1. Install the openssh.

   $ nix-env -iA nixpkgs.openssh
   

2. Disable the built-in macOS SSH agent.

   $ launchctl disable gui/$UID/com.openssh.ssh-agent
   

3. Add definition of the new ssh-agent to ~/Library/LaunchAgents/org.nix.ssh-agent.plist.

   <?xml version="1.0" encoding="UTF-8"?>
   <!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
   <plist version="1.0">
   <dict>
   	<key>Label</key>
   	<string>org.nix.ssh-agent</string>
   	<key>ProgramArguments</key>
   	<array>
   		<string>/bin/sh</string>
   		<string>-c</string>
   		<!-- We reuse SSH_AUTH_SOCK from com.openssh.ssh-agent -->
   		<string>rm -f $SSH_AUTH_SOCK; exec $HOME/.nix-profile/bin/ssh-agent -D -a $SSH_AUTH_SOCK</string>
   	</array>
   	<key>RunAtLoad</key>
   	<true/>
   </dict>
   </plist>
   

4. Enable the new ssh-agent.

   $ launchctl bootstrap gui/$UID ~/Library/LaunchAgents/org.nix.ssh-agent.plist
   

5. Check it's running.

   $ launchctl list | grep ssh