sse_client blocks indefinitely when server has incorrect base URL

Describe the bug
When using the sse_client function to connect to a remote MCP server, if the MCP server sets an incorrect base URL (like http://localhost:8080), the sse_client will raise an exception in the following code:

python-sdk/src/mcp/client/sse.py

Line 81 in c2ca8e0

raise ValueError(error_msg)

The read_stream_writer then sends this exception to the read_stream:

python-sdk/src/mcp/client/sse.py

Line 107 in c2ca8e0

await read_stream_writer.send(exc)

However, since the read_stream has not been yielded yet, the exception will never be received.
As a result, the sse_client will be blocked indefinitely.

To Reproduce

Set up an MCP server with an incorrect base URL on a remote server, for example:

import express, { Request, Response } from 'express';
import { McpServer } from '@modelcontextprotocol/sdk/server/mcp.js';
import { SSEServerTransport } from '@modelcontextprotocol/sdk/server/sse.js';
import { z } from 'zod';

const server = new McpServer({
    name: 'example-server',
    version: '1.0.0',
});

server.tool(
  "calculate-bmi",
  {
    weightKg: z.number(),
    heightM: z.number()
  },
  async ({ weightKg, heightM }) => ({
    content: [{
      type: "text",
      text: String(weightKg / (heightM * heightM))
    }]
  })
);

const app = express();

const transports: { [sessionId: string]: SSEServerTransport } = {};

app.get('/sse', async (_: Request, res: Response) => {
    // set baseUrl for the transport
    const baseUrl = 'http://localhost:8080';
    const transport = new SSEServerTransport(`${baseUrl}/messages`, res);
    transports[transport.sessionId] = transport;
    res.on('close', () => {
        delete transports[transport.sessionId];
    });
    await server.connect(transport);
});

app.post('/messages', async (req: Request, res: Response) => {
    const sessionId = req.query.sessionId as string;
    const transport = transports[sessionId];
    if (transport) {
        await transport.handlePostMessage(req, res);
    } else {
        res.status(400).send('No transport found for sessionId');
    }
});

app.listen(8080);

Try to connect to the remote MCP server, for example:

import asyncio

from mcp.client.sse import sse_client


async def run():
    try:
        async with sse_client("http://{your_remote_server}:8080/sse"):
            print("Connected to SSE endpoint successfully.")
    except Exception as e:
        print(f"Error: {e}")


if __name__ == "__main__":
    asyncio.run(run())

Expected behavior
The sse_client should raise an exception and exit gracefully.

Logs
None

Additional context
None

@Kudryavkaz

@Kudryavkaz I was able to replicate this issue

The issue in the screenshot I've added must be fixed, as the exception is not handled properly as you've mentioned.

@Kudryavkaz But the above client code, when incorrect URL is added, quits with unhandled error as mentioned in this comment. It is not blocked indefinitely. Can please provide more information about this indefinite block?

Currently debugging this.

@kavinkumar807

@kavinkumar807 Thank you for your reply.

If you start the example MCP server locally and connect the MCP client to "localhost:xxxx", you will not be able to reproduce this issue.

You should start the example MCP server on a remote server, and then use the MCP client to connect to that remote server.

Like this:

"zhangfish.top" is my remote server, and I ran this command on it:

@Kudryavkaz

@Kudryavkaz Debugging the unhandled exception issue. Shall I use your endpoint to test?

@kavinkumar807

@kavinkumar807 Sure, endpoint is "http://zhangfish.top:8080/sse"

@Kudryavkaz

@Kudryavkaz Thanks

@Kudryavkaz

@Kudryavkaz I could see there are two issue when I was debugging,

The issue mentioned here, when I try to connect to the above mentioned url the deadlock situation happens and client is blocked instead of graceful shutdown -> has to be fixed :: I'm working on this
The main reason for this exception is that with the above url client is able to make SSE connection but when checked with endpoint_url and url (as in the below screenshot) there is a mismatch

Logs
endpoint_url :: http://localhost:8080/messages?sessionId=18b0b6fb-b3e9-4faf-9350-e2b0d66d6549
url :: http://zhangfish.top:8080/sse
Endpoint origin does not match connection origin: http://localhost:8080/messages?sessionId=18b0b6fb-b3e9-4faf-9350-e2b0d66d6549
Error in sse_reader: Endpoint origin does not match connection origin: http://localhost:8080/messages?sessionId=18b0b6fb-b3e9-4faf-9350-e2b0d66d6549

@dsp-ant @jspahrsummers the endpoint_url has to be http://zhangfish.top:8080/messages?sessionId=18b0b6fb-b3e9-4faf-9350-e2b0d66d6549 right? is it an issue or am I missing something?

#410 seems like this issue is related to this. Adding here for reference

@Kudryavkaz

On further debugging as I've mentioned in the above comment the first point handling exception must be fixed, handling exception gracefully and quitting the client. But feel free to ignore the second point, that's my misunderstanding.

app.get('/sse', async (_: Request, res: Response) => {
    // set baseUrl for the transport
    const baseUrl = 'http://localhost:8080';
    const transport = new SSEServerTransport(`${baseUrl}/messages`, res);
    transports[transport.sessionId] = transport;
    res.on('close', () => {
        delete transports[transport.sessionId];
    });
    await server.connect(transport);
});

@Kudryavkaz Here the baseUrl for the transport is added intentionally to demonstrate the indefinite blocking scenario right?

@kavinkumar807

@kavinkumar807 yes.

@Kudryavkaz

@Kudryavkaz Raised PR #500 for the above issue.

Cc: @dsp-ant @jspahrsummers @nick-merrill @Kludex @jerome3o-anthropic

Note: adding the above people in cc from top contributors as I'm not sure who is the maintainer.

This issue has been open for a while and the linked PR has been closed #500

I'm curious if this is still a problem?

fix: propagate pre-endpoint errors in sse_client instead of deadlocking

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sse_client blocks indefinitely when server has incorrect base URL #447

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sse_client blocks indefinitely when server has incorrect base URL #447

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kavinkumar807 commented on Apr 10, 2025

Kudryavkaz commented on Apr 11, 2025

kavinkumar807 commented on Apr 11, 2025

Kudryavkaz commented on Apr 11, 2025

kavinkumar807 commented on Apr 11, 2025

kavinkumar807 commented on Apr 11, 2025

kavinkumar807 commented on Apr 12, 2025

kavinkumar807 commented on Apr 13, 2025

Kudryavkaz commented on Apr 13, 2025

kavinkumar807 commented on Apr 14, 2025

felixweinberger commented on Oct 3, 2025

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