API Contract Design

The API contract (api-contract.md) is the shared interface between implementation agents. It defines what the backend provides and what the frontend consumes, ensuring both sides build compatible systems without direct communication.

Purpose

The contract serves as:

• Single source of truth for API shape
• Coordination mechanism between backend and frontend agents
• Validation reference for QA testing
• Documentation for the implemented API

Contract Structure

A complete api-contract.md contains:

# API Contract: [Feature Name]

## Base URL
[API base path, e.g., /api/v1]

## Authentication
[Auth requirements for endpoints]

## Endpoints

### [Endpoint Group]

#### [METHOD] [Route]
[Description]

**Request:**
[Request schema]

**Response:**
[Response schema]

**Errors:**
[Error codes and meanings]

## TypeScript Interfaces
[Shared type definitions]

## Validation Rules
[Input validation requirements]

Writing Endpoints

Endpoint Definition

#### POST /auth/register

Create a new user account.

**Request:**
```json
{
  "email": "string (required, valid email)",
  "password": "string (required, min 8 chars)",
  "name": "string (optional)"
}

Response (201):

{
  "id": "uuid",
  "email": "string",
  "name": "string | null",
  "createdAt": "ISO 8601 datetime"
}

Errors:

• 400: Invalid request body
• 409: Email already exists
• 422: Validation failed

### Key Elements

| Element | Purpose |
|---------|---------|
| Method + Route | HTTP verb and path |
| Description | What the endpoint does |
| Request | Input schema with types and constraints |
| Response | Output schema with status code |
| Errors | Possible error responses |

## TypeScript Interfaces

Define shared types for type safety across agents:

```markdown
## TypeScript Interfaces

```typescript
// User types
interface User {
  id: string;
  email: string;
  name: string | null;
  createdAt: string;
}

interface CreateUserRequest {
  email: string;
  password: string;
  name?: string;
}

interface LoginRequest {
  email: string;
  password: string;
}

interface AuthResponse {
  user: User;
  token: string;
  expiresAt: string;
}

// Error response
interface ApiError {
  code: string;
  message: string;
  details?: Record<string, string[]>;
}

### Type Guidelines

- Use explicit types, not `any`
- Mark optional fields with `?`
- Use union types for nullable: `string | null`
- Include all possible response shapes
- Match types to JSON serialization

## Validation Rules

Document validation requirements explicitly:

```markdown
## Validation Rules

### User Registration
| Field | Rules |
|-------|-------|
| email | Required, valid email format, unique |
| password | Required, min 8 chars, 1 uppercase, 1 number |
| name | Optional, max 100 chars |

### Product Creation
| Field | Rules |
|-------|-------|
| title | Required, 3-200 chars |
| price | Required, positive number, max 2 decimals |
| category | Required, must exist in categories table |

Why Document Validation

• Backend implements the rules
• Frontend can pre-validate before submit
• QA tests edge cases
• All agents have same understanding

Error Handling

Standard Error Format

## Error Response Format

All errors return:
```json
{
  "code": "ERROR_CODE",
  "message": "Human-readable message",
  "details": {
    "field": ["error1", "error2"]
  }
}

Error Codes

### Error Consistency

Use consistent error codes across all endpoints. This helps:
- Frontend implement unified error handling
- QA test all error scenarios
- Users get predictable feedback

## Pagination

For list endpoints:

```markdown
#### GET /products

List products with pagination.

**Query Parameters:**
| Param | Type | Default | Description |
|-------|------|---------|-------------|
| page | integer | 1 | Page number |
| limit | integer | 20 | Items per page (max 100) |
| sort | string | createdAt | Sort field |
| order | string | desc | Sort order (asc/desc) |

**Response (200):**
```json
{
  "data": [Product],
  "pagination": {
    "page": 1,
    "limit": 20,
    "total": 150,
    "totalPages": 8
  }
}

### Pagination Interface

```typescript
interface PaginatedResponse<T> {
  data: T[];
  pagination: {
    page: number;
    limit: number;
    total: number;
    totalPages: number;
  };
}

Contract Evolution

During Sprint

1. Initial: Architect creates contract based on specs
2. Implementation: Agents follow contract exactly
3. Deviations: If agent must deviate, document in report
4. Update: Architect updates contract if deviation justified
5. Final: Contract reflects implemented reality

Contract Changes

When changing an existing contract:

• Note breaking changes
• Consider versioning for major changes
• Update both request and response if affected
• Notify dependent agents via specs

Best Practices

Be Specific

// Good
"email": "string (required, valid email format)"

// Bad
"email": "string"

Include Examples

**Request Example:**
```json
{
  "email": "user@example.com",
  "password": "SecurePass123"
}

### Document All States

Include responses for:
- Success (200, 201, 204)
- Client errors (400, 401, 403, 404, 422)
- Empty states (empty arrays, null values)

### Keep It DRY

Reference shared types instead of duplicating:

```markdown
**Response:** `User` (see TypeScript Interfaces)

No Implementation Details

The contract defines WHAT, not HOW:

• Don't mention database columns
• Don't specify frameworks
• Don't include file paths

Common Patterns

CRUD Endpoints

### Products

| Method | Route | Description |
|--------|-------|-------------|
| GET | /products | List all products |
| GET | /products/:id | Get single product |
| POST | /products | Create product |
| PUT | /products/:id | Update product |
| DELETE | /products/:id | Delete product |

Authentication Endpoints

### Authentication

| Method | Route | Auth | Description |
|--------|-------|------|-------------|
| POST | /auth/register | No | Create account |
| POST | /auth/login | No | Get token |
| POST | /auth/logout | Yes | Invalidate token |
| GET | /auth/me | Yes | Current user |
| POST | /auth/refresh | Yes | Refresh token |

Additional Resources

For complete examples, examine the api-contract.md files generated in sprint directories after running /sprint.