[BUG] Claude Desktop MSIX installation fails with HRESULT 0x80073CFF on Windows 11 Pro Build 26200 — all workarounds exhausted

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 installer consistently fails at the MSIX AddPackage step with HRESULT 0x80073CFF on Windows 11 Pro Build 26200. The error persists across all configurations and workarounds.

System Information

• Windows edition: Professional
• Windows version: 10.0.26200
• Native arch: x64
• Sideloading enabled: true
• Developer Mode: enabled
• S Mode: false
• Claude Setup version: 390d6ca38de808ca81aeccd2945d9c8be800f47d

What Should Happen?

What Should Happen

Claude Desktop should install successfully and launch after the installation process completes. If the full MSIX installation with Cowork fails due to system policy restrictions, the "install without Cowork" fallback (Squirrel installer) should reliably install the application and write files to disk.

Error Messages/Logs

MSIX installation failed: AddPackage failed: AddPackage failed with HRESULT 0x80073CFF
Deployment of package Claude_1.9659.2.0_x64__pzs8sxrjxfjjc with package origin Unknown failed because no valid license or sideloading policy could be applied.

Steps to Reproduce

Steps to Reproduce

1. Download Claude Desktop installer from https://claude.ai/download (Windows x64)
2. Run the installer (Claude Setup.exe)
3. Installer downloads MSIX package successfully (220354768 bytes, SHA256 verified)
4. Installer attempts AddPackage — fails with HRESULT 0x80073CFF
5. Error dialog appears: "Administrator access is required to install Claude with full features"
6. Clicking "Try again" reproduces the same error
7. Running installer with --no-msix flag silently completes but writes no files to disk

Claude Model

Sonnet (default)

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

1.9659.2.0

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

PowerShell

Additional Information

• This issue affects multiple users on Windows 11 Pro Build 26200, see related issues: #60319, #58174, #53634
• MSIX signature verifies successfully every time (WinVerifyTrust: MSIX signature is valid)
• The Squirrel fallback path (--no-msix flag) silently reports "Installation complete" but writes no files to disk
• ms-appinstaller protocol is disabled on this system (Windows policy)
• AllowAllTrustedApps and AllowDevelopmentWithoutDevLicense registry values set to 1, Developer Mode enabled — error persists after reboot
• Anthropic authenticode certificate manually imported to Cert:\LocalMachine\TrustedPeople — no effect
• System is Windows 11 Insider Preview (now unenrolled), which may be contributing to the strict MSIX policy enforcement