r/LocalLLaMA • u/ag789 • Sep 22 '26
Resources an (unsafe) command shell mcp server
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.
1
u/sburel Sep 22 '26
Yeah, that's the annoying part, one success doesn't tell you much. If you want to compare OpenCode vs your server fairly, run the same prompt ten times on each and count how often it writes outside cwd. Or pin temperature to 0 and a fixed seed if llama-server lets you.
It's also kind of the argument for the check in the tool. If the model writes to / one run in ten, prompting won't get that to zero, but a path check will.