How to fix error: commit abc1234 is a merge but no -m option was given in Git

First, confirm the commit you want to revert is actually a merge commit:

git log --oneline <commit-hash> -1

If it's a merge, you'll see something like:

abc1234 Merge branch 'feature-x' into main

To see the parent commits:

git cat-file -p <commit-hash>

This shows both parents. The first listed is parent 1, the second is parent 2.

The parent number determines which branch's state Git reverts to:

- Parent 1 (-m 1): The branch you were on when you ran git merge. Usually main/master.
- Parent 2 (-m 2): The branch that was merged in. Usually your feature branch.

You can visualize this with:

git log --graph --oneline <commit-hash>^1 <commit-hash>^2 -5

Or check the merge commit message which typically says "Merge branch 'X' into Y" - Y is parent 1, X is parent 2.

In most cases, you want to undo the feature branch changes while keeping main's history. Use:

git revert -m 1 <merge-commit-hash>

This creates a new commit that undoes all changes brought in by the merged branch.

Example:

git revert -m 1 abc1234

Git will open your editor for the commit message. The default message explains what was reverted.

Git generates a default commit message like:

Revert "Merge branch 'feature-x' into main"

This reverts commit abc1234, reversing
changes made to 8989ee0.

You can:
- Accept the default message by saving and closing the editor
- Customize it to explain why you're reverting
- Use --no-edit to skip the editor: git revert -m 1 --no-edit <hash>

After the revert, confirm the changes were undone:

# See what changed
git diff HEAD~1

# Check the log
git log --oneline -3

# Verify specific files are restored
git show HEAD --stat

The reverted files should now match their state before the merge.

Once you've verified the revert is correct, push it:

git push origin <branch-name>

Important: Unlike git reset, this creates a new commit rather than rewriting history, so it's safe to push to shared branches.

Critical caveat: After reverting a merge, you cannot simply re-merge the same branch later. Git sees those commits as already merged.

To re-merge the feature branch later, you must first revert the revert:

# Find the revert commit
git log --oneline | grep "Revert"

# Revert the revert
git revert <revert-commit-hash>

# Now you can merge again (or the original changes are back)
git merge feature-branch  # if there are new commits

If you haven't pushed the merge yet, git reset is simpler:

# Undo the merge entirely (moves HEAD back)
git reset --hard HEAD^

# Or keep changes in working directory
git reset --soft HEAD^

Warning: Only use git reset on commits you haven't shared with others. It rewrites history.