I add "Terse, no commentary" to a request, which it will obey, but then immediately return to yammering in a follow-up message. However, this is in Incognito; maybe there's a setting when logged in.
You can add a custom instruction in settings. That works pretty well, applies to all chats. Memory feature I of course disabled as soon as it was released so I don't know which takes precedence.
Same. The GPT5 “Robot” persona does what no custom “be terse”, “no fluff”, etc. custom prompt ever could. It actually makes ChatGPT terse and to-the-point and eliminates (or at least greatly reduces) fluff. I love it.
