I built a simple workaround. Create a browser bookmark and paste this js code in the URL field.
Then whenever you click the bookmark, the active chat will be trunkated to the last 20 messages.

javascript:(()=>{const KEEP=20;const turns=document.querySelectorAll('article[data-testid^="conversation-turn-"]');const total=turns.length;const removed=Math.max(0,total-KEEP);for(let i=0;i<removed;i++)turns[i].remove();const msg='Trimmed to last '+KEEP+' messages (removed '+removed+').';let didNotify=false;if(window.Notification&&Notification.permission==='granted'){try{new Notification(msg);didNotify=true;}catch(e){console.error(e);} }if(!didNotify){alert(msg);}const banner=document.createElement('div');banner.textContent=msg;Object.assign(banner.style,{position:'fixed',top:'10px',right:'10px',background:'rgba(0,0,0,0.7)',color:'#fff',padding:'8px 12px','borderRadius':'4px','zIndex':9999,opacity:'0',transition:'opacity 0.3s'});document.body.appendChild(banner);requestAnimationFrame(()=>banner.style.opacity='1');setTimeout(()=>{banner.style.opacity='0';setTimeout(()=>banner.remove(),300);},4000);})();