How to maintain ssh-agent login session with Windows 10's new OpenSSH and PowerShell

You must configure OpenSSH Authentication Agent service to automatically start (or you can start it manually everytime when opening your powershell for the first time: Start-Service ssh-agent).

After that, you need to ssh-add C:\path\to\your\ssh\key\id_rsa only once. After that, everytime the ssh-agent is started, the key will be there. You can check with ssh-add -l.

EDIT: To have SSH agent to automatically start with Windows, you can run Set-Service ssh-agent -StartupType Automatic on a super-user powershell prompt.

Not a full answer, but still a solution to the problem that brought me here. (I also see a comment from one other person here that seems to be the same problem.)

If you have Git for Windows or MinGW or anything else which might add GNU utilities to your Windows path, that can interfere with the OpenSSH for Windows binaries. For me, I had to remove ProgramFiles/Git/bin from my PATH environment variable and then restart PowerShell in order to get this to work. Prior to that I was getting "communication with agent failed"