[BUG]

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 Desktop installer consistently fails at the MSIX AddPackage step with HRESULT 0x80073CFF on Windows 11 Professional (build 10.0.26200).

The process runs as a standard user, self-elevates via UAC as expected, the MSIX signature verifies successfully (WinVerifyTrust: MSIX signature is valid), and no existing/conflicting Claude packages are found on the system. Despite this, AddPackage fails within ~2 seconds with HRESULT 0x80073CFF, and the elevated installer process exits with code 1.

The error dialog then states "Administrator access is required to install Claude with full features. You can try again or install without Cowork" — but this message is misleading, since the process log confirms "Is elevated: true" and elevation succeeded. The actual failure is at the AddPackage registration step itself, not a lack of admin rights.

Full setup log attached (ClaudeSetup.log) showing the complete sequence from launch through signature verification to the AddPackage failure.

What Should Happen?

Claude Desktop should install successfully without errors on Windows 11 Professional (build 10.0.26200). The MSIX package should register correctly via AddPackage instead of failing with HRESULT 0x80073CFF, or alternatively the installer should offer a working "install without Cowork" fallback when the MSIX install fails.

Error Messages/Logs

2026/07/15 19:19:28.596241 Log file: C:\Users\***\AppData\Local\Temp\ClaudeSetup.log
2026/07/15 19:19:28.596941 === Claude Setup f7518fa0f482cfd68e6ef7fb78e3924e013b5160 started ===
2026/07/15 19:19:28.599181 AllowAllTrustedApps not found (sideloading assumed enabled): The system cannot find the file specified.
2026/07/15 19:19:28.599181 Windows edition: Professional
2026/07/15 19:19:28.601083 Windows version: 10.0.26200
2026/07/15 19:19:28.601083 Native arch: x64
2026/07/15 19:19:28.601083 Elevation type: Limited
2026/07/15 19:19:28.601083 Is elevated: false
2026/07/15 19:19:28.601083 Sideloading enabled: true
2026/07/15 19:19:28.602226 S Mode: false
2026/07/15 19:19:28.602226 Conflicting service: false
2026/07/15 19:19:28.602226 Running in non-elevated context
2026/07/15 19:19:28.602226 User can elevate — installing MSIX directly
2026/07/15 19:19:28.646143 Splash: 400x300, 8 frame(s), animated=true
2026/07/15 19:19:28.646143 System DPI: 120 (scale: 125%)
2026/07/15 19:19:28.787773 MSIX URL: https://api.anthropic.com/api/desktop/win32/x64/msix/latest/redirect
2026/07/15 19:19:28.788687 Downloading MSIX to C:\Users\***\AppData\Local\Temp\Claude-1206701567.msix
2026/07/15 19:19:28.826736 proxy: WinHttpGetProxyForUrl("https://api.anthropic.com/api/desktop/win32/x64/msix/latest/redirect"): winapi error #12180
2026/07/15 19:19:29.365155 proxy: WinHttpGetProxyForUrl("https://downloads.claude.ai/releases/win32/x64/1.21459.0/Claude-f7518fa0f482cfd68e6ef7fb78e3924e013b5160.msix"): winapi error #12180
2026/07/15 19:19:32.345987 Download progress: 10% (25722752 / 256927827 bytes)
2026/07/15 19:19:34.122119 Download progress: 20% (51396224 / 256927827 bytes)
2026/07/15 19:19:36.233100 Download progress: 30% (77102528 / 256927827 bytes)
2026/07/15 19:19:38.215025 Download progress: 40% (102776064 / 256927827 bytes)
2026/07/15 19:19:39.508179 Download progress: 50% (128465968 / 256927827 bytes)
2026/07/15 19:19:40.900240 Download progress: 60% (154188752 / 256927827 bytes)
2026/07/15 19:19:42.405384 Download progress: 70% (179878576 / 256927827 bytes)
2026/07/15 19:19:44.206533 Download progress: 80% (205552096 / 256927827 bytes)
2026/07/15 19:19:45.989903 Download progress: 90% (231258432 / 256927827 bytes)
2026/07/15 19:19:47.108308 Download progress: 100% (256927827 / 256927827 bytes)
2026/07/15 19:19:47.111241 Download complete: 256927827 bytes, SHA256: 08012d54501d915542873076d0ddc13ddded98195268fd5d6353fe7322ae7bc7
2026/07/15 19:19:47.113847 Download complete
2026/07/15 19:19:47.144996 Self-elevating: C:\Users\***\Downloads\Programs\Claude Setup_2.exe --elevated --msix-path "C:\Users\***\AppData\Local\Temp\Claude-1206701567.msix" --log-path "C:\Users\***\AppData\Local\Temp\ClaudeSetup.log"
2026/07/15 19:19:52.645847 Waiting for elevated process to complete...
2026/07/15 19:19:52.664365 Log file: C:\Users\***\AppData\Local\Temp\ClaudeSetup.log
2026/07/15 19:19:52.665463 === Claude Setup f7518fa0f482cfd68e6ef7fb78e3924e013b5160 started ===
2026/07/15 19:19:52.667611 AllowAllTrustedApps not found (sideloading assumed enabled): The system cannot find the file specified.
2026/07/15 19:19:52.668170 Windows edition: Professional
2026/07/15 19:19:52.670404 Windows version: 10.0.26200
2026/07/15 19:19:52.670404 Native arch: x64
2026/07/15 19:19:52.670404 Elevation type: Full
2026/07/15 19:19:52.670404 Is elevated: true
2026/07/15 19:19:52.670404 Sideloading enabled: true
2026/07/15 19:19:52.670404 S Mode: false
2026/07/15 19:19:52.670404 Conflicting service: false
2026/07/15 19:19:52.670404 Running in elevated context
2026/07/15 19:19:52.670940 Installing MSIX from C:\Users\***\AppData\Local\Temp\Claude-1206701567.msix (all-users: false)
2026/07/15 19:19:52.721693 Splash: 400x300, 8 frame(s), animated=true
2026/07/15 19:19:52.722234 System DPI: 120 (scale: 125%)
2026/07/15 19:19:52.798674 Verifying Authenticode signature on C:\Users\***\AppData\Local\Temp\Claude-1206701567.msix
2026/07/15 19:19:55.001531 WinVerifyTrust: MSIX signature is valid
2026/07/15 19:19:55.894139 Signature verified: MSIX signer matches bootstrapper
2026/07/15 19:19:55.898513 Extracted family name from MSIX manifest: Claude_pzs8sxrjxfjjc
2026/07/15 19:19:55.898513 Squirrel process prefix: c:\users\***\appdata\local\anthropicclaude\
2026/07/15 19:19:55.898513 Checking for running Squirrel Claude processes...
2026/07/15 19:19:55.931199 No Squirrel Claude processes found
2026/07/15 19:19:55.931199 Checking for existing Claude MSIX packages...
2026/07/15 19:19:55.995484 Installing MSIX: C:\Users\***\AppData\Local\Temp\Claude-1206701567.msix
2026/07/15 19:19:55.996016 Standard install (not split-account), using AddPackage
2026/07/15 19:19:55.996016 Installing via AddPackage (current-user)...
2026/07/15 19:19:57.816117 MSIX installation failed: AddPackage failed: AddPackage failed with HRESULT 0x80073CFF
2026/07/15 19:19:57.816117 ERROR dialog (opening log): Installation failed: AddPackage failed: AddPackage failed with HRESULT 0x80073CFF

Please share this log with us.
2026/07/15 19:20:13.120284 Elevated process exited with code 1
2026/07/15 19:20:13.120284 Elevation failed: elevated installer exited with code 1
2026/07/15 19:20:13.120284 ERROR dialog (with log path): Administrator access is required to install Claude with full features. You can try again or install without Cowork.

Steps to Reproduce

1. Download Claude Desktop installer from claude.com/download on Windows 11 Professional (build 10.0.26200).
2. Run "Claude Setup_2.exe" as a standard (non-elevated) user.
3. The installer detects sideloading is enabled and proceeds to download the MSIX package (~256 MB) automatically.
4. After download completes, the installer self-elevates via UAC prompt (Claude Setup_2.exe --elevated --msix-path ...).
5. Accept the UAC prompt.
6. The elevated process verifies the Authenticode signature successfully, then attempts to install the MSIX package via AddPackage (current-user).
7. Installation fails immediately with: "AddPackage failed: AddPackage failed with HRESULT 0x80073CFF"
8. The elevated process exits with code 1, and the installer shows: "Administrator access is required to install Claude with full features. You can try again or install without Cowork."

Full setup log attached (ClaudeSetup.log) showing the complete sequence from download through the AddPackage failure.

Claude Model

None

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

N/A — installation did not complete successfully, so the claude command is not available on this system.

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_