Comment by andai

A little bird may have dropped the entire system prompt on my desk.

They do customisation of the prompt per user and session. (Also apparently per IP address, based on a recent post here...)

You could still get a lot of savings if you put all the custom stuff at the end but they don't seem to do that. (Well with the old prompt anyway I haven't checked the new one yet.)

My guess as for why not, is that this is where a lot of their money comes from. So I'm honestly surprised that they removed 80% of the prompt. But I guess my mental model has some gaps in it. Maybe very few of the sessions are short enough for that to matter.