Merges can be undone. Either you can manually remediate by `jj abandon`ing the merge commit so that it's not visible anymore, or you can restore the entire repo state to a previous point in time with `jj undo` or `jj op restore`, or you can do some remediation in between those two extremes.

Off of the top of my head, `jj new` and occasionally `jj rebase` can create merge commits; I don't recall any others.

You can always undo the most recent action using `jj undo`. To undo older actions, the easiest solution is to look for the state you want to get back to in the operations log `jj op log`, and then restore that state directly using `jj op restore <hash of state>`.

You really can undo every action in Jujutsu (and if you can't, that's a bug), but the `undo` mechanism can be a bit surprising - it doesn't behave like a stack, where undoing multiple times will take you further back in history. Instead, undo always undoes the most recent action, and if the most recent action is an undo, then it undoes the undo. This often catches people off-guard - in future versions, JJ will show a warning when this happens, and in even further future versions, there's a plan to make undo behave more like expected.

But if you use `jj op log` and `jj op restore`, you can always get back to any previous state, including undoing merges and other complicated changes.

Thanks, that makes sense. The cheat sheets either didn’t contain the op log-reflog analog or I missed it.

For the record I’ve seen ‘cannot undo a merge’ after jj undo 3-4 times, can’t remember now. I was trying to squash a change into a single commit for a GitHub pr for the first time and couldn’t figure out how to map jj commits into something acceptable, then decided to undo the whole thing and actually managed to overwrite some of my changes in a way I couldn’t find them, fortunately only a few lines of boilerplate.