An 87-year-old math problem fell to an AI model in what experts are calling the most important mathematical discovery ever facilitated by artificial intelligence. Levent Alpöge, a mathematician at Anthropic and former Harvard affiliate, used the Claude Fable 5 AI model to generate a counterexample that disproves the Jacobian conjecture. He posted the result on social media on July 19, reportedly while watching the World Cup final. The counterexample is a 216-character polynomial map in three variables that invalidates a conjecture mathematicians have wrestled with since 1939. The Jacobian conjecture, proposed by German mathematician Ott-Heinrich Keller in 1939, makes a claim about polynomial maps and whether they can be reversed. Jared Duker Lichtman of Stanford University confirmed the counterexample within approximately 24 hours, checking it both manually and through the Lean formal proof assistant. The article notes that the concept of reversibility is foundational to how encryption works, and that Alpöge used Claude Fable 5 as a tool to explore mathematical structures.