[BUG] Cowork regression — worked on Windows 11 ARM64 (Parallels), broke after Claude Desktop auto-update to v1.3109.0.0

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?

Description

Cowork was working perfectly on a Windows 11 ARM64 VM running inside Parallels Desktop 26.3.1 on Apple Silicon (M4 Max, 36GB RAM). After Claude Desktop auto-updated to v1.3109.0.0 on April 16, 2026, Cowork stopped working with the error: "Virtualization is not available — The Claude workspace requires Virtual Machine Platform, but the virtualization service is not responding."

This is a regression — Cowork was fully functional the day before the update, with multiple successful sessions completed.

Environment

• Host: macOS on Apple M4 Max, 36GB RAM
• Virtualization: Parallels Desktop 26.3.1 (57396) Pro Edition
• Guest OS: Windows 11 ARM64 25H2 (Build 26200.8039)
• Claude Desktop: v1.3109.0.0 (MSIX arm64, Claude_pzs8sxrjxfjjc)
• VM Config: 8 CPUs, 16GB RAM, Nested Virtualization: on
• CoworkVMService: Running (auto-start)
• VirtualMachinePlatform: Enabled
• HypervisorPlatform: Enabled
• Microsoft-Hyper-V-All: Enabled (all sub-features)
• HypervisorPresent: True
• hypervisorlaunchtype: Auto
• vmcompute/vmms services: Running

Error from cowork-service.log

2026/04/17 12:02:35 [HCS] HcsCreateComputeSystem returned: hr=0x0
2026/04/17 12:02:35 [HCS] HcsWaitForOperationResult returned: hr=0x80370102
2026/04/17 12:02:35 [HCS] Result JSON: {
  "Error": -2143878910,
  "ErrorMessage": "Cannot start VM because a required resource is not installed.",
  "ErrorEvents": [{
    "Message": "Cannot create system 'cowork-vm-30bc4944' because Hyper-V is not installed on the host.",
    "EventId": 11008
  }]
}

Root Cause — Hyper-V Hypervisor Boot Failure

Windows Event Log shows hypervisor fails to initialize at every boot:

[Microsoft-Windows-Hyper-V-Hypervisor] [Error]
Hypervisor initialization failed; internal logic failure of the hypervisor boot loader
(BalStatus 0x19, sub status 0xC).

[Microsoft-Windows-Kernel-Boot] [Error]
VBS enablement policy check at phase 0 failed: VSM (Virtual Security Mode) was not initialized.
The hypervisor or VSM may not be present or enabled.

Timeline

1. April 15-16: Cowork working normally with multiple sessions
2. April 16 at 19:11: Claude Desktop auto-updated to v1.3109.0.0 (confirmed by Claude.exe file timestamp)
3. April 17: Cowork shows "Virtualization is not available"

Troubleshooting Done

• Verified all Windows features enabled (Hyper-V-All, VMP, HypervisorPlatform)
• Verified bcdedit /set hypervisorlaunchtype auto
• Verified HypervisorPresent = True and all Hyper-V services running
• Verified Parallels nested virtualization enabled (prlctl list -i shows Nested virtualization: on)
• Disabled ALL virtualization features, rebooted, re-enabled, rebooted — same error
• Tested with 6 CPU/12GB, 8 CPU/14GB, 8 CPU/16GB RAM — same error
• Multiple clean shutdown/restart cycles — same error
• Checked C:\ProgramData\Claude\Logs\cowork-service.log — confirmed HCS_E_HYPERV_NOT_INSTALLED (0x80370102)

Related Issues

• #39045 — Similar "yukonSilver unsupported" on ARM64, but that case never worked. This is a regression from a working state.
• #32004, #45715 — Similar "Virtualization not available" errors

Request

1. Provide a way to download previous Claude Desktop versions for regression testing
2. Investigate what changed in v1.3109.0.0 that may have affected HCS/Hyper-V in nested virtualization
3. Consider alternative sandbox mechanisms for environments where nested Hyper-V is fragile

What Should Happen?

Cowork should work on Windows 11 ARM64 running in Parallels Desktop on Apple Silicon, as it did prior to the v1.3109.0.0 update. If the previous version supported this configuration (and it did — multiple successful sessions were completed on April 15-16), the new version should not break it.

Specifically:

1. The CoworkVMService should be able to create Hyper-V compute systems via HCS in nested virtualization environments that were previously working
2. If nested Hyper-V support is fragile, Cowork should detect this gracefully and provide a clear error message rather than a generic "Virtualization is not available"
3. Anthropic should provide a way to download previous Claude Desktop versions so users can roll back when regressions occur

Error Messages/Logs

From C:\ProgramData\Claude\Logs\cowork-service.log:

2026/04/17 12:02:35.569922 [HCS] Calling HcsCreateComputeSystem with id=cowork-vm-30bc4944
2026/04/17 12:02:35.571150 [HCS] HcsCreateComputeSystem returned: hr=0x0, handle=2230851582944, lastErr=The operation completed successfully.
2026/04/17 12:02:35.571150 [HCS] HcsWaitForOperationResult returned: hr=0x80370102
2026/04/17 12:02:35.571150 [HCS] Result JSON: {
  "Error": -2143878910,
  "ErrorMessage": "Não foi possível iniciar a máquina virtual porque um recurso necessário não foi instalado.",
  "ErrorEvents": [{
    "Message": "Não é possível criar o sistema 'cowork-vm-30bc4944' porque o Hyper-V não está instalado no host.",
    "EventId": 11008
  }]
}

Error code: HCS_E_HYPERV_NOT_INSTALLED (0x80370102)

From Windows Event Log (Microsoft-Windows-Hyper-V-Hypervisor):
"Falha na inicialização do hipervisor; falha na lógica interna do carregador de inicialização do hipervisor (BalStatus 0x19, sub status 0xC)."

From Windows Event Log (Microsoft-Windows-Kernel-Boot):
"A verificação da política de habilitação de segurança baseada em virtualização na fase 0 falhou com o status: O VSM (Modo de Segurança Virtual) não foi inicializado. O hipervisor ou o VSM pode não estar presente ou habilitado."

Steps to Reproduce

1. Set up a Windows 11 ARM64 VM in Parallels Desktop 26.3.1 on Apple Silicon (M4 Max)
2. Enable nested virtualization in Parallels (prlctl set <vm> --nested-virt on)
3. Enable Windows features: Hyper-V, VirtualMachinePlatform, HypervisorPlatform
4. Install Claude Desktop (MSIX arm64) — Cowork works fine on versions prior to v1.3109.0.0
5. Allow Claude Desktop to auto-update to v1.3109.0.0
6. Reboot the VM
7. Open Claude Desktop and try to start a Cowork session
8. Result: "Virtualization is not available" error

Note: The Hyper-V hypervisor fails to initialize at boot with BalStatus 0x19, sub status 0xC. This prevents HCS from creating the Cowork VM sandbox. The exact same VM configuration worked on April 15-16 before the update.

Claude Model

None

Is this a regression?

Yes, this worked in a previous version

Last Working Version

Version prior to v1.3109.0.0 (exact version unknown — auto-updated on April 16)

Claude Code Version

Claude Desktop v1.3109.0.0 (MSIX arm64) — this is Cowork in Claude Desktop, not Claude Code CLI

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

PowerShell

Additional Information

_No response_