api-design

name	api-design
type	skill
description	Design and implement REST APIs with consistent conventions, versioning, error contracts, and security.
related-rules	["architecture.md","security.md"]
allowed-tools	Read, Write, Edit, Bash
agentic	{"generated_by":"agentic","source":"areas/software/backend/skills/api-design/SKILL.md","repository":"https://github.com/sawrus/agent-guides"}

API Design Skill

Expertise: REST API design, HTTP semantics, versioning strategies, error contracts, OpenAPI, pagination, idempotency.

HTTP Method & Status Code Reference

Operation	Method	Success	Error cases
Create resource	POST	201 Created	400 Validation, 409 Conflict
Read resource	GET	200 OK	404 Not Found, 403 Forbidden
Full update	PUT	200 OK	400, 404, 409
Partial update	PATCH	200 OK	400, 404
Delete	DELETE	204 No Content	404, 409 (has dependents)
Async action	POST	202 Accepted	400

URL Design Rules

✅ Nouns, plural, lowercase, kebab-case
   GET  /users/{id}
   POST /orders
   GET  /product-categories

❌ Verbs in path
   POST /createOrder
   GET  /getUser?id=123

✅ Nesting only for true ownership (max 2 levels)
   GET  /orders/{order_id}/items          ✅ items belong to order
   GET  /orders/{order_id}/items/{item_id}/tags   ❌ too deep

✅ Actions as sub-resources
   POST /orders/{id}/cancel
   POST /invoices/{id}/resend

Standard Error Contract

All error responses must follow the same shape:

{
  "error": {
    "code": "ORDER_NOT_FOUND",        // machine-readable, stable
    "message": "Order ord_123 not found",  // human-readable
    "details": [                       // optional: per-field errors
      { "field": "items[0].quantity", "issue": "must be > 0" }
    ],
    "request_id": "req_abc123"        // traceable
  }
}

# FastAPI implementation
from fastapi import HTTPException
from pydantic import BaseModel
from typing import Optional, List

class ErrorDetail(BaseModel):
    field: str
    issue: str

class ErrorResponse(BaseModel):
    code: str
    message: str
    details: Optional[List[ErrorDetail]] = None
    request_id: Optional[str] = None

# Usage
raise HTTPException(
    status_code=404,
    detail=ErrorResponse(
        code="ORDER_NOT_FOUND",
        message=f"Order {order_id} not found",
        request_id=request.state.request_id
    ).model_dump()
)

Pagination Patterns

Cursor-based (preferred for large/live datasets)

class PaginatedResponse(BaseModel, Generic[T]):
    items: List[T]
    next_cursor: Optional[str] = None   # opaque, base64-encoded
    total_count: Optional[int] = None   # only if cheap to compute

# Encode cursor: hide implementation detail from client
import base64, json

def encode_cursor(last_id: int, last_created_at: datetime) -> str:
    payload = {"id": last_id, "ts": last_created_at.isoformat()}
    return base64.b64encode(json.dumps(payload).encode()).decode()

def decode_cursor(cursor: str) -> dict:
    return json.loads(base64.b64decode(cursor).decode())

Offset-based (only for small, static datasets)

# Only when total count is cheap and dataset doesn't change under pagination
@router.get("/products")
async def list_products(page: int = 1, page_size: int = Query(20, le=100)):
    offset = (page - 1) * page_size
    # ...

Versioning Strategy

URL versioning (recommended for major breaking changes):
  /api/v1/orders
  /api/v2/orders  ← new schema, old still supported

Header versioning (for minor variations):
  Accept: application/vnd.myapi.v2+json

Rules:
- v1 stays alive for minimum 6 months after v2 launch
- Deprecated endpoints return: Deprecation: true, Sunset: <date> headers
- Never remove a field from a response without a major version bump

Idempotency

# Idempotency key pattern for POST mutations
@router.post("/orders", status_code=201)
async def create_order(
    body: CreateOrderRequest,
    idempotency_key: Optional[str] = Header(None, alias="X-Idempotency-Key"),
    db: AsyncSession = Depends(get_db),
    redis: Redis = Depends(get_redis),
):
    if idempotency_key:
        cached = await redis.get(f"idempotency:{idempotency_key}")
        if cached:
            return JSONResponse(json.loads(cached), status_code=200)  # 200 = already done

    order = await order_service.create(db, body)
    response = OrderResponse.model_validate(order)

    if idempotency_key:
        await redis.setex(
            f"idempotency:{idempotency_key}",
            86400,  # 24h TTL
            response.model_dump_json()
        )
    return response

Security Checklist

• Auth middleware applied before any handler logic — never inside handler
• Ownership check: if order.user_id != current_user.id: raise 403
• Input validation via Pydantic (FastAPI) or Joi/Zod (Node) on every endpoint
• Rate limiting on public endpoints (especially auth, password reset)
• No sensitive data in URLs (tokens, passwords) — use request body or headers
• CORS configured explicitly: no allow_origins=["*"] in production