When should you use async def vs def for route handlers in FastAPI?

FastAPI supports both async def and plain def route handlers. The choice has real performance implications because of how FastAPI's underlying ASGI server (Uvicorn + Starlette) handles concurrency.

import asyncio
import httpx
from fastapi import FastAPI

app = FastAPI()

# GOOD: truly async I/O — does not block the event loop
@app.get("/async-data")
async def get_data():
    async with httpx.AsyncClient() as client:
        response = await client.get("https://api.example.com/data")
    return response.json()

# ALSO GOOD: synchronous — FastAPI runs this in a thread pool
@app.get("/sync-data")
def get_data_sync():
    import requests  # sync library
    response = requests.get("https://api.example.com/data")
    return response.json()

# DANGEROUS: blocking call inside async def without await
# This blocks the entire event loop, killing concurrency
@app.get("/bad")
async def bad_handler():
    import time
    time.sleep(5)  # NEVER do this in async def
    return {"message": "slow"}

# CORRECT: use asyncio.sleep in async context
@app.get("/good-async-wait")
async def good_handler():
    await asyncio.sleep(5)  # non-blocking
    return {"message": "waited"}

Rule of thumb: if your route calls await somewhere, use async def. If your route only calls synchronous libraries, use plain def — FastAPI will correctly run it in a threadpool, keeping the event loop free.

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