API Contract

Overview

API Contract guides the creation of api-contract.md files that serve as the shared interface between backend and frontend agents during sprint execution. The contract defines request/response schemas, endpoint routes, TypeScript interfaces, and error formats so that implementation agents build to an agreed specification without direct coordination.

Prerequisites

• Sprint directory initialized at .claude/sprint/[N]/
• specs.md with defined feature scope and endpoint requirements
• Familiarity with RESTful API conventions (HTTP methods, status codes, JSON schemas)
• TypeScript knowledge for interface definitions (recommended)

Instructions

1. Create api-contract.md in the sprint directory (.claude/sprint/[N]/api-contract.md). Define each endpoint using the standard format: HTTP method, route path, description, request body, response body with status code, and error codes. See ${CLAUDE_SKILL_DIR}/references/writing-endpoints.md for the full template.
2. Define TypeScript interfaces for all request and response types. Use explicit types instead of any, mark optional fields with ?, and use string | null for nullable values. Reference ${CLAUDE_SKILL_DIR}/references/typescript-interfaces.md for canonical type patterns.
3. For list endpoints, include pagination parameters and the PaginatedResponse<T> wrapper. Standardize on page, limit, sort, and order query parameters as documented in ${CLAUDE_SKILL_DIR}/references/pagination.md.
4. Document all response states: success (200, 201, 204), client errors (400, 401, 403, 404, 422), and empty states. Use a consistent error response format with code, message, and optional details fields.
5. Follow best practices from ${CLAUDE_SKILL_DIR}/references/best-practices.md: be specific about field constraints (e.g., "string, required, valid email format"), include request/response examples, reference shared types instead of duplicating, and omit implementation details (no database columns, framework names, or file paths).
6. Share the contract file path in SPAWN REQUEST blocks so both backend and frontend agents read the same interface definition.

Output

• api-contract.md containing all endpoint definitions with typed request/response schemas
• TypeScript interface declarations for User, CreateUserRequest, LoginRequest, AuthResponse, ApiError, and domain-specific types
• Paginated response wrappers for list endpoints
• Standardized error format across all endpoints

Error Handling

Error	Cause	Solution
Backend and frontend schemas diverge	Contract updated without notifying both agents	Always reference a single api-contract.md; never duplicate endpoint definitions
Missing error response codes	Contract only documents the happy path	Document all status codes: 400, 401, 403, 404, 409, 422 per endpoint
Ambiguous field types	Using string without constraints	Specify format, length, and validation rules (e.g., "string, required, min 8 chars")
Pagination inconsistency	List endpoints use different parameter names	Standardize on the PaginatedResponse<T> interface for all list endpoints
Type mismatch between JSON and TypeScript	Dates serialized inconsistently	Use ISO 8601 datetime strings; document as "createdAt": "ISO 8601 datetime"

Examples

Authentication endpoint contract:

#### POST /auth/register

Create a new user account.

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

**Response (201):**  # HTTP 201 Created
{
  "id": "uuid",
  "email": "string",
  "name": "string | null",
  "createdAt": "ISO 8601 datetime"  # 8601 = configured value
}

**Errors:**
- 400: Invalid request body  # HTTP 400 Bad Request
- 409: Email already exists  # HTTP 409 Conflict
- 422: Validation failed  # HTTP 422 Unprocessable Entity

Paginated list endpoint:

#### 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):**  # HTTP 200 OK
{
  "data": [Product],
  "pagination": { "page": 1, "limit": 20, "total": 150, "totalPages": 8 }
}

Shared TypeScript interface:

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

Resources

• ${CLAUDE_SKILL_DIR}/references/writing-endpoints.md -- Endpoint definition template and key elements
• ${CLAUDE_SKILL_DIR}/references/typescript-interfaces.md -- Canonical type definitions and guidelines
• ${CLAUDE_SKILL_DIR}/references/pagination.md -- Pagination parameters and PaginatedResponse interface
• ${CLAUDE_SKILL_DIR}/references/best-practices.md -- Contract authoring rules (specificity, DRY, no implementation details)