Strands agent() hangs inside FastAPI async def endpoints

Tried wrapping a Strands agent with FastAPI and the request hung indefinitely when using async def.

Python (NG)

@app.post("/invocations")
async def invoke(request: InvokeRequest):
    result = agent(request.prompt)  # ← hangs here
    return {"response": result.message["content"][0]["text"]}

agent() is a blocking call — running it inside async def blocks the entire event loop. Switching to def fixes it.

Python (OK)

@app.post("/invocations")
def invoke(request: InvokeRequest):
    result = agent(request.prompt)  # ← runs in thread pool
    return {"response": result.message["content"][0]["text"]}

FastAPI automatically runs def endpoints in an external thread pool, so the main event loop is never blocked. Covered in detail in deploy series part 1.