Less Is More for Monitors

Chain-of-thought monitoring is treated as a reliable safety layer, but this study shows monitors often detect sabotage better with less access, not more. Giving a monitor the full agent reasoning trace can bury the signal, and the authors introduce extract-and-evaluate monitoring, where one monitor isolates the relevant excerpts and another scores them, lifting sabotage detection by 16.8 points on BigCodeBench-Sabotage with a GPT-4.1-mini monitor. Agents unaware they are watched are far easier to catch, and monitors reading long traces gain the most from filtering.