Using {"branch": "phabricator/diff/689", "commit": "HEAD", "message": "..."} should roughly translate into a git fetch phabricator/diff/689 && git checkout FETCH_HEAD in the Buildkite agent bootstrap script. Git is smart enough to resolve that to the tag. The branch name is really only used in Buildkite for branch filters, on the agent Git does a headless checkout. And a commit value of HEAD means we'll ask Git for the commit sha and back-populate that onto the build. 👍