On July 20, 2026, Levant Alpöge, a mathematician at U.S. AI company Anthropic, used the company's large language model "Claude Fable 5" to discover a counterexample to the Jacobian conjecture, a mathematical puzzle that had remained unsolved for 87 years since 1939. The counterexample, posted on X (formerly Twitter) rather than in a peer-reviewed paper, is a polynomial map in three variables whose Jacobian determinant is identically equal to "-2" at every point in space, satisfying the conjecture's premise of being "locally invertible" yet not globally invertible. The equations are concise enough to fit in three lines, and within hours, mathematicians worldwide conducted independent verifications, and the counterexample's accuracy was immediately upheld. The Jacobian conjecture was proposed by German mathematician Ott-Heinrich Keller in 1939, and the two-dimensional case remains unsolved. Akhil Mathew, a mathematician at the University of Chicago, suggested this problem to Alpöge.