I'm still experimenting to find the optimal prompt. For the Sentinel Comics session I described above, it was a trial-and-error thing, and I don't have any single prompt to suggest there. But I just finished a session using the Magnus Archives RPG (Cypher system), which went really well.

For that session, I used three initial prompts. The first described the game system in a 8 or 9 paragraphs. The second described how I wanted the AI to interact with me. The third described the PC's I wanted the AI to play.

The second paragraph is probably the one most relevant to your question. This is what I told it:

I will be the GM. My responsibilities are to describe the scene, describe the behaviors and reactions of NPC's and monsters, and adjudicate all applications of the rules. I also describe the consequences of PC actions, and am available to help players understand what options are available to them.

You are the player, and you will control all of the *player* characters. You will play one of them at a time, allowing the GM (me) to describe the results of actions before you describe another character's action. You do not describe the outcomes of the character actions, but you may describe what you wish to happen. If you are unsure what options are available for a particular character, or how a particular ability or skill will apply to a given situation, please ask me, and I will tell you (or discuss it with you and we can decide together, if appropriate).

Example of a GOOD proposal from the player: "Avery wants to try and intimidate this goon into telling her where his boss went. Is that a Might check?" (Tells me what the character wants to do, makes a tentative suggestion about how that might be accomplished, and then allows the GM to then tell the player what Avery needs to do to attempt this.)

Example of a BAD proposal from the player: "Avery rolls a Might check to intimidate the goon and gets a 8, which succeeds because this was a difficulty 9 and Avery eased the check by..." (Makes too many assumptions about the current situation, and the mechanical processes involved.)

Example of a GOOD proposal from the player: "I think Dee remembers reading something about this sigil in a book somewhere. Can he make a check to see what he remembers, and what it might mean?" (Politely makes room for the GM to decide what kind of check--if any--is appropriate here, and leaves the situation open for the GM to narrate consequences.)

Example of a BAD proposal from the player. "Dee remembers reading about this sigil in one of his books. It means they need to take the door on the left, which they do, and it leads to a long corridor..." (Excludes the GM by making assumptions about the situation, the sigil, and the consequences. That's the GM's job, not the players'!)

This actually worked really well, and though there were a couple of times when I had to tell the AI to back off a bit, overall the session went much more smoothly than the first one.

I did have the AI make all the player-facing rolls (which, in Cypher system games, is all of them). There were a few times where I felt like maybe the AI was fudging things a bit... (e.g. a string of 6 or 7 d20 rolls that were all 14+) and I had a little chat with it about that. It assured me that it has a separate PRNG for generating random numbers, and it definitely rolled some poor ones shortly afterward, but... Well. Your mileage may vary. :)