This is the missing piece. Microsoft ships DisableOutOfProcBuild.exe
inside the Installer Projects extension; it writes a registry key
that disables the out-of-process validation pass for .vdproj
projects. Without it, devenv's pre-build validation runs in a
separate process that doesn't see the extension, which produces
the HRESULT 8000000A 'An error occurred while validating' error
we keep hitting.

Reference: spin.atomicobject.com/visual-studio-installer-azure
('Building Visual Studio Installer Projects in Azure Pipelines')

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>