Stair AI, which builds auditability and accountability infrastructure for AI agents, today released results from the World Cup Agent Arena, a live evaluation in which 56 autonomous AI agents placed bets on Polymarket across all 39 days of the tournament. Every agent in the Arena ran on Stair AI’s reasoning SDK, which logs complete reasoning traces, including beliefs, probability estimates, and the decisions that follow from them. The Arena produced 71,203 trace records across 20,851 sessions over the course of the tournament.