[BUG] Cowork regression — yukonSilver "unsupported" on fully supported Windows 11 Pro 25H2 x64 after silent auto-update

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?

Cowork was working earlier today (Feb 21, 2026), then stopped without any manual changes. No Windows updates, no BIOS changes, no software installed or removed. Claude Desktop appears to have silently auto-updated to 1.1.3918 (a4b368), which introduced a regression in the yukonSilver platform detection.

The app now incorrectly flags this fully supported system as "unsupported" and refuses to start the VM.

UI errors shown (in order):

1. "Virtualization is not enabled — Claude's workspace requires hardware virtualization (Hyper-V). Enable virtualization in your computer's BIOS/UEFI settings, then restart."
2. "Could not connect to MCP server mcp-registry"
3. "Could not connect to MCP server Claude in Chrome"

This is NOT a virtualization issue. systeminfo confirms a hypervisor is detected and running. Hyper-V, Virtual Machine Platform, and Windows Hypervisor Platform are all enabled. The problem is entirely in Claude Desktop's yukonSilver platform detection logic.

Extensive troubleshooting performed — none resolved the issue:

• Enabled Hyper-V, VMP, WHP in Windows Features
• Ran bcdedit /set hypervisorlaunchtype auto
• Ran dism /online /enable-feature /featurename:Microsoft-Hyper-V-All /all
• Enabled Intel VT-d in BIOS
• Help → Troubleshooting → Clear cache and restart
• Deleted %APPDATA%\Claude\vm_bundles\ (~22 GB), let it fully rebuild — same result
• Multiple full system restarts
• Confirmed Claude Desktop is on latest version (1.1.3918)

Chat mode works perfectly. Only Cowork is affected.

What Should Happen?

The yukonSilver platform detection should correctly identify Windows 11 Pro 25H2 (Build 26200.7840) on Intel i7-8700K with an active Hyper-V hypervisor as a supported platform — exactly as it did earlier the same day before the silent auto-update.

Cowork should start the VM successfully and connect to the Claude API.

Error Messages/Logs

2026-02-21 12:43:31 [info] [cleanupVMBundleIfUnsupported] yukonSilver not supported (status=unsupported), checking for stale bundle...
2026-02-21 12:43:35 [warn] [startVM] VM not supported (win32/x64), skipping
2026-02-21 12:43:35 [info] [Callbacks] Initializing VM event callbacks...
2026-02-21 12:43:35 [info] [VM] Loading vmClient (TypeScript) module...
2026-02-21 12:43:35 [info] [VM] Module loaded successfully
2026-02-21 12:43:35 [info] [Callbacks] VM event callbacks initialized
2026-02-21 12:43:37 [warn] [startVM] VM not supported (win32/x64), skipping
2026-02-21 12:43:39 [warn] [startVM] VM not supported (win32/x64), skipping
2026-02-21 12:43:44 [warn] [startVM] VM not supported (win32/x64), skipping
2026-02-21 12:44:05 [warn] [startVM] VM not supported (win32/x64), skipping
2026-02-21 12:45:22 [info] [cleanupVMBundleIfUnsupported] yukonSilver not supported (status=unsupported), checking for stale bundle...
2026-02-21 12:45:25 [warn] [startVM] VM not supported (win32/x64), skipping
2026-02-21 12:52:14 [info] [cleanupVMBundleIfUnsupported] yukonSilver not supported (status=unsupported), checking for stale bundle...
2026-02-21 12:52:17 [warn] [startVM] VM not supported (win32/x64), skipping

Steps to Reproduce

1. Have a working Cowork setup on Windows 11 Pro 25H2 (Build 26200.7840) with Hyper-V enabled
2. Allow Claude Desktop to silently auto-update to version 1.1.3918 (a4b368)
3. Open Claude Desktop and switch to the Cowork tab
4. Observe: "Virtualization is not enabled" error appears
5. Observe: "Could not connect to MCP server mcp-registry" and "Could not connect to MCP server Claude in Chrome" errors follow
6. Check %APPDATA%\Claude\logs\cowork_vm_node.log — shows yukonSilver not supported (status=unsupported) and VM not supported (win32/x64), skipping repeating indefinitely

Note: No manual changes were made between the working state and the broken state. The system, BIOS, Windows features, and drivers are all identical. The only variable is the Claude Desktop auto-update.

Claude Model

Not sure / Multiple models

Is this a regression?

Yes, this worked in a previous version

Last Working Version

Unknown — Claude Desktop auto-updated silently. Cowork was working earlier on Feb 21, 2026 before the update to 1.1.3918 (a4b368).

Claude Code Version

Claude Desktop 1.1.3918 (a4b368) — this is Cowork on Claude Desktop, not Claude Code CLI

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

PowerShell

Additional Information

System Details

| Component | Details |
|---|---|
| OS | Windows 11 Pro 25H2 |
| OS Build | 26200.7840 |
| CPU | Intel Core i7-8700K @ 3.70GHz |
| Hyper-V | Enabled and running |
| Virtual Machine Platform | Enabled |
| Windows Hypervisor Platform | Enabled |
| VT-d (BIOS) | Enabled |
| systeminfo | "A hypervisor has been detected. Features required for Hyper-V will not be displayed." |

Related Issues

This appears related to the yukonSilver platform detection bugs:

• #25136 — yukonSilver marked as unsupported on Windows 11 Pro x64
• #26090 — Cowork tab not showing, yukonSilver unsupported
• #25206 — VM starts but crashes, service unrecoverable

Key difference: This system was previously working and broke after a silent auto-update, confirming this is a regression — not an initial setup failure.