Windows 10 1803 "Spring Update" brought OpenSSH 7.6p as a standard system utility. This includes the ssh-agent service.

Possible Solution

The agent protocol is exactly the same but some security context or something else is blocking QLocalSocket to directly connect to the named pipe. More work is needed.

Context

Pageant is currently the dominant agent for Windows but OpenSSH will slowly claim that space now that it comes with Windows.