If you get the error:
Could not open a connection to your authentication agent.It is easy to fix. This happened to me when I installed the MacPorts version of OpenSSH on my Mac (because work blocks the native ssh app rather than the port; we won’t get in to the silliness of that). Rather than trying to disable the MacOS ssh-agent (which apparently continues to work even if remote connections are disabled in the System Settings), you can still use it. All you have to do is add the following in your shell’s profile before you attempt to use ssh-add:
eval $( ssh-agent )That will load the SSH_AUTH_SOCK and SSH_AGENT_PID environment variables that ssh-add is looking for to find the socket file.
To use the keys from your Keychain, you can look here.