[BUG] Claude Code extension webview tab restoration hangs Cursor IDE on workspace startup

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

The Claude Code extension for Cursor (anthropic.claude-code) hangs Cursor's renderer process when Cursor attempts to restore previously open Claude Code editor tabs at workspace startup. The same extension, with the same session data, restores cleanly in VS Code — this is a Cursor-specific interaction, but I'm filing here as well in case the extension can mitigate from its side or coordinate with Cursor on a fix.
Cursor forum threads with detailed analysis:

• https://forum.cursor.com/t/claude-code-tabs-cause-cursor-to-freeze-on-startup-stuck-loading/155722/18
• https://forum.cursor.com/t/extension-host-fails-to-initialize-timeout-waiting-for-auth-and-plugins-breaking-ai-agents-and-source-control/158539

Root cause (per Sanskar's analysis in the linked Cursor thread): the hang occurs in Cursor's registerWebviewPanelSerializer restore path for claudeVSCodePanel panels, before the extension's deserializeWebviewPanel handler executes. Even patching the handler to immediately dispose the panel doesn't prevent the hang.

Workarounds:

• In-session: close the Claude Code tabs while Cursor is hung; the extension host completes activation within seconds.
• Preventive: close Claude Code tabs before quitting Cursor.
• Heavy-handed: delete the workspace's state.vscdb or remove claudeVSCodePanel entries from memento/workbench.parts.editor.

Possible mitigations on the extension side:

• Defer or guard webview panel serializer registration so restoration never blocks the renderer.
• Detect Cursor as the host and disable webview tab persistence until the upstream issue is fixed.

Environment:

• Cursor 3.2.16 (also reproduced on 3.0.12, 3.2.11)
• macOS Apple Silicon, also reported on Windows
• Claude Code extension 2.1.126 (and possibly earlier versions)

This bug has had an active Cursor forum thread for over a month with multiple users affected. Filing here for cross-team visibility.

What Should Happen?

Cursor should open and load in Claude Code open chats in the open claude code extension tabs.

Error Messages/Logs

Steps to Reproduce

• Open a project in Cursor with the Claude Code extension installed.
• Open a Claude Code session as an editor tab (not the sidebar).
• Quit Cursor with that tab still open.
• Reopen the project. Cursor hangs on startup — terminal, extensions, settings all become non-functional. Dev tools console shows extension host health-check failure, auth ready timeout, and No Connect transport provider registered cascades.

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

VSCode/Cursor extension (anthropic.claude-code): 2.1.126

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Other

Additional Information

_No response_