Cowork on Windows 11 Home: Fix for API UNREACHABLE (DNS misconfiguration)

Summary

Cowork fails to connect to the Claude API on Windows 11 Home with the error:

Cannot access the Claude API in the Claude workspace.

The VM boots and network shows CONNECTED, but API reachability goes PROBABLY_UNREACHABLE then UNREACHABLE.

Root Cause

cowork-svc.exe reads DNS servers from the host's active network adapter and passes them to the VM via RPC.

The problem: If your active adapter uses private/internal DNS servers (e.g. 10.8.8.8, 10.7.7.7 from a VPN or custom network), those DNS servers are unreachable from inside the VM's NAT subnet (172.16.0.0/24). The VM cannot resolve api.anthropic.com.

Additionally, Windows 11 Home is missing NetNatCim.dll, which prevents the New-NetNat PowerShell cmdlet from working - but the WMI class can be registered manually.

Environment

• OS: Windows 11 Home Single Language (10.0.26200.7840)
• Claude Desktop: 1.1.3363.0 (Microsoft Store)

Fix (step by step)

Step 1: Enable full Hyper-V on Windows 11 Home

Save as Enable-HyperV.bat and Run as Administrator:

@echo off
pushd "%~dp0"
dir /b %SystemRoot%\servicing\Packages\*Hyper-V*.mum >hyper-v.txt
for /f %%i in ('findstr /i . hyper-v.txt 2^>nul') do dism /online /norestart /add-package:"%SystemRoot%\servicing\Packages\%%i"
del hyper-v.txt
Dism /online /enable-feature /featurename:Microsoft-Hyper-V -All /LimitAccess /ALL
pause

Reboot after completion. Some packages may show error 0x8007371b - this is normal.

Step 2: Register the NetNat WMI class

In an Admin PowerShell:

mofcomp C:\Windows\System32\wbem\NetNat.mof

Step 3: Enable IP forwarding

In an Admin PowerShell:

Set-NetIPInterface -InterfaceAlias "vEthernet (cowork-vm-nat)" -Forwarding Enabled
Set-NetIPInterface -InterfaceAlias "Wi-Fi" -Forwarding Enabled

Replace Wi-Fi with your internet-facing adapter name if different. IP forwarding may reset on reboot.

Step 4: Fix DNS (the key fix)

Check your active adapter's DNS:

Get-DnsClientServerAddress -AddressFamily IPv4

If your primary adapter has private DNS (e.g. 10.x.x.x, 192.168.x.x), change it to public DNS:

# Replace "Ethernet" with your adapter name
Set-DnsClientServerAddress -InterfaceAlias "Ethernet" -ServerAddresses ("1.1.1.1","8.8.8.8")

Then restart Claude Desktop and Cowork should work.

How We Diagnosed This

1. cowork_vm_node.log showed: [VM] API reachability: UNREACHABLE
2. Inspecting the HNS endpoint (Get-HnsEndpoint | ConvertTo-Json -Depth 3) revealed DNSServerList: 10.8.8.8,10.7.7.7 - private DNS copied from the host adapter
3. These private DNS servers were unreachable from the VM's 172.16.0.0/24 NAT subnet
4. Changing the host adapter DNS to public servers (1.1.1.1, 8.8.8.8) fixed the issue

Suggested Fix for Anthropic

cowork-svc.exe should:

1. Validate DNS reachability from the VM subnet before assigning DNS servers
2. Fall back to public DNS (1.1.1.1, 8.8.8.8) if the detected DNS servers are private/unreachable
3. Allow DNS override via a config file or environment variable
4. Include DNSServerList in the HNS network creation JSON

Related Issues

• #24962 (DNS config, missing sessiondata.vhdx, no NAT)
• #24918 (Cannot connect to Claude API - Windows 11 Home)
• #25241 (Windows 11 Home issue)
• #25155 (VM boots but cannot reach Claude API)
• #25308 (VM starts but API is UNREACHABLE)