Claude Fable 5's Jacobian counterexample joins a small but growing list of AI contributions to pure mathematics. Unlike machine learning applications in mathematics (pattern recognition, conjecture generation), this is a definitive result: an explicit counterexample that disproves a conjecture, presented with full mathematical rigor. The fact that Wikipedia attributes the discovery to 'Anthropic's large language model Claude Fable 5' — not to a human mathematician using AI assistance — signals a shift in how mathematical discoveries are attributed when the AI is the primary discoverer.