api-design-patterns

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• SKILL.md

SKILL.md (first lines)

# API Design Patterns Skill

## When to Use

Use this skill when:
- Designing new API endpoints
- Reviewing API consistency
- Implementing pagination and filtering
- Creating error response formats
- Writing OpenAPI documentation
- Validating HTTP status codes
- Building request/response schemas

## RESTful Design Principles

### 1. HTTP Methods

```
GET    /users          - List users
GET    /users/{id}     - Get single user
POST   /users          - Create user
PUT    /users/{id}     - Full update
PATCH  /users/{id}     - Partial update
DELETE /users/{id}     - Delete user
```

### 2. Status Codes

```
2xx Success:
- 200 OK              - Successful GET, PUT, PATCH
- 201 Created         - Successful POST
- 204 No Content      - Successful DELETE

4xx Client Errors:
- 400 Bad Request     - Invalid input
- 401 Unauthorized    - Not authenticated
- 403 Forbidden       - Not authorized
- 404 Not Found       - Resource not found
- 409 Conflict        - Duplicate resource
- 422 Unprocessable   - Validation error

5xx Server Errors:
- 500 Internal Error  - Unexpected error
- 503 Service Unavail - Maintenance
```

### 3. Pagination Pattern

```python
from pydantic import BaseModel
from typing import Generic, TypeVar, List

T = TypeVar('T')

class PaginatedResponse(BaseModel, Generic[T]):
    items: List[T]
    total: int
    page: int
    size: int
    pages: int
    
    class Config:
        from_attributes = True

# Endpoint
@router.get("/users", response_model=PaginatedResponse[UserResponse])
async def list_users(
    page: int = Query(1, ge=1),
    size: int = Query(20, ge=1, le=100),
):
    total = await user_service.count()
    items = await user_service.get_page(page, size)
    return PaginatedResponse(
        items=items,
        total=total,
        page=page,
        size=size,
        pages=(total + size - 1) // size
    )
```

### 4. Filtering Pattern

```python
from typing import Optional
from fastapi import Query

@router.get("/products")
async def list_products(
    # Search
    q: Optional[str] = Query(None, min_length=2, max_length=100),
    
    # Filters
    category: Optional[str] = None,
    min_price: Optional[float] = Query(None, ge=0),
    max_price: Optional[float] = Query(None, ge=0),
    in_stock: Optional[bool] = None,
    
    # Sorting
    sort_by: str = Query("created_at", regex="^(name|price|created_at)$"),
    order: str = Query("desc", regex="^(asc|desc)$"),
    
    # Pagination
    page: int = Query(1, ge=1),
    size: int = Query(20, ge=1, le=100),
):
    return await product_service.search(
        q=q, category=category, 
        min_price=min_price, max_price=max_price,
        in_stock=in_stock,
        sort_by=sort_by, order=order,
        page=page, size=size
    )
```

### 5. Error Response Format

```python

From ils15/pantheon-legacy, MIT.

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