Unable to connect to the Julia kernel (Python kernel works)

[Raycosine]

Description

Environment

• VS Code version:

Commit: c3a26841a84f20dfe0850d0a5a9bd01da4f003ea
Date: 2026-02-16T15:35:57.932Z
Electron: 39.3.0
ElectronBuildId: 13313481
Chromium: 142.0.7444.265
Node.js: 22.21.1
V8: 14.2.231.22-electron.0
OS: Windows_NT arm64 10.0.26200

• Colab extension version: Latest available 0.3.0
• Jupyter extension version: Identifier Version 2025.9.1
• OS (Linux | Mac | Windows): Windows 11 Home 25H2 26200.7840

Repro Steps

1. Create an .ipynb file and open this notebook in VS Code
2. Click on "Select Kernel"
3. Then click on "Select Another Kernel..."
4. Then click on "Colab"
5. Then click on "New Colab Server"
6. Then click on "CPU"
7. Type a random name and press "Enter"
8. Select the Julia kernel "julia 1.11 (usr\local\bin\julia)"

Expected Behaviour

Actual Behaviour

Cannot connect to the kernel and cannot perform any operation like "3+5"...

Additional context

If I switch to the Python kernel on the same server by

1. Click on "Select Another Kernel..."
2. Then click on "Colab"
3. Then click on "Auto Connect"

There are no such connection issues.

When connected to the Python kernel, run

!which python
!which julia

prints

!which python
!which julia

Run

!jupyter kernelspec list

It prints

0.00s - Debugger warning: It seems that frozen modules are being used, which may
0.00s - make the debugger miss breakpoints. Please pass -Xfrozen_modules=off
0.00s - to python to disable frozen modules.
0.00s - Note: Debugging will proceed. Set PYDEVD_DISABLE_FILE_VALIDATION=1 to disable this validation.
Available kernels:
  julia      /root/.local/share/jupyter/kernels/julia
  ir         /usr/local/share/jupyter/kernels/ir
  python3    /usr/local/share/jupyter/kernels/python3

!julia -e 'using IJulia; println("IJulia OK")'

works correctly.

Logs

Logs from Colab in the Output panel (View > Output, change the drop-down the upper-right of the Output panel to Colab).

I can find the sessions created in VS Code on the "Active Sessions" tab on Colab's web page.

Logs from Jupyter in the Output panel (View > Output, change the drop-down the upper-right of the Output panel to Jupyter).

22:17:20.938 [error] Widget Error: Failed to access CDN https://unpkg.com/ after 0 attempt(s), TypeError: Failed to fetch
22:17:39.282 [info] Starting Kernel (julia 1.11) for '[file path]' (disableUI=false)
22:17:40.103 [info] https://8080-m-s-1t1luoi00x2n3-c.us-central1-2.prod.colab.dev/: Kernel started: 10c3f9e8-a953-4969-beac-[a string]
22:17:40.238 [error] Error in websocket [Error: Server sent no subprotocol
	at ClientRequest.<anonymous> (~\.vscode\extensions\google.colab-0.3.0\out\extension.js:41:44986)
	at ClientRequest.emit (node:events:519:28)
	at TLSSocket.socketOnData (node:_http_client:654:11)
	at TLSSocket.emit (node:events:519:28)
	at addChunk (node:internal/streams/readable:561:12)
	at readableAddChunkPushByteMode (node:internal/streams/readable:512:3)
	at Readable.push (node:internal/streams/readable:392:5)
	at TLSWrap.onStreamRead (node:internal/stream_base_commons:189:23)]
22:17:43.623 [info] Started session for kernel startUsingRemoteKernelSpec:c172890a4a7d7461c14b21d77fa2cd94a1a[a string]..julia.usr\local\bin\julia.\./usr/local/bin/julia#-i#--color=yes#-e#import ijulia; ijulia.run_kernel()#{connection_file}

[amtoney524]

Thanks for reporting this @Raycosine - I'm also able to repro issues connecting to Julia on my side, where the kernel seems to get stuck in the connecting state. We'll take a closer look and follow up. Filed b/486840082 to track internally.