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+---
+name: api-design-principles
+description: Master REST and GraphQL API design principles to build intuitive, scalable, and maintainable APIs that delight developers. Use when designing new APIs, reviewing API specifications, or establishing API design standards.
+---
+
+# API Design Principles
+
+Master REST and GraphQL API design principles to build intuitive, scalable, and maintainable APIs that delight developers and stand the test of time.
+
+## When to Use This Skill
+
+- Designing new REST or GraphQL APIs
+- Refactoring existing APIs for better usability
+- Establishing API design standards for your team
+- Reviewing API specifications before implementation
+- Migrating between API paradigms (REST to GraphQL, etc.)
+- Creating developer-friendly API documentation
+- Optimizing APIs for specific use cases (mobile, third-party integrations)
+
+## Core Concepts
+
+### 1. RESTful Design Principles
+
+**Resource-Oriented Architecture**
+- Resources are nouns (users, orders, products), not verbs
+- Use HTTP methods for actions (GET, POST, PUT, PATCH, DELETE)
+- URLs represent resource hierarchies
+- Consistent naming conventions
+
+**HTTP Methods Semantics:**
+- `GET`: Retrieve resources (idempotent, safe)
+- `POST`: Create new resources
+- `PUT`: Replace entire resource (idempotent)
+- `PATCH`: Partial resource updates
+- `DELETE`: Remove resources (idempotent)
+
+### 2. GraphQL Design Principles
+
+**Schema-First Development**
+- Types define your domain model
+- Queries for reading data
+- Mutations for modifying data
+- Subscriptions for real-time updates
+
+**Query Structure:**
+- Clients request exactly what they need
+- Single endpoint, multiple operations
+- Strongly typed schema
+- Introspection built-in
+
+### 3. API Versioning Strategies
+
+**URL Versioning:**
+```
+/api/v1/users
+/api/v2/users
+```
+
+**Header Versioning:**
+```
+Accept: application/vnd.api+json; version=1
+```
+
+**Query Parameter Versioning:**
+```
+/api/users?version=1
+```
+
+## REST API Design Patterns
+
+### Pattern 1: Resource Collection Design
+
+```python
+# Good: Resource-oriented endpoints
+GET    /api/users              # List users (with pagination)
+POST   /api/users              # Create user
+GET    /api/users/{id}         # Get specific user
+PUT    /api/users/{id}         # Replace user
+PATCH  /api/users/{id}         # Update user fields
+DELETE /api/users/{id}         # Delete user
+
+# Nested resources
+GET    /api/users/{id}/orders  # Get user's orders
+POST   /api/users/{id}/orders  # Create order for user
+
+# Bad: Action-oriented endpoints (avoid)
+POST   /api/createUser
+POST   /api/getUserById
+POST   /api/deleteUser
+```
+
+### Pattern 2: Pagination and Filtering
+
+```python
+from typing import List, Optional
+from pydantic import BaseModel, Field
+
+class PaginationParams(BaseModel):
+    page: int = Field(1, ge=1, description="Page number")
+    page_size: int = Field(20, ge=1, le=100, description="Items per page")
+
+class FilterParams(BaseModel):
+    status: Optional[str] = None
+    created_after: Optional[str] = None
+    search: Optional[str] = None
+
+class PaginatedResponse(BaseModel):
+    items: List[dict]
+    total: int
+    page: int
+    page_size: int
+    pages: int
+
+    @property
+    def has_next(self) -> bool:
+        return self.page < self.pages
+
+    @property
+    def has_prev(self) -> bool:
+        return self.page > 1
+
+# FastAPI endpoint example
+from fastapi import FastAPI, Query, Depends
+
+app = FastAPI()
+
+@app.get("/api/users", response_model=PaginatedResponse)
+async def list_users(
+    page: int = Query(1, ge=1),
+    page_size: int = Query(20, ge=1, le=100),
+    status: Optional[str] = Query(None),
+    search: Optional[str] = Query(None)
+):
+    # Apply filters
+    query = build_query(status=status, search=search)
+
+    # Count total
+    total = await count_users(query)
+
+    # Fetch page
+    offset = (page - 1) * page_size
+    users = await fetch_users(query, limit=page_size, offset=offset)
+
+    return PaginatedResponse(
+        items=users,
+        total=total,
+        page=page,
+        page_size=page_size,
+        pages=(total + page_size - 1) // page_size
+    )
+```
+
+### Pattern 3: Error Handling and Status Codes
+
+```python
+from fastapi import HTTPException, status
+from pydantic import BaseModel
+
+class ErrorResponse(BaseModel):
+    error: str
+    message: str
+    details: Optional[dict] = None
+    timestamp: str
+    path: str
+
+class ValidationErrorDetail(BaseModel):
+    field: str
+    message: str
+    value: Any
+
+# Consistent error responses
+STATUS_CODES = {
+    "success": 200,
+    "created": 201,
+    "no_content": 204,
+    "bad_request": 400,
+    "unauthorized": 401,
+    "forbidden": 403,
+    "not_found": 404,
+    "conflict": 409,
+    "unprocessable": 422,
+    "internal_error": 500
+}
+
+def raise_not_found(resource: str, id: str):
+    raise HTTPException(
+        status_code=status.HTTP_404_NOT_FOUND,
+        detail={
+            "error": "NotFound",
+            "message": f"{resource} not found",
+            "details": {"id": id}
+        }
+    )
+
+def raise_validation_error(errors: List[ValidationErrorDetail]):
+    raise HTTPException(
+        status_code=status.HTTP_422_UNPROCESSABLE_ENTITY,
+        detail={
+            "error": "ValidationError",
+            "message": "Request validation failed",
+            "details": {"errors": [e.dict() for e in errors]}
+        }
+    )
+
+# Example usage
+@app.get("/api/users/{user_id}")
+async def get_user(user_id: str):
+    user = await fetch_user(user_id)
+    if not user:
+        raise_not_found("User", user_id)
+    return user
+```
+
+### Pattern 4: HATEOAS (Hypermedia as the Engine of Application State)
+
+```python
+class UserResponse(BaseModel):
+    id: str
+    name: str
+    email: str
+    _links: dict
+
+    @classmethod
+    def from_user(cls, user: User, base_url: str):
+        return cls(
+            id=user.id,
+            name=user.name,
+            email=user.email,
+            _links={
+                "self": {"href": f"{base_url}/api/users/{user.id}"},
+                "orders": {"href": f"{base_url}/api/users/{user.id}/orders"},
+                "update": {
+                    "href": f"{base_url}/api/users/{user.id}",
+                    "method": "PATCH"
+                },
+                "delete": {
+                    "href": f"{base_url}/api/users/{user.id}",
+                    "method": "DELETE"
+                }
+            }
+        )
+```
+
+## GraphQL Design Patterns
+
+### Pattern 1: Schema Design
+
+```graphql
+# schema.graphql
+
+# Clear type definitions
+type User {
+  id: ID!
+  email: String!
+  name: String!
+  createdAt: DateTime!
+
+  # Relationships
+  orders(
+    first: Int = 20
+    after: String
+    status: OrderStatus
+  ): OrderConnection!
+
+  profile: UserProfile
+}
+
+type Order {
+  id: ID!
+  status: OrderStatus!
+  total: Money!
+  items: [OrderItem!]!
+  createdAt: DateTime!
+
+  # Back-reference
+  user: User!
+}
+
+# Pagination pattern (Relay-style)
+type OrderConnection {
+  edges: [OrderEdge!]!
+  pageInfo: PageInfo!
+  totalCount: Int!
+}
+
+type OrderEdge {
+  node: Order!
+  cursor: String!
+}
+
+type PageInfo {
+  hasNextPage: Boolean!
+  hasPreviousPage: Boolean!
+  startCursor: String
+  endCursor: String
+}
+
+# Enums for type safety
+enum OrderStatus {
+  PENDING
+  CONFIRMED
+  SHIPPED
+  DELIVERED
+  CANCELLED
+}
+
+# Custom scalars
+scalar DateTime
+scalar Money
+
+# Query root
+type Query {
+  user(id: ID!): User
+  users(
+    first: Int = 20
+    after: String
+    search: String
+  ): UserConnection!
+
+  order(id: ID!): Order
+}
+
+# Mutation root
+type Mutation {
+  createUser(input: CreateUserInput!): CreateUserPayload!
+  updateUser(input: UpdateUserInput!): UpdateUserPayload!
+  deleteUser(id: ID!): DeleteUserPayload!
+
+  createOrder(input: CreateOrderInput!): CreateOrderPayload!
+}
+
+# Input types for mutations
+input CreateUserInput {
+  email: String!
+  name: String!
+  password: String!
+}
+
+# Payload types for mutations
+type CreateUserPayload {
+  user: User
+  errors: [Error!]
+}
+
+type Error {
+  field: String
+  message: String!
+}
+```
+
+### Pattern 2: Resolver Design
+
+```python
+from typing import Optional, List
+from ariadne import QueryType, MutationType, ObjectType
+from dataclasses import dataclass
+
+query = QueryType()
+mutation = MutationType()
+user_type = ObjectType("User")
+
+@query.field("user")
+async def resolve_user(obj, info, id: str) -> Optional[dict]:
+    """Resolve single user by ID."""
+    return await fetch_user_by_id(id)
+
+@query.field("users")
+async def resolve_users(
+    obj,
+    info,
+    first: int = 20,
+    after: Optional[str] = None,
+    search: Optional[str] = None
+) -> dict:
+    """Resolve paginated user list."""
+    # Decode cursor
+    offset = decode_cursor(after) if after else 0
+
+    # Fetch users
+    users = await fetch_users(
+        limit=first + 1,  # Fetch one extra to check hasNextPage
+        offset=offset,
+        search=search
+    )
+
+    # Pagination
+    has_next = len(users) > first
+    if has_next:
+        users = users[:first]
+
+    edges = [
+        {
+            "node": user,
+            "cursor": encode_cursor(offset + i)
+        }
+        for i, user in enumerate(users)
+    ]
+
+    return {
+        "edges": edges,
+        "pageInfo": {
+            "hasNextPage": has_next,
+            "hasPreviousPage": offset > 0,
+            "startCursor": edges[0]["cursor"] if edges else None,
+            "endCursor": edges[-1]["cursor"] if edges else None
+        },
+        "totalCount": await count_users(search=search)
+    }
+
+@user_type.field("orders")
+async def resolve_user_orders(user: dict, info, first: int = 20) -> dict:
+    """Resolve user's orders (N+1 prevention with DataLoader)."""
+    # Use DataLoader to batch requests
+    loader = info.context["loaders"]["orders_by_user"]
+    orders = await loader.load(user["id"])
+
+    return paginate_orders(orders, first)
+
+@mutation.field("createUser")
+async def resolve_create_user(obj, info, input: dict) -> dict:
+    """Create new user."""
+    try:
+        # Validate input
+        validate_user_input(input)
+
+        # Create user
+        user = await create_user(
+            email=input["email"],
+            name=input["name"],
+            password=hash_password(input["password"])
+        )
+
+        return {
+            "user": user,
+            "errors": []
+        }
+    except ValidationError as e:
+        return {
+            "user": None,
+            "errors": [{"field": e.field, "message": e.message}]
+        }
+```
+
+### Pattern 3: DataLoader (N+1 Problem Prevention)
+
+```python
+from aiodataloader import DataLoader
+from typing import List, Optional
+
+class UserLoader(DataLoader):
+    """Batch load users by ID."""
+
+    async def batch_load_fn(self, user_ids: List[str]) -> List[Optional[dict]]:
+        """Load multiple users in single query."""
+        users = await fetch_users_by_ids(user_ids)
+
+        # Map results back to input order
+        user_map = {user["id"]: user for user in users}
+        return [user_map.get(user_id) for user_id in user_ids]
+
+class OrdersByUserLoader(DataLoader):
+    """Batch load orders by user ID."""
+
+    async def batch_load_fn(self, user_ids: List[str]) -> List[List[dict]]:
+        """Load orders for multiple users in single query."""
+        orders = await fetch_orders_by_user_ids(user_ids)
+
+        # Group orders by user_id
+        orders_by_user = {}
+        for order in orders:
+            user_id = order["user_id"]
+            if user_id not in orders_by_user:
+                orders_by_user[user_id] = []
+            orders_by_user[user_id].append(order)
+
+        # Return in input order
+        return [orders_by_user.get(user_id, []) for user_id in user_ids]
+
+# Context setup
+def create_context():
+    return {
+        "loaders": {
+            "user": UserLoader(),
+            "orders_by_user": OrdersByUserLoader()
+        }
+    }
+```
+
+## Best Practices
+
+### REST APIs
+1. **Consistent Naming**: Use plural nouns for collections (`/users`, not `/user`)
+2. **Stateless**: Each request contains all necessary information
+3. **Use HTTP Status Codes Correctly**: 2xx success, 4xx client errors, 5xx server errors
+4. **Version Your API**: Plan for breaking changes from day one
+5. **Pagination**: Always paginate large collections
+6. **Rate Limiting**: Protect your API with rate limits
+7. **Documentation**: Use OpenAPI/Swagger for interactive docs
+
+### GraphQL APIs
+1. **Schema First**: Design schema before writing resolvers
+2. **Avoid N+1**: Use DataLoaders for efficient data fetching
+3. **Input Validation**: Validate at schema and resolver levels
+4. **Error Handling**: Return structured errors in mutation payloads
+5. **Pagination**: Use cursor-based pagination (Relay spec)
+6. **Deprecation**: Use `@deprecated` directive for gradual migration
+7. **Monitoring**: Track query complexity and execution time
+
+## Common Pitfalls
+
+- **Over-fetching/Under-fetching (REST)**: Fixed in GraphQL but requires DataLoaders
+- **Breaking Changes**: Version APIs or use deprecation strategies
+- **Inconsistent Error Formats**: Standardize error responses
+- **Missing Rate Limits**: APIs without limits are vulnerable to abuse
+- **Poor Documentation**: Undocumented APIs frustrate developers
+- **Ignoring HTTP Semantics**: POST for idempotent operations breaks expectations
+- **Tight Coupling**: API structure shouldn't mirror database schema
+
+## Resources
+
+- **references/rest-best-practices.md**: Comprehensive REST API design guide
+- **references/graphql-schema-design.md**: GraphQL schema patterns and anti-patterns
+- **references/api-versioning-strategies.md**: Versioning approaches and migration paths
+- **assets/rest-api-template.py**: FastAPI REST API template
+- **assets/graphql-schema-template.graphql**: Complete GraphQL schema example
+- **assets/api-design-checklist.md**: Pre-implementation review checklist
+- **scripts/openapi-generator.py**: Generate OpenAPI specs from code