Cannot break out of session.initialize() when using streamable-http

When I use streaming-http connection, I can't jump out of the session initialization. However, when I debug, I find that the initialization request has been sent and the message from the other party has been received, but I can't jump out of initialize.

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.

I have a same problem.

        async with streamablehttp_client(url=url) as (
            read_stream,
            write_stream,
            _,
        ):
            log.debug('MCPClient.run::streamablehttp_client done', level=4)
            async with ClientSession(
                read_stream, write_stream,
                elicitation_callback=elicitation_callback
            ) as session:
                log.debug('MCPClient.run::ClientSession done', level=4)
                await session.initialize()
                log.debug('MCPClient.run::initialize done', level=4)

With the code, it could be run two times, and when it called third time, the session.initialize() won't return.

Hi thanks for reporting this! Could you provide a snippet or example code that we can run to debug the specific problem you're facing?

I can not give a simple sample code , but I've already found something.
The case of mine(maybe not as same as the others) is :

Creating a MCP Client B within another MCP Server A, when the MCP Client A gone.
MCP codes called asyncio who throw-ed a asyncio.exceptions.CancelledError:Cancelled by cancel scope xxxxxx
CancelledError is a BaseException which could not be caught by normal try...except and the code blocked.

Reading through here more deeply looks like it might be related to #301

Thanks for the report. The v2 rework changed the code this targets, so it no longer applies as-is. I'm closing this out as part of a backlog cleanup now that v2 is out. Feel free to reopen if this is still relevant.

_{AI Disclaimer}

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Cannot break out of session.initialize() when using streamable-http #874

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Cannot break out of session.initialize() when using streamable-http #874

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jonsampson commented on Jun 6, 2025

zeerd commented on Sep 12, 2025

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