rctneil I haven't used SourceTree, but a few things to double check that have caught me in the past:
- Run
git config --list
to make sure your Git user matches the user and SSH key you are offering to GitHub (https://developer.1password.com/docs/ssh/git-commit-signing#step-1-configure-git-commit-signing-with-ssh)
- Double check to make sure your SSH keys are in your Private vault. I assume they are, but if they aren't, you would need to update the SSH agent config
- Do you have multiple SSH keys in the vault? The SSH agent picks the first one and then runs through them all until one works. This can cause some hosts to reject the keys. If you do, you may need to configure the SSH agent to match your key to a specific host. (https://developer.1password.com/docs/ssh/agent/advanced/#ssh-server-six-key-limit