Pointed Fable at a repo that had nothing in it but the top 20 NES games (as scientifically ranked by Jeff Gerstmann).  All I told it was to have a look at them for context, and create an original game.

In \~20 minutes and using about 60% of my session limit, it had produced "Starfall", which is a pretty ok NES game.  I'm kind of blown away because "pretty ok" is so much better than anything I was getting out of any previous LLM attempts at this task, which ranged from utter failure to bootable but glitchy mess.

It's got a coherent structure, half-decent music, basic but legible sprite work, what seems like fairly thoughtful level design etc.  The version I'm sharing below isn't a true one-shot, because it initially had an issue where you could jump too high off the top of the screen and die instantly, and some jumps were too big to make, but it tidied those up handily.

Here's how Fable summarised what it did:

>I wrote a headless NES harness (tools/headless.py, py65 CPU + my own PPU model) and an automated playthrough suite (tools/test\_e2e.py) that boots the ROM and plays it: title → zone 1 → gate → zone transition → boss spawn with camera lock → defeating the boss only in its vulnerable phases → final gate → victory with correct score (5000 for the boss); plus three pit deaths → game over → back to title, and pause freeze/unfreeze. The harness caught real bugs along the way — a register-clobber that hung the engine past x≈768, an unwinnable boss hovering above bullet height, a charmap quirk that corrupted the ROM header — all fixed and re-verified. The screenshots above are rendered from the actual ROM running in that harness.

.nes file here, and there are a million online emulators out there that let you upload a rom if you want to test it out:  
ufile. io/ p66ha89b

Fable made a pretty ok authentic NES game