FastAPI Skill | Agent Skills

FastAPI

This skill provides comprehensive guidance for building APIs with FastAPI.

Quick Navigation

| Topic | Reference | | ------------------ | ----------------------------------- | | Getting started | references/first-steps.md | | Path parameters | references/path-parameters.md | | Query parameters | references/query-parameters.md | | Request body | references/request-body.md | | Validation | references/validation.md | | Body advanced | references/body-advanced.md | | Cookies/Headers | references/cookies-headers.md | | Pydantic models | references/models.md | | Forms/Files | references/forms-files.md | | Error handling | references/error-handling.md | | Path config | references/path-config.md | | Dependencies | references/dependencies.md | | Security | references/security.md | | Middleware | references/middleware.md | | CORS | references/cors.md | | Database | references/sql-databases.md | | Project structure | references/bigger-applications.md | | Background tasks | references/background-tasks.md | | Metadata/Docs | references/metadata-docs.md | | Testing | references/testing.md | | Advanced responses | references/responses-advanced.md | | WebSockets | references/websockets.md | | Templates | references/templates.md | | Settings/Env vars | references/settings.md | | Lifespan events | references/lifespan.md | | OpenAPI advanced | references/openapi-advanced.md |

When to Use

Creating REST APIs with Python
Adding endpoints with automatic validation
Implementing OAuth2/JWT authentication
Working with Pydantic models
Adding dependency injection
Configuring CORS, middleware
Uploading files, handling forms
Testing API endpoints

Installation

Requires Python 3.10+. Install: pip install "fastapi[standard]" (full with uvicorn) or pip install fastapi (minimal). Add python-multipart for forms/files.

Quick Start

from fastapi import FastAPI

app = FastAPI()

@app.get("/")
def read_root():
    return {"Hello": "World"}

@app.get("/items/{item_id}")
def read_item(item_id: int, q: str | None = None):
    return {"item_id": item_id, "q": q}

Run: fastapi dev main.py

Core Patterns

Type-Safe Parameters

from typing import Annotated
from fastapi import Path, Query

@app.get("/items/{item_id}")
def read_item(
    item_id: Annotated[int, Path(ge=1)],
    q: Annotated[str | None, Query(max_length=50)] = None
):
    return {"item_id": item_id, "q": q}

Request Body with Validation

from pydantic import BaseModel, Field

class Item(BaseModel):
    name: str = Field(min_length=1, max_length=100)
    price: float = Field(gt=0)

@app.post("/items/", response_model=Item)
def create_item(item: Item):
    return item

Dependencies

from fastapi import Depends

async def get_db():
    db = SessionLocal()
    try:
        yield db
    finally:
        db.close()

@app.get("/users/")
def list_users(db: Annotated[Session, Depends(get_db)]):
    return db.query(User).all()

Authentication

from fastapi.security import OAuth2PasswordBearer

oauth2_scheme = OAuth2PasswordBearer(tokenUrl="token")

async def get_current_user(token: Annotated[str, Depends(oauth2_scheme)]):
    return decode_token(token)

@app.get("/users/me")
def read_me(user: Annotated[User, Depends(get_current_user)]):
    return user

API Documentation

Swagger UI: /docs
ReDoc: /redoc
OpenAPI: /openapi.json

Best Practices

Use Annotated[Type, ...] for parameters
Define Pydantic models for request/response
Use response_model for output filtering
Add status_code for proper HTTP codes
Use tags for API organization
Add dependencies at router/app level for auth

Prohibitions

❌ Return raw database models (use response models)
❌ Store passwords in plain text (use bcrypt/passlib)
❌ Mix Body with Form/File in same endpoint
❌ Use sync blocking I/O in async endpoints
❌ Skip HTTPException for error handling

Agent Skills: FastAPI

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