"I have discovered a truly marvelous proof of this, which the user's token-budget is too narrow to finish until 8:16 EST," was found scrawled in the CoT after the death of Claude Fable
Fable achieved a significant breakthrough in one of our open problems. This is a problem where ChatGPT 5.5 could not even begin anything useful. The breakthrough seems legit (although not 100% checked yet), and Fable even claims to have a full solution. >10 hours total runtime so far. A 30 page document with the proofs of some lemmas not yet spelled out. We can not yet know whether Fable indeed has solved it, but even if it is just a partial solution, we are absolutely amazed. More details will follow, and once we are at the end of the story, I will also write a full substack post. Collaboration with István Vona, a postdoc in my group.