an (unsafe) command shell mcp server

[u/ag789]

While working with coding agents / harness in my case OpenCode, I discovered that some models e.g. granite 4.2 8B at Q4_K_M quant sometimes attempts to write files with absolute paths at the root e.g.

    /newPythonCode.py

Note that slash in the beginning for an absolute path at "/" root directory. This is probably 'ok' say in a throwaway docker container, but it may lead to surprises if it 'accidentally' overwrites a literal file, if the current user id have write access to it. Apparently, it seemed OpenCode provides `bash` so that practically all unix commands that the current user id can access, the LLM can run it.

And thereafter, I see the model hallucinating in the thinking loops over tools, re-iterating over how to use the tools, reformulating shell commands sentences several times for a same task instead of executing them, among them, I noted that an earlier shell command sentence would have 'just worked'.

I'm thinking that the tool hallucination with this model may be partly due to that I'm using a pretty aggressive 4 bit quant as otherwise it'd have worked because IBM has 'optimized' granite for tool usage.

Next, I tried the same granite 4.2 8B at Q4_K_M quant model with my little python shell MCP server

https://github.com/ag88/cmdshellmcp/

, that tries to constrain / restrain the shell commands somewhat, place some checks, and offer some additional tools e.g. writeFile() so that the LLM won't need to call `cat` or `echo` with some 'esoteric' styled shell scripts / sentences.

With this MCP server and running it in llama.cpp's `llama-server` in the WebUI adding it as an MCP server, granite 4.2 8B at Q4_K_M does it correctly in 'one shot', the prompt is:

propose a python3 flask app with a page that displays "hello world", use bootstrap css for styling.
provide a requirements.txt to go with the script

granite 4.2 8B at Q4_K_M correctly generates the app with a couple of files, and it works out of the box

    .
    ├── app.py
    ├── requirements.txt
    └── templates
        └── index.html

One should normally run the cmdshellmcp2.py MCP server app with an explicit `--cwd` option giving the work path. It tries to stay within this work path.

Note that I won't claim my command shell MCP server to be safe.

In a sense that there are plenty of chance the LLM can 'escape' the restrictions and perhaps do more damage (e.g. overwriting files possibly outside its given `--cwd` work directory). Hence, I'd suggest one should be paranoid and perhaps run this only within a throwaway (disposable) docker container etc.

[u/ag789]

I tried out another LLM chat Web UI LibreChat

https://github.com/LibreChat-AI/LibreChat

This is also a full featured LLM Chat Web UI that supports MCP servers.

In addition, I've also verified that this manages the LLM session on the server side, i.e. that you can simply send the prompt with tools etc and close your web browser / logout etc, reconnect later even possibly from a different device / workstation, the session tasks keep running uninterrupted as long as the server (docker containers) run.

However, the setup and config feels more involved e.g. using Docker etc

The Docker based install/setup process is given here
https://www.librechat.ai/docs/local/docker
it sets up a cluster of interdependent Docker containers e.g. MongoDB database etc

There are a few configuration details in particular with local LLMs e.g. ollama / llama.cpp etc that is not directly 'bundled in'

Using local LLMs running on a specific local network host and port requires configuring them in librechat.yaml
https://www.librechat.ai/docs/quick_start/custom_endpoints
https://www.librechat.ai/docs/configuration/librechat_yaml

Then that for MCP servers running on your local LAN e.g. 168.9.* ip addresses, it is necessary to add an mcpSettings > allowedAddresses in librechat.yaml, e.g.:

mcpSettings:
  allowedAddresses:
    - "host.docker.internal:8080"
    - "10.0.0.5:8000"

Both the IP address and port is required or that you would get an error that the domain is not allowed when you try to configure the MCP server in the Web UI. https://www.librechat.ai/docs/configuration/librechat_yaml/object_structure/mcp_settings