I'm going to guess this could be for a variety of reasons, so posting some of your related code might be helpful. I confronted something similar today.

When you create a ClientSession and call initialize(), it sends the request successfully (which you can see in your debugging), but then hangs forever waiting for the response. This could happen because the _receive_loop task, which is responsible for reading responses from the stream and routing them to the appropriate handlers, never starts.

Make sure you're entering the session as a context manager:

#  This will hang:
session = ClientSession(read_stream, write_stream)
await session.initialize()  # Hangs forever

# This works:
session = ClientSession(read_stream, write_stream)
await session.__aenter__()  # Starts the receive loop
await session.initialize()  # Now completes successfully

Or better yet, use it as a proper context manager:

async with ClientSession(read_stream, write_stream) as session:
    await session.initialize()
    # ... use the session

You can try adding logging to _receive_loop to confirm it's starting:

# find `BaseSession` in mcp/shared/session.py

async def _receive_loop(self) -> None:
    print("Starting receive loop")  # Add this
    async with (self._read_stream, self._write_stream):
        async for message in self._read_stream:
            print(f"Received message: {message}")  # And this
            # ... et cetra

Hope this helps! I spent quite a while debugging this exact issue myself.

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