api-design

SKILL.md

API Design Patterns

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

When to Activate

Designing new API endpoints

Reviewing existing API contracts

Adding pagination, filtering, or sorting

Implementing error handling for APIs

Planning API versioning strategy

Building public or partner-facing APIs

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
HTTP/1.1 404 Not Found
{ "error": { "code": "not_found", "message": "User not found" } }

# BAD: 500 for validation errors
# GOOD: 400 or 422 with field-level details

# 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

{
  "error": {
    "code": "validation_error",
    "message": "Request validation failed",
    "details": [
      {
        "field": "email",
        "message": "Must be a valid email address",
        "code": "invalid_format"
      },
      {
        "field": "age",
        "message": "Must be between 0 and 150",
        "code": "out_of_range"
      }
    ]
  }
}

Response Envelope Variants

// Option A: Envelope with data wrapper (recommended for public APIs)
interface ApiResponse<T> {
  data: T;
  meta?: PaginationMeta;
  links?: PaginationLinks;
}

interface ApiError {
  error: {
    code: string;
    message: string;
    details?: FieldError[];
  };
}

// Option B: Flat response (simpler, common for internal APIs)
// Success: just return the resource directly
// Error: return error object
// Distinguish by HTTP status code

Pagination

Offset-Based (Simple)

GET /api/v1/users?page=2&per_page=20

# Implementation
SELECT * FROM users
ORDER BY created_at DESC
LIMIT 20 OFFSET 20;

Pros: Easy to implement, supports "jump to page N"
Cons: Slow on large offsets (OFFSET 100000), inconsistent with concurrent inserts

Cursor-Based (Scalable)

GET /api/v1/users?cursor=eyJpZCI6MTIzfQ&limit=20

# Implementation
SELECT * FROM users
WHERE id > :cursor_id
ORDER BY id ASC
LIMIT 21;  -- fetch one extra to determine has_next

{
  "data": [...],
  "meta": {
    "has_next": true,
    "next_cursor": "eyJpZCI6MTQzfQ"
  }
}

Pros: Consistent performance regardless of position, stable with concurrent inserts
Cons: Cannot jump to arbitrary page, cursor is opaque

When to Use Which

Use Case
Pagination Type

Admin dashboards, small datasets (<10K)
Offset

Infinite scroll, feeds, large datasets
Cursor

Public APIs
Cursor (default) with offset (optional)

Search results
Offset (users expect page numbers)

Filtering, Sorting, and Search

Filtering

# Simple equality
GET /api/v1/orders?status=active&customer_id=abc-123

# Comparison operators (use bracket notation)
GET /api/v1/products?price[gte]=10&price[lte]=100
GET /api/v1/orders?created_at[after]=2025-01-01

# Multiple values (comma-separated)
GET /api/v1/products?category=electronics,clothing

# Nested fields (dot notation)
GET /api/v1/orders?customer.country=US

Sorting

# Single field (prefix - for descending)
GET /api/v1/products?sort=-created_at

# Multiple fields (comma-separated)
GET /api/v1/products?sort=-featured,price,-created_at

Full-Text Search

# Search query parameter
GET /api/v1/products?q=wireless+headphones

# Field-specific search
GET /api/v1/users?email=alice

Sparse Fieldsets

# Return only specified fields (reduces payload)
GET /api/v1/users?fields=id,name,email
GET /api/v1/orders?fields=id,total,status&include=customer.name

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
HTTP/1.1 429 Too Many Requests
Retry-After: 60
{
  "error": {
    "code": "rate_limit_exceeded",
    "message": "Rate limit exceeded. Try again in 60 seconds."
  }
}

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

Implementation Patterns

TypeScript (Next.js API Route)

import { z } from "zod";
import { NextRequest, NextResponse } from "next/server";

const createUserSchema = z.object({
  email: z.string().email(),
  name: z.string().min(1).max(100),
});

export async function POST(req: NextRequest) {
  const body = await req.json();
  const parsed = createUserSchema.safeParse(body);

  if (!parsed.success) {
    return NextResponse.json({
      error: {
        code: "validation_error",
        message: "Request validation failed",
        details: parsed.error.issues.map(i => ({
          field: i.path.join("."),
          message: i.message,
          code: i.code,
        })),
      },
    }, { status: 422 });
  }

  const user = await createUser(parsed.data);

  return NextResponse.json(
    { data: user },
    {
      status: 201,
      headers: { Location: `/api/v1/users/${user.id}` },
    },
  );
}

Python (Django REST Framework)

from rest_framework import serializers, viewsets, status
from rest_framework.response import Response

class CreateUserSerializer(serializers.Serializer):
    email = serializers.EmailField()
    name = serializers.CharField(max_length=100)

class UserSerializer(serializers.ModelSerializer):
    class Meta:
        model = User
        fields = ["id", "email", "name", "created_at"]

class UserViewSet(viewsets.ModelViewSet):
    serializer_class = UserSerializer
    permission_classes = [IsAuthenticated]

    def get_serializer_class(self):
        if self.action == "create":
            return CreateUserSerializer
        return UserSerializer

    def create(self, request):
        serializer = CreateUserSerializer(data=request.data)
        serializer.is_valid(raise_exception=True)
        user = UserService.create(**serializer.validated_data)
        return Response(
            {"data": UserSerializer(user).data},
            status=status.HTTP_201_CREATED,
            headers={"Location": f"/api/v1/users/{user.id}"},
        )

Go (net/http)

func (h *UserHandler) CreateUser(w http.ResponseWriter, r *http.Request) {
    var req CreateUserRequest
    if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
        writeError(w, http.StatusBadRequest, "invalid_json", "Invalid request body")
        return
    }

    if err := req.Validate(); err != nil {
        writeError(w, http.StatusUnprocessableEntity, "validation_error", err.Error())
        return
    }

    user, err := h.service.Create(r.Context(), req)
    if err != nil {
        switch {
        case errors.Is(err, domain.ErrEmailTaken):
            writeError(w, http.StatusConflict, "email_taken", "Email already registered")
        default:
            writeError(w, http.StatusInternalServerError, "internal_error", "Internal error")
        }
        return
    }

    w.Header().Set("Location", fmt.Sprintf("/api/v1/users/%s", user.ID))
    writeJSON(w, http.StatusCreated, map[string]any{"data": user})
}

API Design Checklist

Before shipping a new endpoint:

 Resource URL follows naming conventions (plural, kebab-case, no verbs)

 Correct HTTP method used (GET for reads, POST for creates, etc.)

 Appropriate status codes returned (not 200 for everything)

 Input validated with schema (Zod, Pydantic, Bean Validation)

 Error responses follow standard format with codes and messages

 Pagination implemented for list endpoints (cursor or offset)

 Authentication required (or explicitly marked as public)

 Authorization checked (user can only access their own resources)

 Rate limiting configured

 Response does not leak internal details (stack traces, SQL errors)

 Consistent naming with existing endpoints (camelCase vs snake_case)

 Documented (OpenAPI/Swagger spec updated)