Agent Skills: Implement

Implement

Write the implementation code to make all tests pass. This is the "green" phase of TDD.

Purpose

Implementation transforms stubs into working code:

• Make tests pass with minimal code
• Follow the design architecture
• Adhere to project standards
• Produce working, tested functionality

Input

Expect from orchestrator:

• Stubs (interfaces and stub implementations to complete)
• Tests (failing tests that define expected behaviour)
• Design (architecture decisions and data flow)
• Project standards (patterns, naming, style)

Process

1. Review failing tests

Understand what each test expects:

# Run tests to see current failures
npm test -- --verbose

Group tests by component and prioritise:

• Start with lowest-level components (repositories)
• Move up to services
• Finish with controllers/handlers

2. Implement incrementally

For each component, implement one method at a time:

1. Pick one failing test
2. Write minimal code to pass it
3. Run tests to verify
4. Repeat until all tests for that method pass
5. Move to next method

3. Replace stub implementations

Transform stubs into real implementations:

Before (stub):

async save(user: User): Promise<User> {
  throw new Error('Not implemented: UserRepository.save');
}

After (implementation):

async save(user: User): Promise<User> {
  const result = await this.db.collection('users').insertOne(user);
  return { ...user, id: result.insertedId.toString() };
}

4. Follow design patterns

Implement according to the design document:

• Respect layer boundaries
• Use dependency injection as designed
• Follow error handling strategy
• Implement data transformations as specified

5. Handle errors

Implement error handling as specified in design:

async createUser(input: CreateUserInput): Promise<User> {
  // Check for duplicate
  const existing = await this.userRepository.findByEmail(input.email);
  if (existing) {
    throw new DuplicateEmailError(input.email);
  }

  // Create user
  const user: User = {
    id: generateId(),
    email: input.email.toLowerCase(),
    name: input.name,
    createdAt: new Date(),
  };

  // Persist
  const saved = await this.userRepository.save(user);

  // Publish event
  await this.eventPublisher.publish(new UserCreatedEvent(saved));

  return saved;
}

6. Run tests continuously

After each change, run tests:

# Watch mode
npm test -- --watch

# Or run manually
npm test

Track progress: 0/12 → 3/12 → 7/12 → 12/12

7. Achieve green

Continue until all tests pass:

PASS  tests/services/user.service.test.ts
  UserService
    createUser
      ✓ should return user with generated id (5ms)
      ✓ should persist user to repository (2ms)
      ✓ should throw DuplicateEmailError when email exists (3ms)
      ✓ should normalise email to lowercase (2ms)
      ...

Test Suites: 1 passed, 1 total
Tests:       12 passed, 12 total

8. Database migrations (if applicable)

If the feature requires schema changes to make tests pass:

1. Create migration file using the project's migration framework
2. Create corresponding rollback (down) migration
3. Apply migration to dev database
4. Verify schema matches implementation expectations
5. Record migration files in output

Migrations are implementation detail — they exist to make tests pass. They are NOT a separate phase.

If migration fails:

1. Roll back the migration
2. Fix the migration file
3. Re-apply and re-verify

Implementation guidelines

Write minimal code

• Only write code needed to pass tests
• Avoid premature optimisation
• Don't add features not covered by tests

Follow SOLID principles

• Single responsibility: One reason to change
• Open/closed: Open for extension, closed for modification
• Liskov substitution: Subtypes substitutable for base types
• Interface segregation: Specific interfaces over general
• Dependency inversion: Depend on abstractions

Handle edge cases

Tests should cover edge cases; implementation should handle them:

// Normalise email
const normalisedEmail = input.email.toLowerCase().trim();

// Validate length
if (normalisedEmail.length > MAX_EMAIL_LENGTH) {
  throw new ValidationError('Email too long');
}

Keep it simple

• Prefer clarity over cleverness
• Use descriptive variable names
• Avoid deep nesting
• Extract helper functions when complexity grows

Output

implement:
  story_id: "US-001"
  files_modified:
    - path: "src/repositories/user.repository.ts"
      changes: "Full implementation of IUserRepository"
      methods_implemented: ["save", "findById", "findByEmail"]
    - path: "src/services/user.service.ts"
      changes: "Full implementation of UserService"
      methods_implemented: ["createUser", "findById"]
    - path: "src/controllers/user.controller.ts"
      changes: "Full implementation of UserController"
      methods_implemented: ["createUser", "getUser"]

  files_created:
    - path: "src/errors/duplicate-email.error.ts"
      reason: "Custom error type for duplicate email"

  run_result:
    command: "npm test"
    total: 12
    passed: 12
    failed: 0
    status: "green"  # Required

  migrations:
    created: true  # or false if no schema changes needed
    files:
      - path: "migrations/20240115_add_users_table.sql"
        direction: "up"
      - path: "migrations/20240115_add_users_table_down.sql"
        direction: "down"
    dev_db_verified: true

  notes: ""

Update manifest:

stories:
  US-001:
    phase: "implement"
    artifacts:
      implementation: "src/services/user.service.ts"

Gate

Must pass before proceeding to refactor phase:

• [ ] All tests pass (100%)
• [ ] No skipped or pending tests
• [ ] Code compiles without warnings
• [ ] No runtime errors in test execution
• [ ] If migrations exist: dev DB schema verified after migration
• [ ] If migrations exist: rollback migration file exists

Troubleshooting

Test still fails after implementation

• Re-read the test assertion carefully
• Check mock setup matches implementation
• Verify return types and shapes
• Check async/await handling

Implementation seems wrong but test passes

• Test may be incomplete; consider adding more tests
• Discuss with design phase if behaviour unclear

Stuck on a test

• Break the problem into smaller steps
• Write a simpler test first
• Check if stub interfaces need adjustment

Tips

• Let tests guide you; don't implement ahead of tests
• Run tests frequently; small feedback loops
• Commit working state before attempting complex changes
• If implementation is hard, design may need revision
• Green means "it works", not "it's perfect" (that's refactor)