Debug: Git push rejected with non-fast-forward error

Git push is rejected with 'non-fast-forward' or 'Updates were rejected because the remote contains work that you do not have locally'.

Fix rejected push:

# The error:
# ! [rejected] main -> main (non-fast-forward)

# CAUSE: Remote has commits you don't have locally

# FIX 1: Pull and merge (safest)
git pull origin main
git push origin main

# FIX 2: Pull with rebase (cleaner history)
git pull --rebase origin main
git push origin main

# FIX 3: For feature branches already pushed
git fetch origin
git rebase origin/main
git push --force-with-lease origin feature-branch
# --force-with-lease is safer than --force
# It fails if someone else pushed to YOUR branch

# CHECK: What's different?
git log HEAD..origin/main --oneline  # Remote commits not in local
git log origin/main..HEAD --oneline  # Local commits not on remote

# PREVENTION:
git config --global pull.rebase true

Git prevents non-fast-forward pushes by default to protect against overwriting others' work. The solution is always to integrate remote changes first, then push.

Git collaboration and pushing