API Design Patterns

Conventions and best practices for designing consistent, developer-friendly REST APIs.

When to Activate

• Before writing any implementation — design the contract first, code second
• Designing new API endpoints or changing existing ones
• Reviewing existing API contracts for consistency
• Adding pagination, filtering, or sorting — see skill api-pagination-filtering
• Implementing error handling for APIs
• Planning API versioning strategy
• Building public or partner-facing APIs

For the full Contract-First workflow (spec writing, code generation, CI breaking-change detection, Pact): see skill api-contract. For API documentation production — platform choice (Mintlify, Docusaurus, Redoc, Scalar), OpenAPI descriptions/examples, interactive playground, changelog automation, Vale prose linting, and Divio structure: see skill api-docs-patterns.

Contract-First Principle

Write the OpenAPI spec before writing any implementation code.

@startuml
:Write spec\napi/v1/openapi.yaml;
note right
  REST → OpenAPI 3.1
  Events → AsyncAPI 3.0
end note
:Lint spec\nspectral lint;
:Generate types / stubs\nfrom spec;
note right
  TS: openapi-typescript
  Go: oapi-codegen
  Java: openapi-generator
  Python: datamodel-codegen
end note
:Implement business logic only;
:Validate in CI\nschemathesis + oasdiff;
@enduml

The spec is the public contract. Consumers depend on it. Code is a private implementation detail.

• Never generate the spec from code (annotations, reflection) — it will drift
• Never write request/response types by hand — generate them from the spec
• Any breaking change requires a new API version (/api/v2/)

See skill api-contract for the complete toolchain and CI setup.

Write descriptions and examples from the start

OpenAPI documentation is easiest to write while you are designing the spec — not after the implementation is shipped.

Minimum documentation requirements per operation (add these when you write each path, not later):

paths:
  /orders:
    post:
      summary: Create an order        # ← one-line summary
      description: |                  # ← full description with scope, side effects, notes
        Places a new order for the authenticated customer.
        The order is created in `pending` status and transitions to
        `processing` once payment is confirmed (async, webhook fired).

        **Scopes required:** `orders:write`
      operationId: createOrder        # ← stable, unique identifier
      tags: [Orders]                  # ← logical grouping
      parameters: []                  # ← every param needs description + example
      requestBody:
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/CreateOrderRequest'
            example:                  # ← concrete, realistic example
              customer_id: "cust_abc123"
              items:
                - product_id: "prod_xyz"
                  quantity: 2
      responses:
        '201':
          description: Order created successfully.
          headers:
            Location:
              description: URL of the newly created order.
              schema: { type: string }
        '400':
          description: Request validation failed.
          content:
            application/problem+json:
              schema: { $ref: '#/components/schemas/ProblemDetails' }

See skill api-docs-patterns for the full documentation workflow: platform setup, interactive playground, code examples in all languages, changelog automation, and CI.

---

Resource Design

URL Structure

# Resources are nouns, plural, lowercase, kebab-case
GET    /api/v1/users
GET    /api/v1/users/:id
POST   /api/v1/users
PUT    /api/v1/users/:id
PATCH  /api/v1/users/:id
DELETE /api/v1/users/:id

# Sub-resources for relationships
GET    /api/v1/users/:id/orders
POST   /api/v1/users/:id/orders

# Actions that don't map to CRUD (use verbs sparingly)
POST   /api/v1/orders/:id/cancel
POST   /api/v1/auth/login
POST   /api/v1/auth/refresh

Naming Rules

# GOOD
/api/v1/team-members          # kebab-case for multi-word resources
/api/v1/orders?status=active  # query params for filtering
/api/v1/users/123/orders      # nested resources for ownership

# BAD
/api/v1/getUsers              # verb in URL
/api/v1/user                  # singular (use plural)
/api/v1/team_members          # snake_case in URLs
/api/v1/users/123/getOrders   # verb in nested resource

HTTP Methods and Status Codes

Method Semantics

Method	Idempotent	Safe	Use For
GET	Yes	Yes	Retrieve resources
POST	No	No	Create resources, trigger actions
PUT	Yes	No	Full replacement of a resource
PATCH	No*	No	Partial update of a resource
DELETE	Yes	No	Remove a resource

*PATCH can be made idempotent with proper implementation

Status Code Reference

# Success
200 OK                    — GET, PUT, PATCH (with response body)
201 Created               — POST (include Location header)
204 No Content            — DELETE, PUT (no response body)

# Client Errors
400 Bad Request           — Validation failure, malformed JSON
401 Unauthorized          — Missing or invalid authentication
403 Forbidden             — Authenticated but not authorized
404 Not Found             — Resource doesn't exist
409 Conflict              — Duplicate entry, state conflict
422 Unprocessable Entity  — Semantically invalid (valid JSON, bad data)
429 Too Many Requests     — Rate limit exceeded

# Server Errors
500 Internal Server Error — Unexpected failure (never expose details)
502 Bad Gateway           — Upstream service failed
503 Service Unavailable   — Temporary overload, include Retry-After

Common Mistakes

# BAD: 200 for everything
{ "status": 200, "success": false, "error": "Not found" }

# GOOD: Use HTTP status codes semantically + RFC 7807 body
HTTP/1.1 404 Not Found
Content-Type: application/problem+json

{
  "type": "https://api.example.com/problems/not-found",
  "title": "Not Found",
  "status": 404,
  "detail": "User abc-123 not found.",
  "instance": "/api/v1/users/abc-123"
}

# BAD: 500 for validation errors
# GOOD: 400 or 422 with field-level details (RFC 7807 errors extension)

# BAD: 200 for created resources
# GOOD: 201 with Location header
HTTP/1.1 201 Created
Location: /api/v1/users/abc-123

Response Format

Success Response

{
  "data": {
    "id": "abc-123",
    "email": "alice@example.com",
    "name": "Alice",
    "created_at": "2025-01-15T10:30:00Z"
  }
}

Collection Response (with Pagination)

{
  "data": [
    { "id": "abc-123", "name": "Alice" },
    { "id": "def-456", "name": "Bob" }
  ],
  "meta": {
    "total": 142,
    "page": 1,
    "per_page": 20,
    "total_pages": 8
  },
  "links": {
    "self": "/api/v1/users?page=1&per_page=20",
    "next": "/api/v1/users?page=2&per_page=20",
    "last": "/api/v1/users?page=8&per_page=20"
  }
}

Error Response — RFC 7807 / RFC 9457 Problem Details

All error responses MUST use Content-Type: application/problem+json and the standard fields:

HTTP/1.1 400 Bad Request
Content-Type: application/problem+json

{
  "type": "https://api.example.com/problems/validation-failed",
  "title": "Validation Failed",
  "status": 400,
  "detail": "One or more fields failed validation.",
  "instance": "/api/v1/users",
  "errors": [
    { "field": "email", "detail": "must be a valid email address" },
    { "field": "age",   "detail": "must be between 0 and 150" }
  ]
}

Field	Required	Description
type	Recommended	URI identifying the problem type. about:blank if no docs exist yet.
title	Recommended	Stable, human-readable summary (don't interpolate dynamic data).
status	Yes	HTTP status code mirrored in the body.
detail	Optional	Occurrence-specific explanation for the client.
instance	Optional	URI of this specific occurrence (e.g., request path or ID).
errors	Extension	RFC 9457 array for multiple sub-problems (validation errors).

See skill: problem-details for full specification and per-language implementation.

Response Envelope Variants

// Success responses: return the resource (or data wrapper for public APIs)
interface ApiResponse<T> {
  data: T;
  meta?: PaginationMeta;
  links?: PaginationLinks;
}

// Error responses: always RFC 7807 ProblemDetails — never { success: false, error: "..." }
interface ProblemDetails {
  type: string;         // URI — link to docs
  title: string;        // Short, stable summary
  status: number;       // HTTP status mirrored
  detail?: string;      // Occurrence-specific detail
  instance?: string;    // URI of this occurrence
  [key: string]: unknown; // Extension fields
}
// Content-Type for errors: application/problem+json (NOT application/json)

For pagination (offset/cursor), filtering, sorting, and sparse fieldsets — see skill api-pagination-filtering.

Authentication and Authorization

Token-Based Auth

# Bearer token in Authorization header
GET /api/v1/users
Authorization: Bearer eyJhbGciOiJIUzI1NiIs...

# API key (for server-to-server)
GET /api/v1/data
X-API-Key: sk_live_abc123

Authorization Patterns

// Resource-level: check ownership
app.get("/api/v1/orders/:id", async (req, res) => {
  const order = await Order.findById(req.params.id);
  if (!order) return res.status(404).json({ error: { code: "not_found" } });
  if (order.userId !== req.user.id) return res.status(403).json({ error: { code: "forbidden" } });
  return res.json({ data: order });
});

// Role-based: check permissions
app.delete("/api/v1/users/:id", requireRole("admin"), async (req, res) => {
  await User.delete(req.params.id);
  return res.status(204).send();
});

Rate Limiting

Headers

HTTP/1.1 200 OK
X-RateLimit-Limit: 100
X-RateLimit-Remaining: 95
X-RateLimit-Reset: 1640000000

# When exceeded — RFC 7807 body
HTTP/1.1 429 Too Many Requests
Content-Type: application/problem+json
Retry-After: 60

{
  "type": "https://api.example.com/problems/too-many-requests",
  "title": "Too Many Requests",
  "status": 429,
  "detail": "Rate limit exceeded. Try again in 60 seconds.",
  "retryAfter": 60
}

Rate Limit Tiers

Tier	Limit	Window	Use Case
Anonymous	30/min	Per IP	Public endpoints
Authenticated	100/min	Per user	Standard API access
Premium	1000/min	Per API key	Paid API plans
Internal	10000/min	Per service	Service-to-service

Versioning

URL Path Versioning (Recommended)

/api/v1/users
/api/v2/users

Pros: Explicit, easy to route, cacheable Cons: URL changes between versions

Header Versioning

GET /api/users
Accept: application/vnd.myapp.v2+json

Pros: Clean URLs Cons: Harder to test, easy to forget

Versioning Strategy

1. Start with /api/v1/ — don't version until you need to
2. Maintain at most 2 active versions (current + previous)
3. Deprecation timeline:
   - Announce deprecation (6 months notice for public APIs)
   - Add Sunset header: Sunset: Sat, 01 Jan 2026 00:00:00 GMT
   - Return 410 Gone after sunset date
4. Non-breaking changes don't need a new version:
   - Adding new fields to responses
   - Adding new optional query parameters
   - Adding new endpoints
5. Breaking changes require a new version:
   - Removing or renaming fields
   - Changing field types
   - Changing URL structure
   - Changing authentication method

For per-language implementation patterns (TypeScript, Go), anti-patterns, and the full API Design Checklist, see skill api-design-advanced.