Windows runners run as SYSTEM with the workspace under C:\Windows\System32\config\systemprofile. cl.exe is 32-bit, so WOW64 file redirection sends its System32 access to SysWOW64 and it cannot see the checked-out sources (C1083: aegis.h not found), while 64-bit processes (git, python) see them fine. Move the Windows checkout/build to C:\aeg-build.