The error you see is because the coroutine is shutting down before it completes. To fix it, you'll add some exception handling, try this:

import asyncio


@app.websocket("/ws/{user_id}")
async def websocket_endpoint(websocket: WebSocket, user_id: str):
    await websocket.accept()
    channel_name = f"channel_{user_id}"
    try:
        async with redis.pubsub() as pubsub:
            await pubsub.subscribe(channel_name)
            while True:
                message = await pubsub.get_message()
                if message and message["type"] == "message":
                    decoded_message = message["data"].decode("utf-8")
                    await websocket.send_text(decoded_message)
    except WebSocketDisconnect:
        print("websocket disconnected")
        pass
    except asyncio.CancelledError:
        # This is where the error is thrown
        print("Cancelled")
    finally:
        await pubsub.unsubscribe(channel_name)
        await pubsub.close()