fastapi
Modern high-performance Python web API framework — async-first REST API framework with automatic OpenAPI/Swagger documentation, Pydantic v2 validation, and dependency injection. FastAPI features: @app.get/post/put/delete/patch decorators, Pydantic models for request/response validation, automatic JSON serialization, OpenAPI schema generation at /docs and /redoc, Depends() for dependency injection, BackgroundTasks, OAuth2/JWT security, File/UploadFile, HTTPException, APIRouter for route organization, middleware, websockets, streaming responses, async/sync handlers, and excellent editor support via type annotations.
Score Breakdown
⚙ Agent Friendliness
🔒 Security
Web framework. Enable HTTPS via reverse proxy (nginx/Caddy). Add rate limiting middleware (slowapi). Use CORS middleware carefully — allow specific origins not *. Input validation via Pydantic is automatic. Implement authentication on all protected endpoints. SQL injection: use ORM or parameterized queries.
⚡ Reliability
Best When
Building REST APIs, microservices, and agent backends needing type safety and automatic documentation — FastAPI is the standard for modern Python APIs with best-in-class developer experience.
Avoid When
Server-side HTML apps (use Flask+Jinja2), batteries-included needs (use Django), or synchronous-only constraints.
Use Cases
- • Agent REST API — from fastapi import FastAPI; from pydantic import BaseModel; app = FastAPI(); class Item(BaseModel): name: str; price: float; @app.post('/items/', response_model=Item); async def create_item(item: Item): return save(item) — typed API; agent creates type-safe REST endpoint with automatic validation and docs; request body validated by Pydantic
- • Agent dependency injection — from fastapi import Depends, FastAPI; async def get_db(): db = Database(); try: yield db; finally: db.close(); @app.get('/users/{id}'); async def get_user(id: int, db=Depends(get_db)): return await db.get_user(id) — DI; agent uses dependency injection for database connections, auth, and shared resources
- • Agent OAuth2 JWT auth — from fastapi.security import OAuth2PasswordBearer; oauth2_scheme = OAuth2PasswordBearer(tokenUrl='token'); async def get_current_user(token: str = Depends(oauth2_scheme)): payload = verify_jwt(token); return payload; @app.get('/me'); async def read_me(user=Depends(get_current_user)): return user — auth; agent adds JWT authentication
- • Agent background tasks — from fastapi import BackgroundTasks; def send_email(email: str, body: str): send(email, body); @app.post('/signup'); async def signup(data: SignupData, background_tasks: BackgroundTasks): user = create_user(data); background_tasks.add_task(send_email, data.email, 'Welcome!'); return user — background; agent defers slow operations after response
- • Agent file upload — from fastapi import UploadFile, File; @app.post('/upload/'); async def upload(file: UploadFile = File(...)): contents = await file.read(); process(contents); return {'filename': file.filename, 'size': len(contents)} — file upload; agent handles multipart file uploads with streaming
Not For
- • Traditional server-side HTML — FastAPI is API-focused; for server-side HTML rendering use Flask/Jinja2 or add Jinja2 templates
- • Synchronous-only applications — FastAPI works sync but is designed for async; for pure sync use Flask
- • Batteries-included framework — FastAPI is lean; for admin panels, ORM integration, auth views use Django
Interface
Authentication
FastAPI provides OAuth2 schemes (OAuth2PasswordBearer, OAuth2AuthorizationCodeBearer). JWT validation via dependencies. API key via APIKeyHeader/APIKeyQuery.
Pricing
FastAPI is MIT licensed. Free for all use. Requires uvicorn or another ASGI server to run.
Agent Metadata
Known Gotchas
- ⚠ ASGI server required to run — FastAPI is an ASGI framework; running python app.py does not start server; run with: uvicorn main:app --host 0.0.0.0 --port 8080; or: fastapi run main.py (FastAPI CLI in 0.111+); agent code: Dockerfile CMD: uvicorn main:app; for development: uvicorn main:app --reload
- ⚠ Sync functions run in thread pool — @app.get('/') def endpoint(): sync operations — FastAPI runs sync functions in thread pool automatically; @app.get('/') async def endpoint(): async operations — pure async; mixing sync blocking in async function blocks event loop; agent code: use async def for async operations; use def for CPU/sync operations (FastAPI handles thread pool)
- ⚠ Response model strips extra fields — @app.get('/', response_model=UserPublic) — Pydantic model with fields subset; response_model filters output to declared fields; extra fields not in UserPublic are excluded automatically; agent code: define response_model to control what's returned; omit sensitive fields by not including in response model
- ⚠ HTTPException detail is JSON-serializable only — HTTPException(status_code=422, detail='message') or detail={'field': 'error'}; detail must be JSON-serializable; agent code: raise HTTPException(status_code=404, detail=f'User {id} not found'); for complex errors: detail={'code': 'NOT_FOUND', 'msg': '...'}; do NOT pass objects
- ⚠ Pydantic model in path/query — def endpoint(user_id: int = Path(), page: int = Query(default=1, ge=1, le=100)): from fastapi import Path, Query; add validation and metadata; agent code: always use Query() for query params with validation; Path() for path params; Body() for body fields; Depends() for injected dependencies
- ⚠ OpenAPI docs at /docs break with large schemas — automatic Swagger UI at /docs generates from all routes; very large number of routes or deeply nested models slow browser; agent code: separate routers with APIRouter(prefix='/v1'); can disable docs: FastAPI(docs_url=None) for production; redoc at /redoc as alternative
Alternatives
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Scores are editorial opinions as of 2026-03-06.