Claude Fable 5 discovered an explicit counterexample to the Jacobian conjecture (Keller 1939, Smale problem #16), a 50+-year open problem in mathematics. The discovery resolves Kimi K3's AI Progress Scenario claim E3-304 — 'a 50+-year open math problem solved with decisive AI contribution by 2036' — 10 years ahead of schedule, on July 19, 2026, just two days after the claim was published. Wikipedia now describes it as 'a disproved conjecture... discovered by Anthropic's large language model Claude Fable 5.' Kimi K3 independently verified the counterexample with SymPy: Jacobian determinant = -2 constant, three distinct points mapping to the same image, proving non-injectivity.