UnhandledPromiseRejection and attach failure in MATLAB Language Server

[prabhakar993]

Describe the bug
The MATLAB Language Server (matlabls) fails to attach properly to MATLAB R2024b when running under Node.js v22.19.0. The process logs show repeated connection attempts, followed by an UnhandledPromiseRejection error and automatic server restart.

To Reproduce

1. Launch VS Code with MATLAB extension enabled.
2. Open any .m file in the workspace.
3. Wait for MATLAB Language Server to initialize.
4. Observe repeated "Retrying attach" and "UnhandledPromiseRejection" errors in the output log.

Expected behavior
The MATLAB Language Server should attach successfully to MATLAB R2024b without throwing unhandled promise rejections or restarting repeatedly.

Screenshots
(node:7576) [DEP0040] DeprecationWarning: The punycode module is deprecated. Please use a userland alternative instead.
(Use Code --trace-deprecation ... to show where the warning was created)
(10:01:51) matlabls: Log Directory: C:\Users\prabh\AppData\Local\Temp\matlabls_7576
(10:10:28) matlabls: Launching MATLAB...
(10:10:28) matlabls: Found cached licensing information...
(10:10:28) matlabls: Directory created or already exists: C:\Users\prabh.matlab\MWI\hosts\prabha
(10:10:30) matlabls: Successfully fetched entitlements
Using cached Online Licensing to launch MATLAB.
(10:10:30) matlabls: File written successfully to C:\Users\prabh.matlab\MWI\hosts\prabha\proxy_app_config.json
(10:10:38) matlabls: Started MATLAB (session 1)
(10:10:38) matlabls: MATLAB session 1 connected to R2024b
[Error - 10:10:41 AM] (10:10:41) matlabls - ERROR: Timeout when attaching to release: 24b
(10:10:41) matlabls: Retrying attach
(10:10:41) matlabls: MVM attach success
(10:10:41) matlabls: Adding workspace folder(s) to the MATLAB Path:
d:\PRE
(10:10:42) matlabls: CWD set to: d:\PRE
node:internal/process/promises:392
new UnhandledPromiseRejection(reason);
^

UnhandledPromiseRejection: This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). The promise rejected with the reason "undefined".
at throwUnhandledRejectionsMode (node:internal/process/promises:392:7)
at processPromiseRejections (node:internal/process/promises:475:17)
at process.processTicksAndRejections (node:internal/process/task_queues:106:32) {
code: 'ERR_UNHANDLED_REJECTION'
}

Node.js v22.19.0
[Info - 10:10:43 AM] Connection to server got closed. Server will restart.
true
(10:10:44) matlabls: Log Directory: C:\Users\prabh\AppData\Local\Temp\matlabls_17024
(node:17024) [DEP0040] DeprecationWarning: The punycode module is deprecated. Please use a userland alternative instead.
(Use Code --trace-deprecation ... to show where the warning was created)

Useful Information

• MATLAB Version: R2024b
• OS Version: Windows 10 (x64)
• VS Code Version: 1.105

Additional context
Add any other context about the problem here.

[dklilley]

Thanks for reaching out about this!

Can you share what version of the MATLAB extension you have installed?

Does this issue happen every time you try to connect, or only sporadically?

[prabhakar993]

I’m currently using MATLAB extension v1.3.6.
The issue occurs every time I try to connect — the Language Server consistently throws the UnhandledPromiseRejection error and restarts, so I haven’t been able to establish a stable connection yet.

[dklilley]

Can you try installing v1.3.5 instead, and letting me know if that resolves the problem? This will help to narrow down the set of changes which this may be coming from.

You can do this by:

Go to the Extensions side panel in VS Code
Click the ⚙️ icon next to the MATLAB extension
Select "Install Specific Version..."
After selecting v1.3.5, I would recommend restarting VS Code to ensure the change takes effect.

[prabhakar993]

I’ve installed v1.3.5 of the MATLAB extension and restarted VS Code as recommended, but unfortunately the issue still persists. The Language Server continues to show the same UnhandledPromiseRejection error and restarts repeatedly when trying to attach to MATLAB R2024b.

(02:40:35) matlabls: Log Directory: C:\Users\prabh\AppData\Local\Temp\matlabls_3224
(node:3224) [DEP0040] DeprecationWarning: The punycode module is deprecated. Please use a userland alternative instead.
(Use Code --trace-deprecation ... to show where the warning was created)
(02:40:54) matlabls: Launching MATLAB...
(02:40:54) matlabls: Found cached licensing information...
(02:40:54) matlabls: Directory created or already exists: C:\Users\prabh.matlab\MWI\hosts\prabha
(02:40:56) matlabls: Successfully fetched entitlements
Using cached Online Licensing to launch MATLAB.
(02:40:56) matlabls: File written successfully to C:\Users\prabh.matlab\MWI\hosts\prabha\proxy_app_config.json
(02:41:06) matlabls: Started MATLAB (session 1)
(02:41:06) matlabls: MATLAB session 1 connected to R2024b
[Error - 2:41:09 AM] (02:41:09) matlabls - ERROR: Timeout when attaching to release: 24b
(02:41:09) matlabls: Retrying attach
(02:41:09) matlabls: MVM attach success
(02:41:09) matlabls: Adding workspace folder(s) to the MATLAB Path:
d:\PRE
(02:41:11) matlabls: CWD set to: d:\PRE
node:internal/process/promises:392
new UnhandledPromiseRejection(reason);
^

UnhandledPromiseRejection: This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). The promise rejected with the reason "undefined".
at throwUnhandledRejectionsMode (node:internal/process/promises:392:7)
at processPromiseRejections (node:internal/process/promises:475:17)
at process.processTicksAndRejections (node:internal/process/task_queues:106:32) {
code: 'ERR_UNHANDLED_REJECTION'
}

Node.js v22.19.0
[Info - 2:41:13 AM] Connection to server got closed. Server will restart.
true
(02:41:14) matlabls: Log Directory: C:\Users\prabh\AppData\Local\Temp\matlabls_12996
(node:12996) [DEP0040] DeprecationWarning: The punycode module is deprecated. Please use a userland alternative instead.
(Use Code --trace-deprecation ... to show where the warning was created)

[dklilley]

Thanks for the extra info! We are looking into this.

One more question in the meantime:

It appears, based on the above error log, that you have a workspace set up with at least one folder.

Does this issue only occur when you have a workspace set up in VS Code, or are you able to connect to MATLAB when there is no workspace?