Edit profile">
Edit
</button>
</header>
<section>
<p>{user.email}</p>
{user.bio && <p>{user.bio}</p>}
</section>
</div>
);
};
Accessibility, interaction, additional features added incrementally.
Decision Frameworks
Framework 1: Fake vs. Real Implementation
When to Fake It:
- First test for a new feature
- Complex external dependencies (payment gateways, APIs)
- Implementation approach is still uncertain
- Need to validate test structure first
- Time pressure to see all tests green
When to Go Real:
- Second or third test reveals pattern
- Implementation is obvious and simple
- Faking would be more complex than real code
- Need to test integration points
- Tests explicitly require real behavior
Decision Matrix:
Complexity Low | High
↓ | ↓
Simple → REAL | FAKE first, real later
Complex → REAL | FAKE, evaluate alternatives
Framework 2: Complexity Trade-off Analysis
Simplicity Score Calculation:
Score = (Lines of Code) + (Cyclomatic Complexity × 2) + (Dependencies × 3)
< 20 → Simple enough, implement directly
20-50 → Consider simpler alternative
> 50 → Defer complexity to refactor phase
Example Evaluation:
// Option A: Direct implementation (Score: 45)
function calculateShipping(weight: number, distance: number, express: boolean): number {
let base = weight * 0.5 + distance * 0.1;
if (express) base *= 2;
if (weight > 50) base += 10;
if (distance > 1000) base += 20;
return base;
}
// Option B: Simplest for green phase (Score: 15)
function calculateShipping(weight: number, distance: number, express: boolean): number {
return express ? 50 : 25; // Fake it until more tests drive real logic
}
Choose Option B for green phase, evolve to Option A as tests require.
Framework 3: Performance Consideration Timing
Green Phase: Focus on Correctness
❌ Avoid:
- Caching strategies
- Database query optimization
- Algorithmic complexity improvements
- Premature memory optimization
✓ Accept:
- O(n²) if it makes code simpler
- Multiple database queries
- Synchronous operations
- Inefficient but clear algorithms
When Performance Matters in Green Phase:
- Performance is explicit test requirement
- Implementation would cause timeout in test suite
- Memory leak would crash tests
- Resource exhaustion prevents testing
Performance Testing Integration:
// Add performance test AFTER functional tests pass
describe('Performance', () => {
it('should handle 1000 users within 100ms', () => {
const start = Date.now();
for (let i = 0; i < 1000; i++) {
userService.create({ email: `user${i}@test.com`, name: `User ${i}` });
}
expect(Date.now() - start).toBeLessThan(100);
});
});
Framework-Specific Patterns
React Patterns
Simple Component → Hooks → Context:
// Green Phase: Props only
const Counter = ({ count, onIncrement }) => (
<button
);
// Refactor: Add hooks
const Counter = () => {
const [count, setCount] = useState(0);
return <button => setCount(c => c + 1)}>{count}</button>;
};
// Refactor: Extract to context
const Counter = () => {
const { count, increment } = useCounter();
return <button
};
Django Patterns
Function View → Class View → Generic View:
# Green Phase: Simple function
def product_list(request):
products = Product.objects.all()
return JsonResponse({'products': list(products.values())})
# Refactor: Class-based view
class ProductListView(View):
def get(self, request):
products = Product.objects.all()
return JsonResponse({'products': list(products.values())})
# Refactor: Generic view
class ProductListView(ListView):
model = Product
context_object_name = 'products'
Express Patterns
Inline → Middleware → Service Layer:
// Green Phase: Inline logic
app.post('/api/users', (req, res) => {
const user = { id: Date.now(), ...req.body };
users.push(user);
res.json(user);
});
// Refactor: Extract middleware
app.post('/api/users', validateUser, (req, res) => {
const user = userService.create(req.body);
res.json(user);
});
// Refactor: Full layering
app.post('/api/users',
validateUser,
asyncHandler(userController.create)
);
Refactoring Resistance Patterns
Pattern 1: Test Anchor Points
Keep tests green during refactoring by maintaining interface contracts:
// Original implementation (tests green)
function calculateTotal(items: Item[]): number {
return items.reduce((sum, item) => sum + item.price, 0);
}
// Refactoring: Add tax calculation (keep interface)
function calculateTotal(items: Item[]): number {
const subtotal = items.reduce((sum, item) => sum + item.price, 0);
const tax = subtotal * 0.1;
return subtotal + tax;
}
// Tests still green because return type/behavior unchanged
Pattern 2: Parallel Implementation
Run old and new implementations side by side:
def process_order(order):
# Old implementation (tests depend on this)
result_old = legacy_process(order)
# New implementation (testing in parallel)
result_new = new_process(order)
# Verify they match
assert result_old == result_new, "Implementation mismatch"
return result_old # Keep tests green
Pattern 3: Feature Flags for Refactoring
class PaymentService {
processPayment(amount) {
if (config.USE_NEW_PAYMENT_PROCESSOR) {
return this.newPaymentProcessor(amount);
}
return this.legacyPaymentProcessor(amount);
}
}
Performance-First Green Phase Strategies
Strategy 1: Type-Driven Development
Use types to guide minimal implementation:
// Types define contract
interface UserRepository {
findById(id: string): Promise<User | null>;
save(user: User): Promise<void>;
}
// Green phase: In-memory implementation
class InMemoryUserRepository implements UserRepository {
private users = new Map<string, User>();
async findById(id: string) {
return this.users.get(id) || null;
}
async save(user: User) {
this.users.set(user.id, user);
}
}
// Refactor: Database implementation (same interface)
class DatabaseUserRepository implements UserRepository {
constructor(private db: Database) {}
async findById(id: string) {
return this.db.query('SELECT * FROM users WHERE id = ?', [id]);
}
async save(user: User) {
await this.db.insert('users', user);
}
}
Strategy 2: Contract Testing Integration
// Define contract
const userServiceContract = {
create: {
input: { email: 'string', name: 'string' },
output: { id: 'string', email: 'string', name: 'string' }
}
};
// Green phase: Implementation matches contract
class UserService {
create(data: { email: string; name: string }) {
return { id: '123', ...data }; // Minimal but contract-compliant
}
}
// Contract test ensures compliance
describe('UserService Contract', () => {
it('should match create contract', () => {
const result = userService.create({ email: 'test@test.com', name: 'Test' });
expect(typeof result.id).toBe('string');
expect(typeof result.email).toBe('string');
expect(typeof result.name).toBe('string');
});
});
Strategy 3: Continuous Refactoring Workflow
Micro-Refactoring During Green Phase:
# Test passes with this
def calculate_discount(price, customer_type):
if customer_type == 'premium':
return price * 0.8
return price
# Immediate micro-refactor (tests still green)
DISCOUNT_RATES = {
'premium': 0.8,
'standard': 1.0
}
def calculate_discount(price, customer_type):
rate = DISCOUNT_RATES.get(customer_type, 1.0)
return price * rate
Safe Refactoring Checklist:
- ✓ Tests green before refactoring
- ✓ Change one thing at a time
- ✓ Run tests after each change
- ✓ Commit after each successful refactor
- ✓ No behavior changes, only structure
Modern Development Practices (2024/2025)
Type-Driven Development
Python Type Hints:
from typing import Optional, List
from dataclasses import dataclass
@dataclass
class User:
id: str
email: str
name: str
class UserService:
def create(self, email: str, name: str) -> User:
return User(id="123", email=email, name=name)
def find_by_email(self, email: str) -> Optional[User]:
return None # Minimal implementation
TypeScript Strict Mode:
// Enable strict mode in tsconfig.json
{
"compilerOptions": {
"strict": true,
"noUncheckedIndexedAccess": true,
"exactOptionalPropertyTypes": true
}
}
// Implementation guided by types
interface CreateUserDto {
email: string;
name: string;
}
class UserService {
create(data: CreateUserDto): User {
// Type system enforces contract
return { id: '123', email: data.email, name: data.name };
}
}
AI-Assisted Green Phase
Using Copilot/AI Tools:
- Write test first (human-driven)
- Let AI suggest minimal implementation
- Verify suggestion passes tests
- Accept if truly minimal, reject if over-engineered
- Iterate with AI for refactoring phase
AI Prompt Pattern:
Given these failing tests:
[paste tests]
Provide the MINIMAL implementation that makes tests pass.
Do not add error handling, validation, or features beyond test requirements.
Focus on simplicity over completeness.
Cloud-Native Patterns
Local → Container → Cloud:
// Green Phase: Local implementation
class CacheService {
private cache = new Map();
get(key) { return this.cache.get(key); }
set(key, value) { this.cache.set(key, value); }
}
// Refactor: Redis-compatible interface
class CacheService {
constructor(private redis) {}
async get(key) { return this.redis.get(key); }
async set(key, value) { return this.redis.set(key, value); }
}
// Production: Distributed cache with fallback
class CacheService {
constructor(private redis, private fallback) {}
async get(key) {
try {
return await this.redis.get(key);
} catch {
return this.fallback.get(key);
}
}
}
Observability-Driven Development
Add observability hooks during green phase:
class OrderService {
async createOrder(data: CreateOrderDto): Promise<Order> {
console.log('[OrderService] Creating order', { data }); // Simple logging
const order = { id: '123', ...data };
console.log('[OrderService] Order created', { orderId: order.id }); // Success log
return order;
}
}
// Refactor: Structured logging
class OrderService {
constructor(private logger: Logger) {}
async createOrder(data: CreateOrderDto): Promise<Order> {
this.logger.info('order.create.start', { data });
const order = await this.repository.save(data);
this.logger.info('order.create.success', {
orderId: order.id,
duration: Date.now() - start
});
return order;
}
}
Tests to make pass: $ARGUMENTS