Jupyter Extension Fails to Attach Kernels on Windows - Invalid Path Format

Bug Reports
1

Where does the bug appear (feature/product)?

Cursor IDE

Describe the Bug

Cursor’s Jupyter extension fails to attach kernels to .ipynb files on Windows due to path handling issues. The extension appears to construct runtime paths using Unix-style path format (/c:/Users/...) instead of Windows-native paths (C:\Users\...), causing kernel attachment to completely fail.

This affects all Jupyter notebooks in Cursor, regardless of which kernel is selected.

Steps to Reproduce

  1. Open Cursor on Windows (Windows 10/11)
  2. Open any .ipynb file
  3. Observe “Select Kernel” in top-right corner
  4. Attempt to select a kernel - no kernels appear in dropdown
  5. Check output logs for Jupyter extension errors

You should see Error Message:

Failed to start the Kernel 'aws (Python 3.12.4)'. 
View Jupyter log for further details. 
/c:/Users/[USERNAME]/AppData/Roaming/Cursor/User/globalStorage/ms-toolsai.jupyter/version-2025.3.0/jupyter/runtime contains invalid WIN32 path characters.

Key Issue: The path uses Unix-style format /c:/Users/... instead of Windows format C:\Users\...

Expected Behavior

Jupyter notebooks should attach to available kernels (similar to VS Code behavior), and the kernel should be selectable and functional from the kernel picker in the top-right corner of notebook files.

Actual Behavior

  • Kernel picker shows “Select Kernel” but kernels never attach
  • Error message appears: contains invalid WIN32 path characters
  • PowerShell commands run automatically on Cursor startup (visible in PowerShell extension output)
  • Brief win32 CLI box appears and disappears immediately
  • No kernels are available for selection, even though they are properly registered

Operating System

Windows 10/11

Current Cursor Version (Menu → About Cursor → Copy)

Version: 2.0.50
Commit: a788ca5efbb89cf7c24e873ea3623b2e47b42490

Additional Information

OS: Windows 10/11 (10.0.22631)
Architecture: x64
Jupyter Extension: Built-in (ms-toolsai.jupyter)

Verification That Setup is Correct

The same notebooks and kernels work perfectly in VS Code, confirming this is a Cursor-specific issue:

Kernels Properly Registered

Available kernels:
  python3         C:\Users\...\anaconda3\envs\code_mapping\share\jupyter\kernels\python3
  aws             C:\Users\...\AppData\Roaming\jupyter\kernels\aws
  code_mapping    C:\Users\...\AppData\Roaming\jupyter\kernels\code_mapping

Verified via: jupyter kernelspec list

Working in VS Code

  • Same notebooks attach kernels successfully in VS Code
  • Same kernels are available and functional
  • No path-related errors in VS Code

Additional Context

Automatic PowerShell Commands

On Cursor startup, two PowerShell commands run automatically (visible in PowerShell extension output). These appear to be the extension attempting to:

  1. Detect or fix the path issue
  2. Launch kernels using shell commands as a workaround

Both attempts fail immediately due to the invalid path format, causing a brief win32 CLI window to appear and disappear.

Affected Users

This bug prevents all Jupyter notebook functionality in Cursor on Windows. Users must either:

  • Use VS Code for Jupyter notebooks instead
  • Use JupyterLab/Notebook Server in browser
  • Avoid using Jupyter notebooks in Cursor entirely

Suggested Fix

The Jupyter extension should:

  1. Use Windows-native path format (C:\Users\...) on Windows
  2. Avoid Unix-style path normalization (/c:/Users/...) on Windows
  3. Properly handle Windows path construction in the runtime directory setup

Workaround

Use VS Code for Jupyter notebooks until this is fixed. The same setup works correctly there.

Does this stop you from using Cursor

Yes - Cursor is unusable

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8 posts were merged into an existing topic: Kernel Selection doesn’t work