[BUG] Claude Desktop MSIX install fails with HRESULT 0x80073CFF despite full elevation, sideloading enabled, and clean WindowsApps\Deleted folder

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?

Claude Desktop installation consistently fails at the MSIX AddPackage step with
HRESULT 0x80073CFF, even after exhausting every fix I could find: running as
administrator, enabling sideloading via both Settings and Group Policy
("Allow all trusted apps to install" set to Enabled), rebooting, and clearing
66 stuck leftover files from C:\Program Files\WindowsApps\Deleted via
takeown/icacls/rmdir. The error is identical before and after every fix attempt.

Environment

• OS: Windows 11 Professional
• Windows version: 10.0.26200
• Architecture: x64
• Sideloading enabled: true (confirmed in log, both before and after manual GP change)
• Elevation: Full / Is elevated: true (confirmed in log — the "Administrator

access is required" dialog appears despite this)

• S Mode: false
• Conflicting service: false
• No previous Claude MSIX package found on the system

Notes

This appears to match existing reports #60319, #64029, and #58174 — same
error code, same "Administrator access required" message despite confirmed
elevation, same fast-fail signature at the AddPackage step. Happy to provide
the full ClaudeSetup.log on request.

Possibly related to #73505, though that report's root cause (locked Helium hives from a failed update) differs from mine (fresh install, no existing package, stuck WindowsApps\Deleted files).

What Should Happen?

Claude Desktop should install successfully given full elevation, confirmed
sideloading, valid signature, and no pre-existing package or stuck deletion
files.

Error Messages/Logs

### Log excerpt

Windows edition: Professional
Windows version: 10.0.26200
Native arch: x64
Elevation type: Full
Is elevated: true
Sideloading enabled: true
S Mode: false
Conflicting service: false
Running in elevated context
Verifying Authenticode signature on Claude-*.msix
WinVerifyTrust: MSIX signature is valid
Signature verified: MSIX signer matches bootstrapper
Extracted family name from MSIX manifest: Claude_pzs8sxrjxfjjc
Checking for existing Claude MSIX packages...
Installing via AddPackage (current-user)...
MSIX installation failed: AddPackage failed: AddPackage failed with HRESULT 0x80073CFF

### Additional diagnostic step taken

Checked Event Viewer → Applications and Services Logs → Microsoft → Windows → 
AppXDeploymentServer → Operational. Found:
Event ID: 493
Level: Warning
Description: There were 66 additional files that failed to be deleted under
the folder \?\C:\Program Files\WindowsApps\Deleted.

Cleared these via elevated `takeown /f /r /d y`, `icacls /grant administrators:F /t`, 
and `rmdir /s /q` on `WindowsApps\Deleted` specifically, then rebooted and 
retried. Same HRESULT 0x80073CFF failure recurred immediately.

Steps to Reproduce

1. Download Claude Desktop installer from claude.ai/download
2. Right-click → Run as administrator
3. Installer downloads MSIX successfully, signature verifies (WinVerifyTrust:

MSIX signature is valid, signer matches bootstrapper)

1. AddPackage (current-user) step fails almost immediately (~1.3s) with

HRESULT 0x80073CFF

1. Dialog then shows "Administrator access is required to install Claude with

full features. You can try again or install without Cowork" — despite logs
confirming Is elevated: true

Claude Model

None

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

N/A — this is a Claude Desktop (not Claude Code CLI) installer bug

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

Windows Terminal

Additional Information

_No response_