WebSocket closed with code 4001: Not authorized when connecting to AssemblyAI real-time transcription API

I'm working on integrating AssemblyAI's real-time transcription API with WebSocket in Python, using the websockets library. However, when I attempt to connect to the WebSocket endpoint, I get the following error:

WebSocket closed with code 4001: Not authorized
Connecting to WebSocket at wss://api.assemblyai.com/v2/realtime/ws?sample_rate=16000
WebSocket connected. Receiving SessionBegins...
SessionBegins: {"error":"Not authorized"}
Starting to send and receive messages...
Receive: WebSocket closed with code 4001: Not authorized
Send: WebSocket closed with code 4001: Not authorized

Here is the code I'm using:

import websockets
import asyncio
import base64
import json
from configure import auth_key
import pyaudio

# Audio configuration
FRAMES_PER_BUFFER = 3200
FORMAT = pyaudio.paInt16
CHANNELS = 1
RATE = 16000

# Initialize PyAudio
p = pyaudio.PyAudio()

# Start recording
stream = p.open(
    format=FORMAT,
    channels=CHANNELS,
    rate=RATE,
    input=True,
    frames_per_buffer=FRAMES_PER_BUFFER
)

# AssemblyAI WebSocket endpoint
URL = "wss://api.assemblyai.com/v2/realtime/ws?sample_rate=16000"
async def send_receive():
    print(f"Connecting to WebSocket at {URL}")

    try:
        auth_url = f"{URL}&auth_key={auth_key}"  # Append API key to URL
        async with websockets.connect(
            auth_url,
            ping_interval=5,
            ping_timeout=20
        ) as _ws:
            await asyncio.sleep(0.1)
            print("WebSocket connected. Receiving SessionBegins...")

            # Receive SessionBegins
            session_begins = await _ws.recv()
            print(f"SessionBegins: {session_begins}")
            print("Starting to send and receive messages...")

            # Define the send coroutine
            async def send():
                while True:
                    try:
                        # Read audio data, encode and send
                        data = stream.read(FRAMES_PER_BUFFER)
                        data = base64.b64encode(data).decode("utf-8")
                        json_data = json.dumps({"audio_data": data})
                        await _ws.send(json_data)
                        print("Sent audio data")
                    except websockets.ConnectionClosedError as e:
                        print(f"Send: WebSocket closed with code {e.code}: {e.reason}")
                        break
                    except Exception as e:
                        print(f"Send: Unexpected error: {e}")
                        break
                    await asyncio.sleep(0.01)

            # Define the receive coroutine
            async def receive():
                while True:
                    try:
                        # Receive and print text data
                        result_str = await _ws.recv()
                        print(f"Received: {json.loads(result_str).get('text', '')}")
                    except websockets.ConnectionClosedError as e:
                        print(f"Receive: WebSocket closed with code {e.code}: {e.reason}")
                        break
                    except Exception as e:
                        print(f"Receive: Unexpected error: {e}")
                        break

            # Run send and receive concurrently
            await asyncio.gather(send(), receive())

    except Exception as e:
        print(f"Connection failed: {e}")

while True:
    try:
        asyncio.run(send_receive())
    except KeyboardInterrupt:
        print("Program interrupted by user. Exiting...")
        break
    except Exception as e:
        print(f"Unexpected error: {e}")
        break

When I attempt to connect to the WebSocket server, I receive the following response in the SessionBegins message:

SessionBegins: {"error":"Not authorized"}

Steps Taken:

1. I verified that the API key (auth_key) is correct and has been included properly in the request.

2. I checked that the URL is correctly formatted with the auth_key appended as a query parameter.

3. I have confirmed that the API key has the necessary permissions to access the real-time transcription service.

Why am I receiving a "Not authorized" error with WebSocket code 4001 when connecting to the AssemblyAI WebSocket endpoint, and how can I resolve it?