MCP Inspector Fails to Connect with stdio Transport and --connect Flag (macOS) #106

dazzaji · 2024-12-07T10:11:07Z

Description:

When attempting to connect the MCP Inspector to the "everything" server using stdio transport and the --connect flag (with the server running in a separate terminal), the connection fails. The Inspector frontend consistently shows "Connection Error," and the browser console logs repeated ERR_CONNECTION_REFUSED. No MCP messages are exchanged.

Steps to Reproduce:

Environment:
- Node.js v, npm v, mcp-inspector v
- Python 3.11.1 (via pyenv), macOS Ventura 13.5, VS Code 1.95.3

Create and activate a Python virtual environment:

python3.11 -m venv .venv
source .venv/bin/activate

Install mcp and trio packages:
```
pip install mcp trio
```
Terminal 1 (external terminal):
- Navigate to your project directory:
```
cd /path/to/your/directory
source .venv/bin/activate
python -m mcp.server
```
- Leave this terminal running.
Terminal 2 (VS Code integrated terminal):
- Open VS Code, set Python interpreter to .venv/bin/python.
- Run the following:
```
cd /path/to/your/directory
source .venv/bin/activate
mcp-inspector --transport stdio --connect
```
Open Inspector in a browser at http://localhost:5173 (or the URL displayed in Terminal 2). Observe the "Connection Error."
External Terminal Test (Optional):
Repeat the steps above using only external terminals (instead of the VS Code integrated terminal) to rule out VS Code as a factor.

Expected Behavior:

The MCP Inspector connects to the externally running server, and the frontend UI (including the "Tools" tab) is fully functional.

Actual Behavior:

The Inspector frontend displays "Connection Error," the browser console logs repeated ERR_CONNECTION_REFUSED, and no MCP messages are exchanged. The server appears to restart repeatedly.

Additional Information:

VS Code's terminal.integrated.closeWhenInactive setting is disabled.
No launch.json (debugger configuration) is present.
No firewall or port conflicts on localhost (verified using lsof -i :3000 and lsof -i :5173).

ATTACHMENTS

Terminal 1 (server)

Last login: Sat Dec 7 00:10:52 on ttys009
-bash: chruby: command not found
-bash: chruby: command not found

The default interactive shell is now zsh.
To update your account to use zsh, please run chsh -s /bin/zsh.
For more details, please visit https://support.apple.com/kb/HT208050.
Users-iMac:~ User$ cd /Users/User/test-mcp
Users-iMac:test-mcp User$ rm -rf .venv
Users-iMac:test-mcp User$ python3.11 -m venv .venv
Users-iMac:test-mcp User$ source .venv/bin/activate
(.venv) Users-iMac:test-mcp User$ pip install --upgrade mcp trio
Collecting mcp
Using cached mcp-1.1.0-py3-none-any.whl (36 kB)
Collecting trio
Using cached trio-0.27.0-py3-none-any.whl (481 kB)
Collecting anyio>=4.6
Using cached anyio-4.7.0-py3-none-any.whl (93 kB)
Collecting httpx-sse>=0.4
Using cached httpx_sse-0.4.0-py3-none-any.whl (7.8 kB)
Collecting httpx>=0.27
Using cached httpx-0.28.1-py3-none-any.whl (73 kB)
Collecting pydantic>=2.8.0
Using cached pydantic-2.10.3-py3-none-any.whl (456 kB)
Collecting sse-starlette>=2.0
Using cached sse_starlette-2.1.3-py3-none-any.whl (9.4 kB)
Collecting starlette>=0.39
Using cached starlette-0.41.3-py3-none-any.whl (73 kB)
Collecting attrs>=23.2.0
Using cached attrs-24.2.0-py3-none-any.whl (63 kB)
Collecting sortedcontainers
Using cached sortedcontainers-2.4.0-py2.py3-none-any.whl (29 kB)
Collecting idna
Using cached idna-3.10-py3-none-any.whl (70 kB)
Collecting outcome
Using cached outcome-1.3.0.post0-py2.py3-none-any.whl (10 kB)
Collecting sniffio>=1.3.0
Using cached sniffio-1.3.1-py3-none-any.whl (10 kB)
Collecting typing_extensions>=4.5
Using cached typing_extensions-4.12.2-py3-none-any.whl (37 kB)
Collecting certifi
Using cached certifi-2024.8.30-py3-none-any.whl (167 kB)
Collecting httpcore==1.*
Using cached httpcore-1.0.7-py3-none-any.whl (78 kB)
Collecting h11<0.15,>=0.13
Using cached h11-0.14.0-py3-none-any.whl (58 kB)
Collecting annotated-types>=0.6.0
Using cached annotated_types-0.7.0-py3-none-any.whl (13 kB)
Collecting pydantic-core==2.27.1
Using cached pydantic_core-2.27.1-cp311-cp311-macosx_11_0_arm64.whl (1.8 MB)
Collecting uvicorn
Using cached uvicorn-0.32.1-py3-none-any.whl (63 kB)
Collecting click>=7.0
Using cached click-8.1.7-py3-none-any.whl (97 kB)
Installing collected packages: sortedcontainers, typing_extensions, sniffio, idna, httpx-sse, h11, click, certifi, attrs, annotated-types, uvicorn, pydantic-core, outcome, httpcore, anyio, trio, starlette, pydantic, httpx, sse-starlette, mcp
Successfully installed annotated-types-0.7.0 anyio-4.7.0 attrs-24.2.0 certifi-2024.8.30 click-8.1.7 h11-0.14.0 httpcore-1.0.7 httpx-0.28.1 httpx-sse-0.4.0 idna-3.10 mcp-1.1.0 outcome-1.3.0.post0 pydantic-2.10.3 pydantic-core-2.27.1 sniffio-1.3.1 sortedcontainers-2.4.0 sse-starlette-2.1.3 starlette-0.41.3 trio-0.27.0 typing_extensions-4.12.2 uvicorn-0.32.1

[notice] A new release of pip available: 22.3.1 -> 24.3.1
[notice] To update, run: pip install --upgrade pip
(.venv) Users-iMac:test-mcp User$ python -m mcp.server
INFO:server:Starting receive loop

Terminal 2 (inspector)

bash: chruby: command not found
bash: chruby: command not found

The default interactive shell is now zsh.
To update your account to use zsh, please run chsh -s /bin/zsh.
For more details, please visit https://support.apple.com/kb/HT208050.
(.venv) Users-iMac:test-mcp User$ cd /Users/User/test-mcp
(.venv) Users-iMac:test-mcp User$ rm -rf .venv
(.venv) Users-iMac:test-mcp User$ python3.11 -m venv .venv
(.venv) Users-iMac:test-mcp User$ source .venv/bin/activate
(.venv) (.venv) Users-iMac:test-mcp User$ mcp-inspector --transport stdio --connect
Starting MCP inspector...
node:internal/util/parse_args/parse_args:87
throw new ERR_PARSE_ARGS_INVALID_OPTION_VALUE(errorMessage);
^

TypeError [ERR_PARSE_ARGS_INVALID_OPTION_VALUE]: Option '--env' argument is ambiguous.
Did you forget to specify the option argument for '--env'?
To specify an option argument starting with a dash use '--env=-XYZ'.
at checkOptionLikeValue (node:internal/util/parse_args/parse_args:87:11)
at node:internal/util/parse_args/parse_args:382:9
at Array.forEach ()
at parseArgs (node:internal/util/parse_args/parse_args:378:3)
at file:///usr/local/lib/node_modules/@modelcontextprotocol/inspector/server/build/index.js:14:20
at ModuleJob.run (node:internal/modules/esm/module_job:222:25)
at async ModuleLoader.import (node:internal/modules/esm/loader:316:24)
at async asyncRunEntryPointWithESMLoader (node:internal/modules/run_main:123:5) {
code: 'ERR_PARSE_ARGS_INVALID_OPTION_VALUE'
}

Node.js v20.16.0

🔍 MCP Inspector is up and running at http://localhost:5173 🚀

Console output from browser on Inspector page

Failed to load resource: net::ERR_CONNECTION_REFUSEDUnderstand this errorAI
index-BKN3G2r3.js:22136 Error fetching default environment: TypeError: Failed to fetch
at index-BKN3G2r3.js:22127:5
at Qj (index-BKN3G2r3.js:5012:22)
at Hk (index-BKN3G2r3.js:6247:19)
at Ek (index-BKN3G2r3.js:5811:3)
at jg (index-BKN3G2r3.js:2725:16)
at Wk (index-BKN3G2r3.js:6151:3)
at Pk (index-BKN3G2r3.js:6094:36)
at Gk (index-BKN3G2r3.js:5750:11)
at J2 (index-BKN3G2r3.js:442:19)
at MessagePort.R2 (index-BKN3G2r3.js:470:14)
(anonymous) @ index-BKN3G2r3.js:22136Understand this errorAI
index-BKN3G2r3.js:22353 Error: SSE error: {"isTrusted":true}
at _eventSource.onerror (index-BKN3G2r3.js:12410:23)
connectMcpServer @ index-BKN3G2r3.js:22353Understand this errorAI
:3000/sse?transportType=stdio&command=python&args=-m+mcp.server&env=%7B%7D:1

   Failed to load resource: net::ERR_CONNECTION_REFUSEDUnderstand this errorAI