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SSE Connection Failure Described as "throw new Error("Not connected")" #433
Description
Describe the bug
MCP initialization stucked at "Set up MCP Proxy" with node_modules/@modelcontextprotocol/sdk/dist/esm/server/sse.js:103
throw new Error("Not connected") on Windows 11 24H2. I doubt that this shouldn't be a bug but could be misconfiguration.
To Reproduce
Steps to reproduce the behavior:
- Minimal walkthrough of installation.
- npm cache clean --force
- Copy&Paste simple MCP server.py at QuickStart. (https://github.com/modelcontextprotocol/python-sdk?tab=readme-ov-file#quickstart)
- mcp dev server.py
- Error with SSE. Logs below:
Starting MCP inspector...
⚙️ Proxy server listening on port 6277
🔍 MCP Inspector is up and running at http://127.0.0.1:6274 🚀
Stdio transport: command=C:\Program Files\Python311\Scripts\uv.exe, args=run,--with,mcp,mcp,run,test_server.py
Spawned stdio transport
Connected MCP client to backing server transport
Created web app transport
Created web app transport
Set up MCP proxy
file:///C:/.../node_modules/@modelcontextprotocol/sdk/dist/esm/server/sse.js:103
throw new Error("Not connected");
^
Error: Not connected
at SSEServerTransport.send (file:///C:/.../node_modules/@modelcontextprotocol/sdk/dist/esm/server/sse.js:103:19)
at Socket.<anonymous> (file:///C:/.../node_modules/@modelcontextprotocol/inspector/server/build/index.js:101:33)
at Socket.emit (node:events:507:28)
at addChunk (node:internal/streams/readable:559:12)
at readableAddChunkPushByteMode (node:internal/streams/readable:510:3)
at Readable.push (node:internal/streams/readable:390:5)
at Pipe.onStreamRead (node:internal/stream_base_commons:189:23)
Node.js v23.11.0
Expected behavior
Connection through SSE should be established.
Screenshots
I shall provide detailed log output and actively participating discussion. I'd be glad to hear if there's anything I could provide.
Desktop (please complete the following information):
- OS: Windows 11 24H2
- Browser Thorium / Edge / Chromium
- Version 26100.3624
Additional context
Gratitude on active development. Shame on me triggering low-level weird bugs.
Activity
yychanlee commentedon Apr 5, 2025
Would you mind sharing how you installed it in your minimum walkthrough?
Ishiharaerika commentedon Apr 5, 2025
Huge thanks for the reply.
It seems that I solved the problem by installing basic requirements. It could be indeed completely user-side problem. Please take my apologies.
Another practice needs to be mentioned is that Killer NIC (now hold by Intel) with Killer Performance Suite UWD installed slows down SSE connection significantly. I would recommend those having toubles try uninstalling Killer Performance Suite UWD for stablized connection.
Tylersuard commentedon May 8, 2025
This is happening to me too. Installing those programs did not help.
Ishiharaerika commentedon May 8, 2025
Hmm🧐... What I could recommend would be checking basic prerequisite for MCP installation first.
For example, if Windows, https://gist.github.com/feveromo/7a340d7795fca1ccd535a5802b976e1f