MCP Inspector: "Error Connecting to MCP Inspector Proxy - Check Console logs" fix?
I'm using MCP Inspector (modelcontextprotocol/inspector) with a simple FastMCP server. I start it with
npx @modelcontextprotocol/inspector uv run main.pyormcp dev server.py; the console shows "Proxy server listening on port 6277" and the UI at http://127.0.0.1:6274, but the page reports "Error Connecting to MCP Inspector Proxy - Check Console logs". No OS/version details are given. How do I resolve this?Originally asked on GitHub: https://github.com/modelcontextprotocol/inspector/issues/484
6 answers
Status: The issue is closed. There is no single official fix in the thread, but one user reported it was resolved for them in a later release of
inspector@latest(they closed their own issue #561), and others found workarounds.What worked for people in the thread:
-
Make port 6277 reachable from the browser. One user found the root cause was that the Inspector UI (port 6274) also calls the proxy on port 6277, which failed when only 6274 was exposed. They fixed it in Kubernetes with
hostNetwork: trueand bothcontainerPort: 6274andcontainerPort: 6277, or with Docker:docker run --network host -p 6274:6274 -p 6277:6277 ghcr.io/aaronpowell/inspector:latest. -
Pin the Python MCP packages. Another user resolved it by setting
fastmcp==2.10.1andmcp[cli]==1.10.1inpyproject.toml. An older working container in the thread usedmcp/python-sdk v1.9.5.dev, which suggests version drift can cause this. -
Try a different browser. One commenter suggested DuckDuckGo or VS Code Simple Browser, but this was not confirmed.
Also reported: After a protocol update, some users saw the proxy fail on the
/healthcall (HTML 200 response that the client can't parse) and neededDANGEROUSLY_OMIT_AUTH, while another user said that flag did not help. The console showed HTTP 503upstream connect errorin one reported case. This flag disables authentication, so use it only for local testing.The original reporter's own setup (console fine, UI error) had no reply from a maintainer, so if the above don't help, check whether your browser can reach port 6277 and try
inspector@latest.Did this work for you?-
What a GitHub user tried on GitHub (Didn't work)
Started the inspector with
npx @modelcontextprotocol/inspector uv run main.pyandmcp dev server.py. Console showed the proxy on port 6277 and UI on port 6274, but the UI still reported the proxy connection error.Source: https://github.com/modelcontextprotocol/inspector/issues/484 (opens external site)
Did this work for you?What @Codecaver tried on GitHub (Worked)
Deployed the inspector in Kubernetes with
hostNetwork: trueand exposed both containerPort 6274 and 6277, or randocker run --network host -p 6274:6274 -p 6277:6277 ghcr.io/aaronpowell/inspector:latest. The fix worked because port 6277 became reachable.Did this work for you?What @GoodGuyGregory tried on GitHub (Worked)
Pinned
fastmcp==2.10.1andmcp[cli]==1.10.1inpyproject.toml, which resolved the connection error for that user.Did this work for you?What @vitaliy-sharandin tried on GitHub (Outcome unclear)
Switched the browser to DuckDuckGo or VS Code Simple Browser to avoid the proxy error.
Did this work for you?What @hesreallyhim tried on GitHub (Partly worked)
Used
DANGEROUSLY_OMIT_AUTHbecause the proxy's/healthcall returned an HTML 200 response the client couldn't parse; one commenter said this flag did not help for them.Did this work for you?
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