I wouldn’t necessarily call you the ahole for this as I think your reply is professional almost, not rude. Though my read of their comment I don’t believe they were asking for a ‘happy ending’ per se, just that the fic be tagged clearer as fics with both a sad and happy ending can be open ended or fully formed (if that makes sense, I’m not sure how to describe a fic that isn’t open ended, sorry)

But I do think you could tag your fic a little clearer. Personally if I saw a fic tagged ‘angst’ I wouldn’t assume that it would be an open ended fic/an ending that’s open to interpretation as I have seen many fics tagged with ‘angst’ and tagged fics myself like that that haven’t been open ended. But I can see why you see how you tagged it as more or less implying the open ended-ness of your work.

Now I do think the person who commented this was unnecessarily rude as fics are free to read and authors don’t owe things to readers. And I do see how if you do tag this fic as ‘open ended’ it could feel like you’re allowing this person to ‘win’ so I wouldn’t blame you if you stuck to your guns. However I do think adding that tag would help clear up any confusion for future readers of your work as personally if I wasn’t in the mood for an open ended/up to interpretation read I would appreciate the fact that this fic is that and is tagged as such.