[BUG] Cowork sandbox blocked by yukonSilver feature flag on Windows 11 Pro (win32/x64)

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?

This is the same yukonSilver failure signature reported at least four times (#28227, #27417, #48733, and separately #55649 which is a different root cause). All were closed without a documented working fix. Filing fresh with full diagnostic evidence rather than commenting on a closed/stale thread.

Related Issues (same failure signature, none resolved)

• #28227 — "yukonSilver marked as unsupported on Windows 11 Pro x64 with virtualization enabled" — closed, no documented fix
• #27417 — "yukonSilver not supported on Windows 11 Pro x64 with Hyper-V fully enabled" — closed, no documented fix
• #48733 — "HYPERVISOR_SERVICE_ERROR ... yukonSilver unsupported despite Hyper-V fully enabled" — closed, no documented fix
• #55649 — different root cause (CoworkVMService crash), included for contrast — a community-posted "fix" in that thread does not apply here, confirmed no such service exists on this machine

This issue has recurred across multiple reports over an extended period without resolution. Requesting engineering visibility into what yukonSilver actually gates and why it continues to evaluate as unsupported on properly configured Windows 11 Pro x64 machines.

What's Wrong?

App Version: 1.25927.0
CCD Version: 2.1.221 → 2.1.222 (update installed this morning)
OS: Windows 11 Pro, x64
Date first observed: 2026-08-07, immediately after this morning's update

Description

Cowork's Linux sandbox will not start. Every bash call returns "Workspace unavailable. The isolated Linux environment failed to start."

This is not a Hyper-V/virtualisation configuration issue — all prerequisites are confirmed present and working (see below). The app logs show the sandbox is being blocked by a feature flag before it ever attempts to provision a VM:

[cleanupVMBundleIfUnsupported] yukonSilver not supported (status=unsupported), checking for stale bundle...
[Bundle:status] rootfs.vhdx missing
[Bundle:status] vmlinuz missing
[Bundle:status] initrd missing
[startVM] VM not supported (win32/x64), skipping

The yukonSilver flag is loaded via GrowthBook ([growthbook] loaded 240 features) and evaluates to unsupported for this account/device on every launch. Because of this, the VM bundle (rootfs.vhdx, vmlinuz, initrd) is never downloaded and startVM exits immediately — the failure happens before any Hyper-V/VM provisioning is attempted.

Environment Confirmed Working (ruled out as cause)

• Hyper-V, Virtual Machine Platform, Windows Hypervisor Platform, Windows Sandbox, WSL — all enabled in Windows Features
• Task Manager confirms Virtualization: Enabled
• 12 cores / 14 logical processors, 5.8GB free memory, 0% disk utilisation at time of failure
• Windows Event Viewer: no application errors correlating with the failure (ruled out the secureconnectionservice.exe crash pattern seen in other reports)
• No CoworkVMService present on this machine at all (confirms this is a different failure mode from #55649)

What Should Happen?

The Linux sandbox should provision and start normally on Windows 11 Pro x64 with Hyper-V/VMP/WHP enabled, matching documented requirements.

Error Messages/Logs

Workspace unavailable. The isolated Linux environment failed to start.


### Relevant Log Excerpt (cowork_vm_node.log)


2026-08-06 09:13:08 [info] [cleanupVMBundleIfUnsupported] yukonSilver not supported (status=unsupported), checking for stale bundle...
2026-08-06 09:16:01 [info] [Bundle:status] rootfs.vhdx missing
2026-08-06 09:16:01 [info] [Bundle:status] vmlinuz missing
2026-08-06 09:16:01 [info] [Bundle:status] initrd missing
2026-08-06 09:27:05 [warn] [startVM] VM not supported (win32/x64), skipping
2026-08-07 07:37:46 [warn] [vm-client] getNetworkDrives unavailable; keeping prior snapshot: Error: VM service not running. The service failed to start.

Steps to Reproduce

1. Windows 11 Pro x64, Hyper-V/Virtual Machine Platform/Windows Hypervisor Platform all enabled
2. Launch Claude for Windows, open Cowork mode
3. Issue any bash command
4. Observe: "Workspace unavailable. The isolated Linux environment failed to start."
5. Check %APPDATA%\Claude\logs\cowork_vm_node.log — see yukonSilver not supported and VM not supported (win32/x64), skipping before any VM bundle download is attempted

Claude Model

Sonnet (default)

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2026-08-07, CCD 2.1.221 → 2.1.222

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

PowerShell

Additional Information

This looks like a GrowthBook feature-flag targeting issue (yukonSilver) rather than a local environment problem — flagging in case this account/device fell out of an intended rollout cohort after today's update.

cowork_vm_node.log