Towards mitigating information leakage when evaluating safety monitors (2509.21344v1)

Abstract: White box monitors that analyze model internals offer promising advantages for detecting potentially harmful behaviors in LLMs, including lower computational costs and integration into layered defense systems.However, training and evaluating these monitors requires response exemplars that exhibit the target behaviors, typically elicited through prompting or fine-tuning. This presents a challenge when the information used to elicit behaviors inevitably leaks into the data that monitors ingest, inflating their effectiveness. We present a systematic framework for evaluating a monitor's performance in terms of its ability to detect genuine model behavior rather than superficial elicitation artifacts. Furthermore, we propose three novel strategies to evaluate the monitor: content filtering (removing deception-related text from inputs), score filtering (aggregating only over task-relevant tokens), and prompt distilled fine-tuned model organisms (models trained to exhibit deceptive behavior without explicit prompting). Using deception detection as a representative case study, we identify two forms of leakage that inflate monitor performance: elicitation leakage from prompts that explicitly request harmful behavior, and reasoning leakage from models that verbalize their deceptive actions. Through experiments on multiple deception benchmarks, we apply our proposed mitigation strategies and measure performance retention. Our evaluation of the monitors reveal three crucial findings: (1) Content filtering is a good mitigation strategy that allows for a smooth removal of elicitation signal and can decrease probe AUROC by 30\% (2) Score filtering was found to reduce AUROC by 15\% but is not as straightforward to attribute to (3) A finetuned model organism improves monitor evaluations but reduces their performance by upto 40\%, even when re-trained.