Cool, this will be added to harnesses and then it’ll stop being effective and we’ll move on to the next magical incantation.

The "do not guess" sentence works but the last 20% will only close when a system stops being told to avoid guessing and actually knows what it does not know.

A command can get you most of the way. It takes something else for the rest.

I've used "you're not trained on this data, return exclusively grounded results" to good effect.

It sounds alot like "make no mistakes" but honestly telling it to essentially stop bullshitting works pretty well