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

[olaservo]

If you're still having this issue, could you double check that this still happens with the latest version? eg using npx @modelcontextprotocol/inspector@latest

[Lawrence-Godfrey]

I have the same issue
• OS: Ubuntu 24.04.2 LTS (Noble)
• Kernel: Linux 6.11.0-29-generic
• Architecture: x86_64
• Hardware: Dell G15-5530

npm view @modelcontextprotocol/inspector version
0.16.8

npx @modelcontextprotocol/inspector@latest   
Starting MCP inspector...

uv run /home/.../server.py
...
Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)

# server.py

"""
FastMCP Echo Server
"""

from fastmcp import FastMCP

# Create server
app = FastMCP("Echo Server")


@app.tool
def echo(text: str) -> str:
    """Echo the input text"""
    return text


if __name__ == "__main__":
    app.run(transport="http", host="127.0.0.1", port=8000)


__all__ = [
    "app",
]

In the inspector (running on http://localhost:6274/):

[Lawrence-Godfrey]

GET http://localhost:6277/health net::ERR_CONNECTION_REFUSED
checkProxyHealth @ index-3LzLs-Yz.js:25194
connect @ index-3LzLs-Yz.js:25253
Nb @ index-3LzLs-Yz.js:1324
Tb @ index-3LzLs-Yz.js:1336
Ub @ index-3LzLs-Yz.js:1339
nf @ index-3LzLs-Yz.js:2276
se @ index-3LzLs-Yz.js:2299
(anonymous) @ index-3LzLs-Yz.js:2571
Qk @ index-3LzLs-Yz.js:5875
Jb @ index-3LzLs-Yz.js:1277
hd @ index-3LzLs-Yz.js:2372
fd @ index-3LzLs-Yz.js:1770
ed @ index-3LzLs-Yz.js:1753Understand this error
index-3LzLs-Yz.js:25200 Couldn't connect to MCP Proxy Server TypeError: Failed to fetch
    at checkProxyHealth (index-3LzLs-Yz.js:25194:41)
    at connect (index-3LzLs-Yz.js:25253:13)
    at Object.Nb (index-3LzLs-Yz.js:1324:9)
    at Tb (index-3LzLs-Yz.js:1336:8)
    at Ub (index-3LzLs-Yz.js:1339:8)
    at nf (index-3LzLs-Yz.js:2276:5)
    at se (index-3LzLs-Yz.js:2299:11)
    at index-3LzLs-Yz.js:2571:7
    at Qk (index-3LzLs-Yz.js:5875:14)
    at Jb (index-3LzLs-Yz.js:1277:14)

There's nothing running on port 6277

[MB-MuratBayraktar]

any progress here? still having the same issue.

[chaithuchowdhary]

any progress here? still having the same issue.

Still same issue. Not able to start the proxy server. I tried clearing cache. But still same issue

[TidyMaze]

I have a similar issue

➜  ~ npx @modelcontextprotocol/inspector@latest
Need to install the following packages:
@modelcontextprotocol/inspector@0.17.2
Ok to proceed? (y)

npm warn deprecated node-domexception@1.0.0: Use your platform's native DOMException instead
Starting MCP inspector...
⚙️ Proxy server listening on localhost:6277
🔑 Session token: 6fc0da25d35c27137217483bc6a1988dcb11d80df08c4a281cb1752e5403b948
   Use this token to authenticate requests or set DANGEROUSLY_OMIT_AUTH=true to disable auth

🚀 MCP Inspector is up and running at:
   http://localhost:6274/?MCP_PROXY_AUTH_TOKEN=6fc0da25d35c27137217483bc6a1988dcb11d80df08c4a281cb1752e5403b948

🌐 Opening browser...
New StreamableHttp connection request
Query parameters: {"url":"https://iago.iadvize.net/iago/mcp","transportType":"streamable-http"}
Created StreamableHttp client transport
Client <-> Proxy  sessionId: fc2b3804-7ad2-4e4d-a0f1-0047a7baf685
file:///Users/yannrolland/.npm/_npx/9eac9498388ae25e/node_modules/node-fetch/src/index.js:216
						if (response_.statusCode !== 303 && request.body && options_.body instanceof Stream.Readable) {
						                                                             ^

TypeError: Cannot read properties of undefined (reading 'body')
    at ClientRequest.<anonymous> (file:///Users/yannrolland/.npm/_npx/9eac9498388ae25e/node_modules/node-fetch/src/index.js:216:68)
    at ClientRequest.emit (node:events:530:35)
    at HTTPParser.parserOnIncomingClient [as onIncoming] (node:_http_client:716:27)
    at HTTPParser.parserOnHeadersComplete (node:_http_common:117:17)
    at Socket.socketOnData (node:_http_client:558:22)
    at Socket.emit (node:events:530:35)
    at addChunk (node:internal/streams/readable:561:12)
    at readableAddChunkPushByteMode (node:internal/streams/readable:512:3)
    at Readable.push (node:internal/streams/readable:392:5)
    at TCP.onStreamRead (node:internal/stream_base_commons:189:23)

Node.js v22.15.0

with Streamable HTTP + Proxy, happening on click on "Connect"

the proxy is crashing and the port 6277 is released (proxy process exits)

lsof -nP -iTCP:6277 -sTCP:LISTEN || echo "nothing on :6277"

[sll521]

I have a similar issue

➜  ~ npx @modelcontextprotocol/inspector@latest
Need to install the following packages:
@modelcontextprotocol/inspector@0.17.2
Ok to proceed? (y)

npm warn deprecated node-domexception@1.0.0: Use your platform's native DOMException instead
Starting MCP inspector...
⚙️ Proxy server listening on localhost:6277
🔑 Session token: 6fc0da25d35c27137217483bc6a1988dcb11d80df08c4a281cb1752e5403b948
   Use this token to authenticate requests or set DANGEROUSLY_OMIT_AUTH=true to disable auth

🚀 MCP Inspector is up and running at:
   http://localhost:6274/?MCP_PROXY_AUTH_TOKEN=6fc0da25d35c27137217483bc6a1988dcb11d80df08c4a281cb1752e5403b948

🌐 Opening browser...
New StreamableHttp connection request
Query parameters: {"url":"https://iago.iadvize.net/iago/mcp","transportType":"streamable-http"}
Created StreamableHttp client transport
Client <-> Proxy  sessionId: fc2b3804-7ad2-4e4d-a0f1-0047a7baf685
file:///Users/yannrolland/.npm/_npx/9eac9498388ae25e/node_modules/node-fetch/src/index.js:216
						if (response_.statusCode !== 303 && request.body && options_.body instanceof Stream.Readable) {
						                                                             ^

TypeError: Cannot read properties of undefined (reading 'body')
    at ClientRequest.<anonymous> (file:///Users/yannrolland/.npm/_npx/9eac9498388ae25e/node_modules/node-fetch/src/index.js:216:68)
    at ClientRequest.emit (node:events:530:35)
    at HTTPParser.parserOnIncomingClient [as onIncoming] (node:_http_client:716:27)
    at HTTPParser.parserOnHeadersComplete (node:_http_common:117:17)
    at Socket.socketOnData (node:_http_client:558:22)
    at Socket.emit (node:events:530:35)
    at addChunk (node:internal/streams/readable:561:12)
    at readableAddChunkPushByteMode (node:internal/streams/readable:512:3)
    at Readable.push (node:internal/streams/readable:392:5)
    at TCP.onStreamRead (node:internal/stream_base_commons:189:23)

Node.js v22.15.0

with Streamable HTTP + Proxy, happening on click on "Connect"

the proxy is crashing and the port 6277 is released (proxy process exits)

lsof -nP -iTCP:6277 -sTCP:LISTEN || echo "nothing on :6277"

i have the same issue, i notice when i add a "/" at the end of the mcp url, click inspector connect button then it work,

[olaservo]

Hi! This issue has been waiting on additional information. Could you provide an update on whether this is still occurring with the latest version? If we don't hear back in 14 days, we'll close this issue. Feel free to reopen if needed.

---

This comment was posted by Claude during issue triage.

[sruthi223]

I am having same issue. It doesnt work for me when I add '/' in the URL

npm view @modelcontextprotocol/inspector version
0.21.0

lsof -nP -iTCP:6277 -sTCP:LISTEN || echo "nothing on :6277"
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
node 1931590 admin 17u IPv4 81055381 0t0 TCP 127.0.0.1:6277 (LISTEN)

index-BNqYF5Na.js:37604
GET http://10.xxxxxx:6277/config net::ERR_CONNECTION_REFUSED

index-BNqYF5Na.js:37613 Error fetching default environment: TypeError: Failed to fetch
at index-BNqYF5Na.js:37604:5
at Qj (index-BNqYF5Na.js:5054:23)
at Hk (index-BNqYF5Na.js:6289:21)
at Ek (index-BNqYF5Na.js:5853:5)
at jg (index-BNqYF5Na.js:2767:17)
at Wk (index-BNqYF5Na.js:6193:5)
at Pk (index-BNqYF5Na.js:6136:36)
at Gk (index-BNqYF5Na.js:5792:13)
at J2 (index-BNqYF5Na.js:473:21)
at MessagePort.R (index-BNqYF5Na.js:501:15)
index-BNqYF5Na.js:24658
GET http://10xxxxxxxxxxxxx:6277/health net::ERR_CONNECTION_REFUSED
index-BNqYF5Na.js:24664 Couldn't connect to MCP Proxy Server TypeError: Failed to fetch
at checkProxyHealth (index-BNqYF5Na.js:24658:41)
at connect (index-BNqYF5Na.js:24699:13)
at Object.Nb (index-BNqYF5Na.js:1322:9)
at Tb (index-BNqYF5Na.js:1334:8)
at Ub (index-BNqYF5Na.js:1337:8)
at nf (index-BNqYF5Na.js:2274:5)
at se (index-BNqYF5Na.js:2297:11)
at index-BNqYF5Na.js:2569:7
at Qk (index-BNqYF5Na.js:5873:14)
at Jb (index-BNqYF5Na.js:1275:14)

[remyleone]

I have a similar issue and adding the / to the URL does not work

[galovics]

I'm having the same issue, latest version as well (0.21.2). Tried with Node22.