SSE Connection Failure Described as "throw new Error("Not connected")"

[Ishiharaerika]

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:

1. Minimal walkthrough of installation.
2. npm cache clean --force
3. Copy&Paste simple MCP server.py at QuickStart. (https://github.com/modelcontextprotocol/python-sdk?tab=readme-ov-file#quickstart)
4. mcp dev server.py
5. 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.

[codingjenny]

Would you mind sharing how you installed it in your minimum walkthrough?

[Ishiharaerika]

Would you mind sharing how you installed it in your minimum walkthrough?

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.

npm install -g @modelcontextprotocol/server-filesystem
npm install -g @modelcontextprotocol/server-memory
npm install -g @modelcontextprotocol/server-brave-search

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]

This is happening to me too. Installing those programs did not help.

[Ishiharaerika]

This is happening to me too. Installing those programs did not help.

Hmm🧐... What I could recommend would be checking basic prerequisite for MCP installation first.
For example, if Windows, https://gist.github.com/feveromo/7a340d7795fca1ccd535a5802b976e1f