No need for custom header or query param, Ajax (xmlhttp) requests already have an extra header field set automatically, so all is needed is some if statement server side, assuming registering the upvote and rendering the page is not entangled in some super crazy way.

HN works perfectly with javascript dissabled. It’s a feature

It can still easily work with JavaScript disabled and implement the change.

We're talking about it working better when JS is enabled.

You'd need some frontend code to hide the upvote icon though.

That frontend code already exists and does that every time you vote (line 27 of https://news.ycombinator.com/hn.js). The response content isn't used for anything at all.

Nice! I hadn't actually looked at the code before writing my comment. That does seem relatively trivial, so I stand corrected. I didn't realize the clientside code was discarding the response; I sorta naively assumed it was doing a DOM replacement or something equally straightforward.

What a pleasantly tiny script file. More site code should be this easy to read.

If you have JavaScript disabled, the response content is used to hide the upvote sign.

We're talking here about something simple like:

  (if *ajax-request*
     (upvote)
     (upvote-and-generate-page))