René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"sse_client blocks indefinitely when server has incorrect base URL","articleBody":"**Describe the bug**\nWhen 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:\nhttps://github.com/modelcontextprotocol/python-sdk/blob/c2ca8e03e046908935d089a2ceed4e80b0c29a24/src/mcp/client/sse.py#L81\n\nThe `read_stream_writer` then sends this exception to the `read_stream`:\nhttps://github.com/modelcontextprotocol/python-sdk/blob/c2ca8e03e046908935d089a2ceed4e80b0c29a24/src/mcp/client/sse.py#L107\n\nHowever, since the `read_stream` has not been yielded yet, the exception will never be received.\nAs a result, the `sse_client` will be blocked indefinitely.\n\n**To Reproduce**\n1. Set up an MCP server with an incorrect base URL on a remote server, for example:\n```typescript\nimport express, { Request, Response } from 'express';\nimport { McpServer } from '@modelcontextprotocol/sdk/server/mcp.js';\nimport { SSEServerTransport } from '@modelcontextprotocol/sdk/server/sse.js';\nimport { z } from 'zod';\n\nconst server = new McpServer({\n    name: 'example-server',\n    version: '1.0.0',\n});\n\nserver.tool(\n  \"calculate-bmi\",\n  {\n    weightKg: z.number(),\n    heightM: z.number()\n  },\n  async ({ weightKg, heightM }) =\u003e ({\n    content: [{\n      type: \"text\",\n      text: String(weightKg / (heightM * heightM))\n    }]\n  })\n);\n\nconst app = express();\n\nconst transports: { [sessionId: string]: SSEServerTransport } = {};\n\napp.get('/sse', async (_: Request, res: Response) =\u003e {\n    // set baseUrl for the transport\n    const baseUrl = 'http://localhost:8080';\n    const transport = new SSEServerTransport(`${baseUrl}/messages`, res);\n    transports[transport.sessionId] = transport;\n    res.on('close', () =\u003e {\n        delete transports[transport.sessionId];\n    });\n    await server.connect(transport);\n});\n\napp.post('/messages', async (req: Request, res: Response) =\u003e {\n    const sessionId = req.query.sessionId as string;\n    const transport = transports[sessionId];\n    if (transport) {\n        await transport.handlePostMessage(req, res);\n    } else {\n        res.status(400).send('No transport found for sessionId');\n    }\n});\n\napp.listen(8080);\n```\n\n2. Try to connect to the remote MCP server, for example:\n```python\nimport asyncio\n\nfrom mcp.client.sse import sse_client\n\n\nasync def run():\n    try:\n        async with sse_client(\"http://{your_remote_server}:8080/sse\"):\n            print(\"Connected to SSE endpoint successfully.\")\n    except Exception as e:\n        print(f\"Error: {e}\")\n\n\nif __name__ == \"__main__\":\n    asyncio.run(run())\n```\n\n**Expected behavior**\nThe sse_client should raise an exception and exit gracefully.\n\n**Logs**\nNone\n\n**Additional context**\nNone","author":{"url":"https://github.com/Kudryavkaz","@type":"Person","name":"Kudryavkaz"},"datePublished":"2025-04-07T12:24:44.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":11},"url":"https://github.com/447/python-sdk/issues/447"}