fatal: refusing to merge unrelated histories

GitHub Actions' default checkout configuration causes Unrelated Histories Error more often than developers expect, because the default is a shallow clone — a clone with only the most recent commit in the history. Shallow clones (created with fetch-depth: 1 in actions/checkout) lack the full commit graph. Operations that need ancestry information — such as git log, git merge-base, git describe, version bumping scripts, and changelog generation — will fail with Unrelated Histories Error because the required ancestor commits are not present. The fix: set fetch-depth: 0 in your checkout action to fetch the full history, or set it to a specific depth like 50 if full history is too large. For monorepos detecting which packages changed, shallow clones also break git diff HEAD~1 comparisons. Additionally, pipelines running Git operations that write back to the repository (tagging, version bumps) need token: ${{ secrets.GITHUB_TOKEN }} with write permissions explicitly granted in the workflow file.