Extension Development Host window closes immediately — "Error received from starting extension host (kind: LocalWebWorker)"

[sharpninja (Payton Byrd)]

Where does the bug appear (feature/product)?

Cursor IDE

Describe the Bug

When launching the Extension Development Host (F5 with an “extensionHost” launch configuration), the new window opens briefly then closes. The extension host fails to start with an unspecified error. The failure is tied to LocalWebWorker; the logged error does not include a message or stack trace.

Steps to Reproduce

1. Open a workspace that has an extension in a folder (e.g. extensions/my-extension with a valid package.json and main).
2. Add or use a launch configuration:
   • Type: extensionHost
   • Args: --extensionDevelopmentPath=${workspaceFolder}/extensions/my-extension, ${workspaceFolder}
3. Select that configuration and press F5.
4. A second window (Extension Development Host) appears and closes within about one second.

Expected Behavior

The Extension Development Host window stays open and loads the extension so it can be debugged.

Actual behavior

The second window closes almost immediately. In the first (main) window, the Debug Console does not show a clear error. In the logs for the second window, the extension host fails to start.

Log evidence

Log directory: %APPDATA%\Cursor\logs\<timestamp>\

For the Extension Development Host window (often window2, or the next unused window number), in windowN\renderer.log:

[info] Started local extension host with pid XXXXX.
[error] An unknown error occurred. Please consult the log for more details.: undefined An unknown error occurred. Please consult the log for more details.
[error] Error received from starting extension host (kind: LocalWebWorker)
[error] []

• The error message is undefined and the stack/details are an empty array [], so the underlying cause is not visible in the log.
• The Extension Development Host window never creates an exthost folder or exthost.log; the failure happens during “starting extension host” before the host can log anything.

In main.log (main process):

Extension host with pid XXXXX exited with code: 0, signal: unknown.

Operating System

Windows 10/11

Version Information

2.4.31 (or current stable)

Additional Information

Notes

• The same pattern appears for every new Extension Development Host window (window2, 3, 4, 5, 6, …) in the same session.
• Reproduced with a minimal extension (single view, no native modules, no network at startup; only vscode and a tree data provider).
• The main Cursor window’s extension host (e.g. window1) uses a different kind (e.g. LocalProcess) and works. The failure appears specific to the Extension Development Host and LocalWebWorker.
• Installing the same extension via Developer: Install Extension from Location… in the main window works; the extension runs in the main window’s extension host.

Does this stop you from using Cursor

No - Cursor works, but with this issue

[deanrie]

Hey, thanks for the report.

The fact that the error shows kind: LocalWebWorker with an undefined message and an empty stack suggests the web worker extension host in the Extension Development Host window is crashing before it even initializes. The main window uses LocalProcess, so everything works fine there.

A couple things you can try:

1. In your launch.json, try adding "debugWebWorkerExtensions": false to the extensionHost configuration. This should make the Extension Development Host completely skip the LocalWebWorker host.

2. If that doesn’t help, try adding --disable-extensions to the launch args to rule out interference from other extensions:

   "args": [
       "--extensionDevelopmentPath=${workspaceFolder}/extensions/my-extension",
       "--disable-extensions",
       "${workspaceFolder}"
   ]
   

3. Also, can you confirm:

   • Does the same extension project launch via F5 in VS Code (not in Cursor)?

The team is aware of a few Extension Host stability issues, but this specific LocalWebWorker crash looks like a separate case. Your report helps with prioritization, and I’ve flagged it.

Let me know how these workarounds go.