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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Initialize call hangs forever if MCP server does not return a `Content-Type: text/plain`","articleBody":"### Initial Checks\n\n- [x] I confirm that I'm using the latest version of MCP Python SDK\n- [x] I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue\n\n### Description\n\nWe had some timeouts on our server while using MCP client from the SDK.\nWhen users try to integrate our client with their MCP server, we do an `initialize` call to the server and we are waiting infinitely to the following:\n```\nsync with self:\n    ...\n```\n\nAfter investigation, it seems the error happens when the MCP server is misconfigured, and returns something different than `Content-Type: text/event-stream` as a response from the `initialize` call.\n\nAlthough it is on the server side, the client should be able to fail immediately with a understandable error to warn the user something went wrong with the server's `initialize` output.\n   \n\n### Example Code\n\n```Python\n# server.py\n\nfrom http.server import BaseHTTPRequestHandler, HTTPServer\n\nclass PlainTextHandler(BaseHTTPRequestHandler):\n    def do_POST(self):\n        length = int(self.headers.get(\"Content-Length\", 0))\n        self.rfile.read(length)\n\n        body = b\"this is not json\"\n        self.send_response(200)\n        self.send_header(\"Content-Type\", \"text/plain\")\n        self.send_header(\"Content-Length\", str(len(body)))\n        self.end_headers()\n        self.wfile.write(body)\n\n    def log_message(self, format, *args):\n        print(f\"[server] {format % args}\")\n\nserver = HTTPServer((\"localhost\", 9000), PlainTextHandler)\nprint(\"[server] listening on http://localhost:9000\")\nserver.serve_forever()\n\n# client.py\nimport asyncio\nimport mcp\nfrom mcp.client.streamable_http import streamable_http_client\n\nasync def main():\n    print(\"connecting…\")\n    async with streamable_http_client(\"http://localhost:9000/mcp\") as (\n        read_stream, write_stream, _,\n    ):\n        async with mcp.ClientSession(read_stream, write_stream) as session:\n            await session.initialize()  # ← hangs here forever\n\n    print(\"done (you should never see this)\")\n```\n\n### Python \u0026 MCP Python SDK\n\n```Text\n- Python 3.12.12\n- MCP - 1.27.0\n```","author":{"url":"https://github.com/HenriChabert","@type":"Person","name":"HenriChabert"},"datePublished":"2026-04-13T09:34:14.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":3},"url":"https://github.com/2432/python-sdk/issues/2432"}