GitHub recently started supporting signing commits via SSH opposed to via GPG.
I created a SSH key specifically for signing commits, and added it to the ssh agent using ssh-add. I can see it when listing the public keys:
$ ssh-add -L
ecdsa-sha2-abc123 ABC123/abc123+abc123+abc123== (stdin)
However, git for some reason cannot find this key:
$ git config gpg.format ssh
$ git config user.signingKey 'ecdsa-sha2-abc123 ABC123/abc123+abc123+abc123=='
$ git commit --allow-empty --message='signing using ssh!'
error: Couldn't load public key ecdsa-sha2-abc123 ABC123/abc123+abc123+abc123==: No such file or directory?
fatal: failed to write commit object
How come git cannot find the key which clearly is present in ssh-add?