[BUG] Cowork tab missing on Windows 11 Home 25H2 (Build 26200) - yukonSilver status=unsupported persists across 1.18286.0/1.18286.2/1.19367.0 despite virtualization fully enabled

Summary

The Cowork tab disappeared after a recent auto-update. It worked fine for weeks on this exact machine, then broke a few days ago with no OS change. The VM backend reports yukonSilver status=unsupported even though all virtualization features are enabled. Reinstalling (including as Administrator), rebooting, and updating to newer builds do not help.

Environment

• OS: Windows 11 Home, 25H2, Build 26200.8655, x64
• Claude Desktop versions tested (all reproduce): 1.18286.0, 1.18286.2, 1.19367.0
• Plan: Max
• Last Windows quality update: 2026-06-10 (~1 month ago) — so this is NOT triggered by an OS update
• Regression started after a Claude Desktop auto-update a few days ago; the earlier version worked

Virtualization / security state (elevated PowerShell)

• HypervisorPlatform : Enabled
• VirtualMachinePlatform : Enabled
• Microsoft-Hyper-V-All : (not available — Home edition)
• HyperVisorPresent : True
• VBS (VirtualizationBasedSecurityStatus) : 2 (running)
• SecurityServicesRunning : 3,4 (System Guard Secure Launch, SMM firmware measurement)
• Note: Memory Integrity / HVCI (code 2) is NOT running, so the usual "disable Memory Integrity" workaround does not apply.

Relevant logs

cowork_vm_node.log (repeats on every launch, all versions):

[cleanupVMBundleIfUnsupported] yukonSilver not supported (status=unsupported), checking for stale bundle...

main.log:

[warm] Skipping VM warm download - YukonSilver not supported
[LocalAgentModeSessionManager] no VMAPI; cliPluginBridge not registered

Additionally, Get-Service CoworkVMService returns "service not found" even after an Administrator reinstall — the VM service is never registered.

Impact

Because cleanupVMBundleIfUnsupported deletes the previously-working VM bundle when it (incorrectly) decides the platform is unsupported, the working state cannot be recovered by the user. Every version from 1.18286.x onward re-triggers this.

Steps taken (none resolved it)

1. Clean uninstall + reinstall latest installer
2. Reinstall via "Run as administrator"
3. Full quit from tray + relaunch into newer build (1.19367.0)
4. Reboot; confirmed all virtualization features enabled
5. Verified BIOS virtualization active (HyperVisorPresent=True)

Likely related

#25136, #27316, #27445, #28238, #32004, #47327 (all "yukonSilver unsupported" on Build 26200)

Ask

This looks like a platform-detection regression: yukonSilver is classified as unsupported on Windows Build 26200 even with HypervisorPlatform + VirtualMachinePlatform enabled and a hypervisor present. Please review the yukonSilver support check for Build 26200 / VBS-active configurations.