MPC call entering debug mode can results in a locked state

[av-archer]

If the mcp call gets stuck in a debugger there appears to be no way to recover the matlab sessions and the process must be terminated.

Setup condition: MCP with matlab in foreground with sharing enabled.

This was specifically an issue when dbstop if error was set. There was no way to exit the debugger from within matlab.

[ybenouag]

Hi @av-archer, thank you for reporting this issue!

To help us investigate, could you share a few details about your setup:

1. MATLAB MCP Core Server version — which version are you using? (You can check by running matlab-mcp-core-server --version.)
2. MATLAB version — which release of MATLAB are you using? (e.g. R2024b, R2025a)
3. Session mode — it sounds like you are using --matlab-session-mode=existing with a shared MATLAB session. Can you confirm?
4. Reproducing the issue — could you share the MATLAB code (or a minimal example) that triggered dbstop if error and caused the locked state? This would help us reproduce and debug the problem.

In the meantime, as a workaround, you may be able to recover by typing dbquit in the MATLAB Command Window, though we understand this may not always be possible depending on the state.

Thank you for your help!
Yannis

[av-archer]

1. Using the latest release for the MCP server v0.9.0
2. MATLAB 23.2.0.2485118 (R2023b) Update 6
3. Yes --matlab-session-mode=existing
4. Do dbstop if error if the mcp call is caught by the debugger, there is no way i could figure out to exit the debugger. Closing claude code cli, dbclear all, stop button in the debugger GUI, Ctrl+C

[rrmaram2000]

Yes, I have also experienced a similar issue.

[ybenouag]

Hi @av-archer, @rrmaram2000 — thanks for reporting this and confirming the behavior.

We've reproduced the issue and identified the root cause. When dbstop if error is active and MCP evaluates code that throws an error, MATLAB enters debug mode inside our eval call, which causes it to hang indefinitely. Line-based breakpoints (dbstop in file at line) don't have this problem — only conditional breakpoints (dbstop if error, dbstop if warning, dbstop if naninf) trigger the locked state.

We're working on a fix for this.

In the meantime, if you're using an agent that supports skills (e.g., Claude Code), you can install the matlab-debugging skill from the MATLAB Agentic Toolkit. It teaches the LLM to use breakpoints safely and avoid patterns that cause the hang. It won't fix the underlying bug, but it significantly reduces the chance of the agent triggering it.

Manual workaround: If you do get stuck, typing dbquit in the MATLAB Command Window should recover the session. Running dbclear all before MCP sessions also prevents the issue.

Thanks again for the report — we'll update here when the fix ships.