Local nbextensions can't be found for kernel launched from local kernelspec

[egormkn]

Applies To

• Notebooks (.ipynb files)
• Interactive Window and/or Cell Scripts (.py files with #%% markers)

What happened?

Hello!
I found a bug in vscode-jupyter extension: it does not load local nbextensions if the kernel was launched from a local kernelspec. In this case I get "No nbextensions folder found for kernel" in Jupyter log and widgets do not work if CDN access was not granted, even though I have an nbextensions directory with needed javascript in <venv>/share/jupyter. If I launch a kernel from venv interpreter, everything works as expected.

It seems that happens because NbExtensionsPathProvider does not process the startUsingLocalKernelSpec kind.

I encountered this problem while trying to fix the pythreejs bug with a patch, so you should be able to reproduce the issue with something like pip install git+https://github.com/jupyter-widgets/pythreejs.git@b41067fa9347c55d30821f3e3a85c01aab1726e9 and then executing:

import pythreejs
pythreejs.Group(children=[pythreejs.ArrowHelper(dir=[0, 0, 1]), pythreejs.ArrowHelper(dir=[0, 1, 0])])

(this fails until the patched version of pythreejs is uploaded to CDN)

There was also a relevant discussion about local development of ipywidgets, but I think the author there used an interpreter-based kernel, not a kernelspec-based.

VS Code Version

1.102.1

Jupyter Extension Version

2025.6.0

Jupyter logs

Visual Studio Code (1.102.1, undefined, desktop)
Jupyter Extension Version: 2025.6.0.
Python Extension Version: 2025.10.0.
Python Environment Extension not installed.
Pylance Extension Version: 2025.6.2.
Platform: linux (x64).
Home = /home/<username>
Temp Storage folder ~/.config/Code/User/globalStorage/ms-toolsai.jupyter/version-2025.6.0
Workspace folder ~/workspace
09:50:04.868 [debug] Start refreshing Kernel Picker (1753080604868)
09:50:05.000 [debug] Get Custom Env Variables, Class name = Hm, completed in 132ms, has a truthy return value, Arg 1: undefined, Arg 2: "RunPythonCode"
09:50:05.001 [warn] No interpreter with path ~/venv/bin/python3 found in Python API, will convert Uri path to string as Id ~/venv/bin/python3
09:50:05.044 [debug] Jupyter Paths /kernels: ~/venv/share/jupyter/kernels
09:50:05.044 [debug] Kernel Spec Root Paths, ~/venv/share/jupyter/kernels, /usr/share/jupyter/kernels, /usr/local/share/jupyter/kernels, ~/.local/share/jupyter/kernels
09:50:05.764 [trace] Conda file is conda
09:50:05.778 [trace] Python API env change detected, add => '~/.pyenv/versions/3.12.9/bin/python'
09:50:05.779 [trace] Search for KernelSpecs in Interpreter ~/.pyenv/versions/3.12.9/bin/python
09:50:05.801 [trace] Python API env change detected, add => '/bin/python'
09:50:05.801 [trace] Search for KernelSpecs in Interpreter /bin/python
09:50:05.811 [trace] Python API env change detected, add => '/usr/bin/python'
09:50:05.811 [trace] Search for KernelSpecs in Interpreter /usr/bin/python
09:50:05.817 [trace] Python API env change detected, add => '~/venv/bin/python'
09:50:05.817 [trace] Search for KernelSpecs in Interpreter ~/venv/bin/python
09:50:05.847 [debug] KernelProvider switched kernel to id = .custom.~/venv/bin/python3.~/venv/python.-m#jupytercloud.custom_kernel.lib.kernel
09:50:05.848 [debug] start the kernel, options.disableUI=true for ~/workspace/Untitled-2.ipynb
09:50:05.869 [trace] Registering commtarget jupyter.widget
09:50:05.870 [debug] Controller selection change completed
09:50:05.878 [debug] End refreshing Kernel Picker (1753080604868)
09:50:05.879 [info] Starting Kernel (Python Path: ~/venv/bin/python, Venv, 3.12.11) for '~/workspace/Untitled-2.ipynb' (disableUI=true)
09:50:05.879 [trace] Creating raw notebook for resource '~/workspace/Untitled-2.ipynb'
09:50:05.904 [debug] Get Custom Env Variables, Class name = Hm, completed in 1ms, has a truthy return value, Arg 1: "~/workspace", Arg 2: "RunPythonCode"
09:50:05.904 [debug] Get Custom Env Variables, Class name = Hm, completed in 1ms, has a truthy return value, Arg 1: "~/workspace/Untitled-2.ipynb", Arg 2: "RunPythonCode"
09:50:05.906 [debug] Launching kernel .custom.~/venv/bin/python3.~/venv/python.-m#jupytercloud.custom_kernel.lib.kernel for ~/workspace/Untitled-2.ipynb in ~/workspace with ports 9001, 9000, 9004, 9002, 9003
09:50:05.906 [info] Launching Raw Kernel Custom Python 3 # ~/venv/bin/python3
09:50:05.906 [debug] Create ProcessService, Class name = qm (started execution), Arg 1: "~/workspace/Untitled-2.ipynb", Arg 2: {"a":false,"b":{"A":3,"z":{"i":-1,"end":0},"w":[{"id":870},{"id":871},null,{"id":874}]}}
09:50:05.909 [debug] Get Custom Env Variables, Class name = Hm, completed in 3ms, has a truthy return value, Arg 1: "~/workspace/Untitled-2.ipynb", Arg 2: "RunNonPythonCode", Arg 3: {"a":false,"b":{"A":3,"z":{"i":-1,"end":0},"w":[{"id":870},{"id":871},null,{"id":874}]}}
09:50:05.909 [debug] Create ProcessService, Class name = qm, completed in 3ms, has a truthy return value, Arg 1: "~/workspace/Untitled-2.ipynb", Arg 2: {"a":false,"b":{"A":3,"z":{"i":-1,"end":0},"w":[{"id":870},{"id":871},null,{"id":874}]}}
09:50:06.309 [debug] Got env vars from Python Ext in 405ms for ~/venv/bin/python, with env var count 89.
09:50:06.309 [trace] Prepend PATH with python bin for ~/venv/bin/python
09:50:06.309 [debug] Getting activated env variables, Class name = Fd, completed in 406ms, has a truthy return value, Arg 1: "~/workspace", Arg 2: "~/venv/bin/python", Arg 3: undefined
09:50:06.315 [info] Process Execution: ~/venv/bin/python -m pip list
09:50:06.322 [debug] Got env vars from Python Ext in 418ms for ~/venv/bin/python, with env var count 89.
09:50:06.322 [trace] Prepend PATH with python bin for ~/venv/bin/python
09:50:06.322 [debug] Getting activated env variables, Class name = Fd, completed in 419ms, has a truthy return value, Arg 1: "~/workspace/Untitled-2.ipynb", Arg 2: "~/venv/bin/python", Arg 3: undefined
09:50:06.329 [info] Process Execution: ~/venv/bin/python -c "import ipykernel; print(ipykernel.__version__); print("5dc3a68c-e34e-4080-9c3e-2a532b2ccb4d"); print(ipykernel.__file__)"
09:50:06.336 [info] Process Execution: ~/venv/bin/python3 -m jupytercloud.custom_kernel.lib.kernel -f /run/user/1001/jupyter/runtime/kernel-v359184539e08736779641732b1abea7bcdf19207a.json
    > cwd: ~/workspace
09:50:06.336 [debug] Kernel process 392076.
09:50:07.195 [trace] ipykernel version & path 6.29.5, ~/python/ipykernel/py3/ipykernel/__init__.py for ~/venv/bin/python

09:50:09.313 [debug] KernelProcess error 392076: [I 2025-07-21 09:50:09.312 CustomKernelApp kernelapp:324] Starting the kernel at pid: 392076

09:50:09.320 [debug] KernelProcess error 392076: [I 2025-07-21 09:50:09.320 CustomKernelApp kernelapp:445] To connect another client to this kernel, use:
[I 2025-07-21 09:50:09.320 CustomKernelApp kernelapp:445]     --existing /run/user/1001/jupyter/runtime/kernel-v359184539e08736779641732b1abea7bcdf19207a.json

09:50:09.321 [debug] Kernel output 392076: To connect another client to this kernel, use:
--existing /run/user/1001/jupyter/runtime/kernel-v359184539e08736779641732b1abea7bcdf19207a.json
09:50:09.395 [debug] Waiting for Raw Session to be ready in postStartRawSession
09:50:09.395 [debug] Waiting for Raw session to be ready, status: connected
09:50:09.396 [trace] Raw session connected
09:50:09.397 [debug] Waited for Raw session to be ready & got status: connected
09:50:09.397 [debug] Successfully waited for Raw Session to be ready in postStartRawSession
09:50:09.397 [debug] Kernel status is 'unknown' before requesting kernel info and after ready
09:50:09.397 [debug] Sending request for kernelInfo
09:50:09.399 [debug] KernelProcess error 392076: [I 2025-07-21 09:50:09.356 CustomKernelApp shellapp:325] Loading IPython extension: storemagic

09:50:09.402 [trace] Got response for requestKernelInfo
09:50:09.402 [debug] Successfully completed postStartRawSession after 1 attempt(s) in 5ms
09:50:09.405 [trace] Executing silently Code (idle) = import sys as _VSCODE_sys\nprint(_VSCODE_sys.executable); del _VSCODE_sys
09:50:09.411 [trace] Executing silently Code (completed) = import sys as _VSCODE_sys\nprint(_VSCODE_sys.executable); del _VSCODE_sys with 1 output(s)
09:50:09.412 [trace] Started running kernel initialization for ~/workspace/Untitled-2.ipynb
09:50:09.412 [trace] Executing silently Code (idle) = try:\nimport ipywidgets as _VSCODE_ipywidgets\nprint("e976ee50-99ed-4aba-9b6b-9dcd5634d07d:IPy
09:50:09.510 [trace] Executing silently Code (completed) = try:\nimport ipywidgets as _VSCODE_ipywidgets\nprint("e976ee50-99ed-4aba-9b6b-9dcd5634d07d:IPy with 1 output(s)
09:50:09.511 [trace] Determined IPyWidgets Version as 8
09:50:09.511 [trace] Version of IPyWidgets 8 determined after 3.642s
09:50:09.511 [trace] Executing silently Code (idle) = def __VSCODE_inject_module():\ndef __VSCODE_call_function(function, callback, data=None):
09:50:09.525 [trace] Executing silently Code (completed) = def __VSCODE_inject_module():\ndef __VSCODE_call_function(function, callback, data=None): with 0 output(s)
09:50:09.525 [debug] Requesting Kernel info
09:50:09.526 [trace] Got Kernel info
09:50:09.526 [trace] End running kernel initialization, now waiting for idle
09:50:09.526 [trace] Waiting for idle on (kernel): 5fb33bae-1b74-45ad-98d1-a5cf6f5cae05 -> idle
09:50:09.526 [trace] Finished waiting for idle on (kernel): 5fb33bae-1b74-45ad-98d1-a5cf6f5cae05 -> idle
09:50:09.527 [trace] End running kernel initialization, session is idle
09:50:09.527 [trace] Registering Kernel Completion Provider from kernel Custom Python 3 (Python 3.12.11) for language python
09:50:09.528 [trace] IPyWidgetScriptSource.initialize
09:50:09.529 [info] Kernel successfully started
09:50:09.529 [warn] No nbextensions folder found for kernel .custom.~/venv/bin/python3.~/venv/python.-m#jupytercloud.custom_kernel.lib.kernel
09:50:10.032 [trace] Web view is ready to receive widget messages
09:50:10.043 [trace] Widget Message: Received IPyWidgets_kernelOptions in ScriptManager
09:50:10.043 [trace] Widget Message: Received IPyWidgetMessages.IPyWidgets_kernelOptions
09:50:10.051 [trace] Widget Message: Received IPyWidgets_kernelOptions in ScriptManager
09:50:10.051 [trace] Widget Message: Received IPyWidgetMessages.IPyWidgets_kernelOptions
09:50:10.202 [error] Widget Error: Failed to access CDN https://unpkg.com/ after 0 attempt(s), TypeError: Failed to fetch

Coding Language and Runtime Version

Python v3.12.9

Language Extension Version (if applicable)

2025.10.0

Anaconda Version (if applicable)

No response

Running Jupyter locally or remotely?

Local

[DonJayamanne]

@egormkn Please can you try installing the latest pre-release version of the Jupyter extension and verify this works as expected, thank you

[egormkn]

@DonJayamanne seems that the pre-release version has not been updated yet, but I installed the VSIX build from the pull request and it works. Thank you for the fix!