Push asks a remote to update a branch. A non-fast-forward rejection protects commits already added by someone else; it is evidence to integrate, not an invitation to overwrite. Fetch, inspect and combine the histories before retrying. The simulated server changes between setup and your action so you practice that race locally. The check requires both your delivery and the teammate's record to remain in the final shared history. After integration, inspect the remote-tracking reference and both changed paths. A forced update may report success while losing the teammate commit. The accepted result must preserve that contribution in the published branch.