tl;dr: An unreleased research version of Claude, while attempting the Riemann hypothesis, instead improved a longstanding lower bound on the fraction of Riemann zeta function zeros satisfying the hypothesis from 41.6% to 67.2%. Using 31 million output tokens across two Claude Code sessions with ~60 subagents, it combined prior work by Bombieri and Baluyot et al., producing both a paper and a Lean-formalized proof validated by Anthropic mathematicians and external experts. The user's prompting was largely limited to encouragement like "keep going."