René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"`RuntimeError: Task group is not initialized` in the simplest possible MCP server with default config","articleBody":"## The bug\n```\nRuntimeError: Task group is not initialized. Make sure to use run().\nResponse code: 500 (Internal Server Error); Time: 9ms (9 ms); Content length: 1677 bytes (1.68 kB)\n```\nReproduces in each subsequent HTTP request (the first one works fine).\n\n**It's very counterintuitive if the default configuration doesn't work out of the box.**\n\n## To Reproduce\n\n```python\nfrom mcp.server.fastmcp import FastMCP\nmcp = FastMCP(port=8010, debug=True, host='0.0.0.0')\n@mcp.tool()\ndef ping(): return \"pong\"\nmcp.run(transport=\"streamable-http\")\n```\n\n```http\n###\nPOST http://localhost:8010/mcp/session\nContent-Type: application/json\nAccept: application/json, text/event-stream\n\n{\n    \"method\": \"initialize\",\n    \"jsonrpc\": \"2.0\",\n    \"id\": 1\n}\n```\n\n## Stack trace\n\n```\nHTTP/1.1 500 Internal Server Error\ndate: Thu, 29 May 2025 14:19:00 GMT\nserver: uvicorn\ncontent-length: 1677\ncontent-type: text/plain; charset=utf-8\n\nTraceback (most recent call last):\n  File \"C:\\Users\\********\\AppData\\Roaming\\Python\\Python313\\site-packages\\starlette\\middleware\\errors.py\", line 164, in __call__\n    await self.app(scope, receive, _send)\n  File \"C:\\Users\\********\\AppData\\Roaming\\Python\\Python313\\site-packages\\starlette\\middleware\\exceptions.py\", line 62, in __call__\n    await wrap_app_handling_exceptions(self.app, conn)(scope, receive, send)\n  File \"C:\\Users\\********\\AppData\\Roaming\\Python\\Python313\\site-packages\\starlette\\_exception_handler.py\", line 64, in wrapped_app\n    raise exc\n  File \"C:\\Users\\********\\AppData\\Roaming\\Python\\Python313\\site-packages\\starlette\\_exception_handler.py\", line 53, in wrapped_app\n    await app(scope, receive, sender)\n  File \"C:\\Users\\********\\AppData\\Roaming\\Python\\Python313\\site-packages\\starlette\\routing.py\", line 758, in __call__\n    await self.middleware_stack(scope, receive, send)\n  File \"C:\\Users\\********\\AppData\\Roaming\\Python\\Python313\\site-packages\\starlette\\routing.py\", line 778, in app\n    await route.handle(scope, receive, send)\n  File \"C:\\Users\\********\\AppData\\Roaming\\Python\\Python313\\site-packages\\starlette\\routing.py\", line 487, in handle\n    await self.app(scope, receive, send)\n  File \"C:\\Users\\********\\AppData\\Roaming\\Python\\Python313\\site-packages\\mcp\\server\\fastmcp\\server.py\", line 786, in handle_streamable_http\n    await self.session_manager.handle_request(scope, receive, send)\n  File \"C:\\Users\\********\\AppData\\Roaming\\Python\\Python313\\site-packages\\mcp\\server\\streamable_http_manager.py\", line 137, in handle_request\n    raise RuntimeError(\"Task group is not initialized. Make sure to use run().\")\nRuntimeError: Task group is not initialized. Make sure to use run().\n```\n\n## Related issues\n\nhttps://github.com/modelcontextprotocol/python-sdk/issues/713","author":{"url":"https://github.com/Nayjest","@type":"Person","name":"Nayjest"},"datePublished":"2025-05-29T14:26:49.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":4},"url":"https://github.com/838/python-sdk/issues/838"}