Copilot CLI Reports 'Sandboxing is enabled but is not supported on this host' for latest Windows 25H2 build

[JohannesZahn]

Describe the bug

Dear Copilot CLI Dev Team,

when I run copilot cli with the sandbox flag

copilot --experimental --sandbox

I receive a warning

Sandboxing is enabled but is not supported on this host, so shell commands and sandboxed services will fail. Update Windows — supported versions at https://aka.ms/ghcp-sandbox-os-support — or turn sandboxing off with /sandbox disable, or set sandbox.enabled to false in settings.json. See https://docs.github.com/en/copilot/concepts/about-cloud-and-local-sandboxes
Windows Version: 25H2 (Build 26200.9106)

The latest Windows updates are installed including the latest update https://support.microsoft.com/de-de/servicing/os/hotpatch/windows-11/2026/kb5123607-windows-11-24h2-25h2-standalone-cbs-hotpatch according to my windows update history

The sandbox was previously functional

Affected version

v1.0.81

Steps to reproduce the behavior

1. Install latest Windows Build
2. Run copilot --experimental --sandbox

Expected behavior

Sandbox should be functional and no warning message should appear

Additional context

No response

[MilkyWay008]

Ran into this exact warning on Win11 25H2 after the hotpatch KB5123607 too. For me the culprit was the Windows Sandbox optional feature getting half-disabled by the update, so the host-support check in copilot treats it as unsupported. Check that first: optionalfeatures \u2192 confirm "Windows Sandbox" and "Virtual Machine Platform" are both still enabled, and confirm virtualization is on in firmware (taskmgr \u2192 Performance \u2192 CPU \u2192 Virtualization). If those are fine, then it's likely the support list in the CLI itself lagging the new build, and the only thing that worked for me was sandbox.enabled: false in %USERPROFILE%\\.copilot\\settings.json until the CLI catches up. Worked the same way for a teammate on 24H2 with a fresh build.

[wullemsb]

I have the same issue. What I find especially strange is that the sandbox was working before.