ar5iv-bookmarklet.js

javascript:(function() {
  let url = window.location.href;
  let match = url.match(/arxiv\.org\/abs\/(\d+\.\d+)/);
  if (match && match[1]) {
    let paperId = match[1];
    let ar5ivUrl = `https://ar5iv.labs.arxiv.org/html/${paperId}`;
    window.location.href = ar5ivUrl;
  } else {
    alert("This doesn't appear to be an arXiv paper page. Please use this bookmarklet on an arXiv paper page (e.g., https://arxiv.org/abs/2402.09171)");
  }
})();

one liner

javascript:window.location.href=window.location.href.replace(/arxiv\.org\/abs\/(\d+\.\d+)/, "ar5iv.labs.arxiv.org/html/$1");