Windows sandbox: dotnet build of a multi-project .NET solution #5421

Describe the bug

When sandbox mode is enabled, there are several problems when the agent want to perform a dotnet build/restore command

1. MSBuild worker processes can't connect (blocks multi-project builds).

   • dotnet build 'solution' exits with code 1 reporting "Build failed", 0 errors, 0 warnings.
   • With MSBUILDDEBUGCOMM=1 , each worker creates its named pipe ( MSBuild ) and waits, but the main process never connects.
   • With -m:1 flag it works but is far too slow for large solutions.
   • No sandbox denial is reported and no bypass prompt appears.

2. NuGet Plugins fail to load (specifically Azure Artifacts Credential Provider)

   • environment variable %LOCALAPPDATA% points to the per-session sandbox folder ( ...\Packages\sandbox.{GUID}\AC ), which isn't writable
   • NuGet's plugins-cache and v3-cache both live under it. NuGet fails to start the credential provider, so private feeds return 401 in my case.
   • Allowlisting C:\Users<user>\AppData\Local\NuGet makes no difference
   • there is a workaround by setting NUGET_PLUGINS_CACHE_PATH and  NUGET_HTTP_CACHE_PATH explicitly.

3. Current users .gitconfig is not readable, so the SDKs builtin SourceLink step fails the build. The workaround is set the p:EnableSourceControlManagerQueries=false flag.

dotnet and nuget are detected as "Dev Tool" when running /sandbox policy:

Affected version

GitHub Copilot CLI 1.0.109

Steps to reproduce the behavior

1. Enable sandbox mode with /sandbox enable
2. Prompting copilot that it should build the solution.

Expected behavior

Agent is able to run a dotnet build command and nuget plugins are working correctly (

Additional context

Environment: Copilot CLI 1.0.105, Windows 11 with the ProcessContainer sandbox, .NET SDK 10.0.404. Private NuGet feeds on Azure DevOps Artifacts, authenticated by the Azure Artifacts Credential Provider.