Python Environments causes invalid "Python 3.14.undefined" kernels and Jupyter hangs on "Connecting to kernel"

[lilkui]

Description

After enabling the Python Environments extension, VS Code's Jupyter kernel picker detects kernels with names similar to:

project-name (3.14.x) (Python 3.14.undefined)

When I select one of these kernels and run a notebook cell, the notebook remains stuck indefinitely at:

Connecting to kernel...

Disabling the Python Environments extension and reloading VS Code resolves the issue. Jupyter notebooks then detect the expected kernels and run normally.

Steps to reproduce

1. Install and enable the Python, Jupyter, and Python Environments extensions.
2. Open a .ipynb file in VS Code.
3. Open the kernel picker.
4. Observe a detected kernel named similar to project-name (3.14.x) (Python 3.14.undefined).
5. Select that kernel.
6. Run any notebook cell.

Actual behavior

• The detected kernel name contains Python 3.14.undefined.
• The notebook remains stuck at Connecting to kernel....
• No cell is executed.

Expected behavior

• Python environments should be detected with the correct Python version and kernel metadata.
• Selecting a valid environment should start the Jupyter kernel successfully.
• Invalid or incomplete kernel entries such as Python 3.14.undefined should not be shown.

Workaround

Disable the Python Environments extension and reload VS Code. Kernel detection and notebook execution then work normally.

Environment

• OS: Windows 11 25H2
• VS Code version: 1.138.0
• Python Environments extension version: 1.36.0
• Python extension version: 2026.4.0
• Jupyter extension version: 2025.9.1
• Python version: 3.14.7
• Environment type: uv
• Workspace type: local

Additional information

The problem appears only while the Python Environments extension is enabled. Enabling or disabling that extension, without changing the notebook or Python environment, consistently reproduces or resolves the issue.