My go to benchmark is a 3d snake game Claude does almost flawlessly (or at least in 3-4 iterations)
The prompt:
write a 3d snake game in js and html. you can use any libraries you want. the game still happens inside a single plane, left arrow turns the snake left, right arrow turns it right. the plane is black and there's a green grid. there are multiple rewards of random colors at a given time. each time a reward is eaten, it becomes the snake's new head. The camera follows the snake's head, it is above an a bit behind it, looking forward. When the snake moves right or left, the camera follows gradually left or right, no snap movements. write everything in a single html file.
EDIT: I'm not trying to shit on GPT-5, so many people here seem to be getting very good results, am I doing something wrong with my prompt?