API Design

API Design

Purpose

Design REST/RPC endpoint contracts with request/response types, error handling, and versioning strategy. Produces TypeScript type definitions and endpoint documentation that serve as the contract between frontend and backend.

Inputs

• Feature requirements (from interview/idea phase)
• Schema design output (entities, relationships — drives endpoint shape)
• Existing endpoints (current route handlers, naming conventions)
• Authentication/authorization model (who can call what)

Process

Step 1: Inventory Existing Endpoints

Read current route handlers and list all existing paths, HTTP methods, request/response shapes, and auth requirements. Note naming conventions and patterns in use.

Step 2: Identify New Endpoints Needed

From the feature requirements and schema design output, determine what operations the frontend needs. Group by resource and map to CRUD operations where applicable.

Step 3: Define Endpoint Contracts

For each endpoint, specify:

• Method: GET, POST, PUT, PATCH, DELETE
• Path: Following existing naming conventions
• Request: Body schema, URL params, query params
• Response: Success shape (with HTTP status), error shape
• Auth: Required role/permission, or public

Step 4: Design Type Definitions

Write TypeScript interfaces for:

• Request body types
• Response types (success and error)
• Shared types (enums, union types, reusable shapes)
• Zod schemas for runtime validation where applicable

Step 5: Plan Authentication/Authorization

For each endpoint, define:

• Whether authentication is required
• What role or permission is needed
• How authorization is enforced (middleware, RLS, inline check)
• Service-to-service auth if applicable

Step 6: Define Error Contract

Design a consistent error response shape:

• Error code enum (application-level codes, not just HTTP status)
• Error response structure (code, message, details)
• HTTP status mapping (which app errors map to which HTTP statuses)
• Validation error format (field-level errors for form submissions)

Step 7: Consider Pagination and Filtering

For list endpoints:

• Pagination strategy (cursor-based preferred for real-time data, offset for static)
• Filter parameters (query string format, supported operators)
• Sort parameters (field + direction)
• Default and maximum page sizes

Step 8: Document Rate Limits and Caching

If applicable:

• Rate limit tiers per endpoint or endpoint group
• Cache-Control headers for GET endpoints
• ETag/If-None-Match for conditional requests
• Stale-while-revalidate strategies

Output Format

# API Design: [Feature Name]

## Endpoint Overview

| Method | Path | Auth | Description |
|--------|------|------|-------------|
| GET    | /api/... | authenticated | ... |
| POST   | /api/... | authenticated | ... |

## Type Definitions

```typescript
// Shared types
interface PaginatedResponse<T> {
  data: T[];
  cursor: string | null;
  hasMore: boolean;
}

// Request types
interface CreateFooRequest {
  name: string;
  // ...
}

// Response types
interface FooResponse {
  id: string;
  name: string;
  createdAt: string;
  // ...
}

Error Contract

enum ErrorCode {
  VALIDATION_ERROR = 'VALIDATION_ERROR',
  NOT_FOUND = 'NOT_FOUND',
  UNAUTHORIZED = 'UNAUTHORIZED',
  FORBIDDEN = 'FORBIDDEN',
  // ...
}

interface ApiError {
  code: ErrorCode;
  message: string;
  details?: Record<string, string[]>; // field-level errors
}

Endpoint Details

[METHOD] [path]

Auth: [requirement]

Request:

// params / body / query

Response (200):

// success shape

Errors:

• 400: [when]
• 404: [when]

Example:

curl -X POST /api/foo \
  -H "Authorization: Bearer ..." \
  -d '{"name": "bar"}'

Pagination Strategy

• Method: [cursor-based / offset]
• Default page size: [N]
• Max page size: [N]

Caching Strategy

## Quality Checks

- [ ] Every endpoint has defined auth requirements
- [ ] Error cases are enumerated for each endpoint
- [ ] Request and response types are fully specified (no `any` types)
- [ ] Naming follows existing project conventions (REST or RPC, plural vs singular)
- [ ] List endpoints include pagination strategy
- [ ] Shared types are extracted (no duplication across endpoints)
- [ ] Error contract is consistent across all endpoints
- [ ] Examples include realistic request/response payloads

## Evolution Notes
<!-- Observations appended after each use -->