Couldn't connect to MCP Proxy Server TypeError: Failed to fetch

[chaithuchowdhary]

Inspector Version
0.16.7

When running the MCP Inspector using either of the following commands:

npx @modelcontextprotocol/inspector

or

mcp dev main.py

The terminal output only shows:

Starting MCP Inspector...

The Inspector web UI launches successfully in the browser. However, when attempting to connect to the MCP server from the Inspector UI, the connection consistently fails.

This issue occurs on Ubuntu 24.04.3 LTS.

Console log from the browser

GET http://localhost:6277/config net::ERR_CONNECTION_REFUSED                           index-Dg7mXHRE.js:38902

(anonymous)	@	index-Dg7mXHRE.js:38902
Qj	@	index-Dg7mXHRE.js:5056
Hk	@	index-Dg7mXHRE.js:6291
Ek	@	index-Dg7mXHRE.js:5855
jg	@	index-Dg7mXHRE.js:2769
Wk	@	index-Dg7mXHRE.js:6195
Pk	@	index-Dg7mXHRE.js:6138
Gk	@	index-Dg7mXHRE.js:5794
J2	@	index-Dg7mXHRE.js:475
R	@	index-Dg7mXHRE.js:503

Error fetching default environment: TypeError: Failed to fetch
    at index-Dg7mXHRE.js:38902:5
    at Qj (index-Dg7mXHRE.js:5056:23)
    at Hk (index-Dg7mXHRE.js:6291:21)
    at Ek (index-Dg7mXHRE.js:5855:5)
    at jg (index-Dg7mXHRE.js:2769:17)
    at Wk (index-Dg7mXHRE.js:6195:5)
    at Pk (index-Dg7mXHRE.js:6138:36)
    at Gk (index-Dg7mXHRE.js:5794:13)
    at J2 (index-Dg7mXHRE.js:475:21)
    at MessagePort.R (index-Dg7mXHRE.js:503:15)

To Reproduce

1. run npx @modelcontextprotocol/inspector or mcp dev your_mcp_server.py

Expected behavior

Screenshots

Environment:

• OS: Ubuntu 24.04.3 LTS
• Browser: Brave, Firefox, Chrome

[olaservo]

It looks like the proxy server might not be running. What happens if you just try to start the proxy server using npm run start-server?

[chaithuchowdhary]

It looks like the proxy server might not be running. What happens if you just try to start the proxy server using npm run start-server?

I am not sure about that. If the proxy server is not running how does it open up the inspector in the browser. Also I ran the command npx @modelcontextprotocol/inspector. I don't know about npm run start-server