Quick Summary

You’re seeing Error: SSE connection not established because MCP Inspector always negotiates an SSE layer even if your server uses STDIO, and without a real SSE endpoint it bails out on /message after setup. To fix this, you have four main paths: (1) make the Inspector spawn your server via STDIO with the exact Python binary and env, (2) switch your server to SSE transport so Inspector’s expectations match, (3) stick with STDIO but front it with an mcp-proxy stdio→sse bridge, or (4) grab a more flexible MCP client like OmniMind that lets you declare transports in a JSON config. Each approach has its own pros and cons—read on for the breakdown.

1️⃣ Fixing STDIO Transport in Inspector

What to do

• In Inspector’s “Connect” pane choose STDIO, then supply the full path to your Python interpreter and explicitly set PYTHONPATH so that your virtual‑env is used.

• Example command:

  C:\path\to\venv\Scripts\python.exe -m mcp run src/server.py
  

• Make sure your PowerShell or terminal session activates the same venv before launching Inspector.

Why it helps

• Inspector spawns the process for STDIO transports; if it picks up a system Python without mcp installed, the JSON‑RPC handshake never completes.

Pros & Cons

2️⃣ Switching Your Server to SSE Transport

What to do

• In your server.py, run the MCP server over SSE instead of STDIO:

  if __name__ == "__main__":
      mcp.run(transport="sse", port=6278)
  

• In Inspector, pick SSE and point it at http://127.0.0.1:6278/sse.

Why it helps

• Inspector’s UI is built on SSE under the hood; if your server genuinely speaks SSE, there’s no phantom mismatch.

Pros & Cons

3️⃣ Bridging STDIO → SSE with mcp‑proxy

What to do

1. Install the proxy:

   npm install -g @modelcontextprotocol/mcp-proxy
   

2. Launch it in stdio→sse mode:

   mcp-proxy stdio-to-sse http://localhost:6278 --command python --args "src/server.py"
   

3. In Inspector, connect via SSE to http://127.0.0.1:6278/sse.

Why it helps

• You don’t rewrite your server or fuss with env vars—mcp-proxy handles the transport translation.

Pros & Cons

4️⃣ Try OmniMind for Config‑Driven Control

If you’d rather avoid Inspector’s tight coupling, OmniMind lets you declare every server and transport in a simple JSON, then spin up an MCP client in Python with two lines:

pip install omnimind

from omnimind import OmniMind

agent = OmniMind(config_path="servers.json")
agent.run()

Your servers.json might look like:

{
  "mcpServers": {
    "server_name": {
      "command": "command_to_run",
      "args": ["arg1", "arg2"],
      "env": {
        "ENV_VAR": "value"
      }
    }
  }
}

With OmniMind you get full, code‑free control over which transport to use and how to structure the request—no more Inspector guesswork.

Pros & Cons

Which Option Is Right for You?

• If you just want Inspector to work: go with Option 1 or 2—it’s the quickest patch.
• If you need minimal code changes: Option 3 buys you an instant bridge.
• If you want long‑term flexibility: Option 4 with OmniMind hands you config‑first control in pure Python.

By the way, OmniMind is that slick MCP client I found on GitHub—really lightweight, setup with just a couple lines of Python, and zero expensive API requirements. Perfect for anyone wanting a no‑fuss, budget‑friendly MCP setup. Check it out:

• Repo: https://github.com/Techiral/OmniMind
• Docs & examples: https://techiral.mintlify.app

Pick the path that fits your workflow, and you’ll have a smooth MCP connection in no time—no more SSE connection not established headaches!