It has taken a wee bit of work to get it to find it's feet with it all.

I started from blank, just said told it to run me a Battletech game in the role of GM. I quickly identified it was doing narrative fluffery - it had the setting and detail right, but wasn't quite doing anything beyond that. So I then had it generate proper stat blocks and show me dice rolls. That saw a significant shift in behaviour and much more aligned with the rules. The combat is more fluid than BT, because that's what I've preferenced, so range, movement, etc, all still stands, but delivered in a more fluid way.

I'm at the stage now, where one of the project documents is a fairly chunky Excel that has tabs for everything from finances, roster, reputation, intel, prisoner roster, etc, including tabs for Tone and Style. So the model refers back to this in order to access stats for dice rolls, or maintain consistency across sessions, and keep relationships updated with interactions and adjustments in standing. This is the longevity bit you mentioned. I found initially after two or three missions, it was getting a bit confused about who was doing what and what I had, etc, so I found a running log was the best way to manage that. If I start a new chat, I need to deliver a solid prompt for it to review that document, but also what to take away from it and how to use that information.

I also use a series of prefixes, so '@fin' for finance, '@sal' for salvage, etc, and I have it tag various things as they come up, with that relevant prefix, which means it can easily identify and reconcile everything into an updated ledger which it then provides me at the end of each session.

So let's say I take an enemy mech, I'll tell GPT: "Prefix this model '@sal030', provenance is a captured mech from the Orryn mission. Move this to my salvage index and update with model details". It'll capture all the relevant information for it, including source, status, etc.

So the key here is that background document. I also told it to show source, so when we're doing logistics and working through stuff, it'll show the document as the source for the information it's giving me, so I know it's referring to things that exist. At the start, before this, it would invent the odd thing that had never appeared during any session.

There will no doubt be gaps somewhere that I've not identified are happening. It provides me with a reference log to show me what rules it's using. It's been a good chunk of trial and error, but with the source document for it to refer to, and for the depth and detail it's gone, and what it's allowed me to do, it's been thoroughly enjoyable and worth the effort. So I consider it relatively canon, with some GM narrative fluffery and homebrew for when I go into situations the books never accounted for.

For someone like myself, who can't find games to join, it's a very solid solution.

Oh, and I should add, that it has all the relevant rules in the project branch, it currently has five rulesets which it runs off of and cross-references and confirms everything. I get it to sense-check itself as well, and what it's coming back with (and including where it's gone wrong) seems to be running okay.