You could argue that if you allowed a harness, and that harness was specific for ARC, then you don’t have AGI, you have something that is definitely not general.

What if the harness is developed by the same model in a prior session?

I think that's a strange way to look at it. The brain also has different regions with different functions, and part of what makes us humans intelligent is that we can use tools like pen and paper to keep notes and help us solve problems.

Similarly LLMs are not just massive uniform artificial neural nets, and now we also have harnesses, which I'd personally view more of an extension of the model itself. The harness is both the executive and also what allows it to keep notes, use a calculator, or maybe even create scripts to help it solve complex deterministic problems.

I think it's unfair to give a human a very complex maths problem and say that they're not intelligent if they can't solve it without pen and paper or a calculate. At least expecting a human to solve complex problems this way doesn't really measure anything useful in the real world.