I could totally see agents "cheating" by finding and exploiting Lean soundness holes. But I'd be surprised if it went unnoticed for long, and finding these holes is highly valuable so overall... would be a win? Plus, I'd expect OpenAI would be careful about that proactively: for PR reasons they'd much prefer saying "we found holes in Lean, here's the fix" than "we proved a major thing! oops no we didn't, our model fooled us".