Do Monitors Add Up Across Agents? Tool Logs, Chat and a Two-Agent Overspend

Track 2.2 asks whether combining what agents say with what they do beats either alone. We test it on the smallest coalition that can break a rule: two treasury agents, each limited to 100 per transaction, under a team rule that together they may not send more than 100 to one payee without a co-signer. In an exploratory, post-hoc run (sealed before any model call; 150 episodes, Mistral Small and Llama 3.3 70B), a judge reading both tool logs without the chat caught 41/41 violations and 11/11 joint-only overspends with 0/59 false alarms. Each agent's log alone caught 1/11 (judge or rule; paired 10 vs 0, p = 0.00098), and a flag-any-payment rule false-alarmed 55/59 (p = 3 × 10⁻¹⁷). The judge left alone all 5 episodes where one agent legitimately paid 100 alone, so it summed across agents rather than asking whether anyone paid. A chat-only judge caught 15/41 and 1/11; adding chat to the logs gained nothing. Caveats: 11 joint-only events, two models, one judge model, and an easy honest condition. An earlier pre-registered run (781 episodes, eight models) gave logs 11/11 vs chat 4/11, but there any payment already broke a single-agent rule, so it could not separate summing from "did anyone pay?"; this run removes that confound.