> Someone has submitted a 4k-line PR to one of my projects.

That wouldn’t be reasonable even if it weren’t vibe coded.

> The person is a long-time user and is very enthusiastic about the app.

Be careful. Don’t let yourself be pushed into an XZ-type situation.

https://en.wikipedia.org/wiki/XZ_Utils_backdoor

> Not sure what to do now. I really want to help my user, but I don't like this code in my codebase.

If you don’t want to accept the PR, don’t. It’s still your project. Reject it politely. Tell them why you don’t feel comfortable accepting it (too many changes at once; impractical to review; general distrust of LLM code; …) but remind them that “of course, if you want to keep that in your own fork, you’re more than welcome”. I’ve been doing that for years (way before vibe coding was a thing) and was always able to keep good relations with my users.

Be polite and make sure they understand your rejection is not an indictment on their submission, while reassuring them that the effort was not wasted because they can keep it for their own use if they really want it that bad.

> This morning, I received another message from another user who is about to submit one more feature.

When you write the message for the first user, try to also make it generic enough (i.e. about the project, not their specific submission) that you can link to it for other users.

What if the feature was coded by hand?

I ususally do, but sometimes I don't. Most of the time I will, but there's always a chance that I just won't allow vibecoded submissions in my program. I think that that's the answer you were looking forward to. Nothing actually matters, y'lnow.

General rule... you have zero obligation to merge any code to your repo, much less bad code, or very large hard-to-review submissions.

I think that it's bad manners for someone to submit a big PR without prior experience with the project. Someone needs to earn trust over time. They might start out with a few small PRs and gradually build up to the point where you might trust them with a larger change. But even so, a 4k-line PR is very unreasonable.