[BUG] Existing Squirrel-based Claude Desktop users cannot upgrade to MSIX — silent failure (0x80073CFA)

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?

Existing Claude Desktop users on Windows with the Squirrel-based installation cannot upgrade to the new MSIX-packaged version (required for Cowork). The ClaudeSetup.exe bootstrapper silently fails without any user-facing error message.

The installer shows a progress bar, requests admin credentials, appears to complete successfully, and even launches Claude Desktop — but the version is unchanged, no Cowork tab appears, and only the old Squirrel installation remains.

Root cause (from Windows Event Viewer):

Applications and Services Logs > Microsoft > Windows > AppXDeploymentServer/Operational shows:

Event ID 404 / 717, Error 0x80073CFA: Windows cannot remove Claude_1.1.2685.0_x64__pzs8sxrjxfjjc because the PreserveApplicationData flag can only be used on a package that was deployed in development mode.

The bootstrapper attempts to remove a previously-installed MSIX package using the PreserveApplicationData flag, but the package was not deployed in development mode, so removal fails and the entire operation silently aborts.

Three additional complications encountered:

1. Sideloading required (undocumented): Initial install attempt failed with "App installation is blocked by IT policy. Contact your administrator to enable app sideloading." Required enabling Developer Mode in Windows Settings first.

1. MSIX installed for wrong user: When a standard (non-admin) user provides admin credentials during UAC, the MSIX package installs for the Administrator user profile, not the calling user. Confirmed via Get-AppxPackage -Name "*Claude*" -AllUsers showing the package registered only for the Administrator account.

1. Ghost package blocks reinstall: The hidden MSIX package (installed for wrong user) blocks all subsequent install attempts with the same 0x80073CFA error. Required manual removal via Remove-AppxPackage -AllUsers.

Squirrel auto-updater does not offer the MSIX version:
The Squirrel updater checks https://downloads.claude.ai/releases/win32/x64/RELEASES?id=AnthropicClaude&localVersion=1.1.2685 and receives no update. The MSIX version is not published through the Squirrel update channel, so existing users have no automatic upgrade path.

What Should Happen?

ClaudeSetup.exe should either:

• Detect the existing Squirrel installation and migrate it to MSIX automatically, or
• Inform the user that uninstallation of the Squirrel version is required before upgrading, or
• At minimum, surface the 0x80073CFA error to the user instead of failing silently

Error Messages/Logs

Event Viewer error (repeated at each install attempt):

AppX Deployment operation failed for package with error 0x80073CFA. Windows cannot remove Claude_1.1.2685.0_x64__pzs8sxrjxfjjc because the PreserveApplicationData flag can only be used on a package that was deployed in development mode.

Initial sideloading error (before enabling Developer Mode):

App installation is blocked by IT policy. Contact your administrator to enable app sideloading.

Error Messages/Logs

Event Viewer error (repeated at each install attempt):

AppX Deployment operation failed for package with error 0x80073CFA. Windows cannot remove Claude_1.1.2685.0_x64__pzs8sxrjxfjjc because the PreserveApplicationData flag can only be used on a package that was deployed in development mode.

Initial sideloading error (before enabling Developer Mode):

App installation is blocked by IT policy. Contact your administrator to enable app sideloading.

Steps to Reproduce

1. Have an existing Squirrel-based Claude Desktop installation (e.g., v1.1.2685 at C:\Users\XX\AppData\Local\AnthropicClaude\)
2. Download ClaudeSetup.exe from https://downloads.claude.ai/releases/win32/ClaudeSetup.exe
3. Run ClaudeSetup.exe
4. Provide admin credentials when prompted (as a standard/non-admin user)
5. Installer shows progress bar and appears to complete
6. Claude Desktop launches — version unchanged, no Cowork tab
7. Check Event Viewer > AppXDeploymentServer/Operational for error 0x80073CFA

Workaround (manual, multi-step):

1. Enable Developer Mode (Settings > System > Advanced > For developers)
2. Remove ghost MSIX package: Remove-AppxPackage -Package "Claude_1.1.2685.0_x64__pzs8sxrjxfjjc" -AllUsers
3. Uninstall old Squirrel version from Settings > Apps
4. Delete remnants in C:\Users\XX\AppData\Local\AnthropicClaude\
5. Ensure calling user has admin privileges (standard users cannot install MSIX for themselves via credential-based UAC)
6. Run ClaudeSetup.exe — MSIX installs for the correct user

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

1.1.2685 (Squirrel-based)

Claude Code Version

2.1.34

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

PowerShell

Additional Information

Windows 11 Home, Version 25H2, Build 26200.7840

• This likely affects all existing Windows users who installed Claude Desktop before the Cowork launch (Feb 10, 2026), as all prior individual installations used the Squirrel framework
• Enterprise users who deployed via MSIX directly may not be affected
• Related: #24918 (Cowork API connectivity issues on Windows)