Configure this skill should be used when the user asks about "API contract", "api-contract.md", "shared interface", "TypeScript interfaces", "request response schemas", "endpoint design", or needs guidance on designing contracts that coordinate backend and frontend agents. Use when building or modifying API endpoints. Trigger with phrases like 'create API', 'design endpoint', or 'API scaffold'.
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
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.
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.
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.
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.
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).
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 }
}
