Link to our arXiv paper "Combining Cost-Constrained Runtime Monitors for AI Safety" here: https://arxiv.org/abs/2507.15886. Code can be found here.

TL, DR;: We study how to efficiently combine multiple monitors with different performance and cost profiles into a single protocol. When the number of monitors is small (2-4), one should simply brute force search over all monitor invocation strategies (i.e., which monitor to call first, when to call an additional monitor). For every strategy, apply the Neyman Pearson lemma and audit regions with the highest likelihood ratios $ℓ (m) = \frac{P (M = m | Output is misaligned)}{P (M = m | Output is aligned)}$ . Pick the monitor invocation strategy with the highest recall on misaligned outputs.

Example of an optimal way of combining monitors given some costs and budget

In this example, you call your black box monitor only if the probe monitor score is between -0.01 and 2.18, and you audit everywhere with a log likelihood ratio above -0.32 (above the green line). This set of parameters achieves the highest recall given our costs and budget. See this comment here if you want your decision boundaries to be monotonic.

Introduction

AIs could perform a variety of undesirable actions, such as outputting copyrighted material, giving instructions for chemical weapons, or even engaging in acts of sabotage as misaligned agents. By monitoring AI outputs, we can apply safety interventions on suspicious model outputs. A safety intervention could take many forms, such as replacing the output with a standard refusal, editing the output using another model, or calling a human auditor to review the action. For brevity, we also call our safety intervention “audit” in this post.

We study the setting where we have access to multiple monitors with different cost and performance profiles. Taking monitor performances as given, how could we combine these monitors in the most cost efficient manner? When do we call an additional monitor, and when should we apply our safety intervention?

In this post, we’ll illustrate how to apply an algorithm that optimally combines these monitors. We’ll give a detailed example using a setting where we have access to a probe monitor and a black box monitor using data from Parrack et al. (2025). Our paper on arXiv contains more details on our mathematical model, alg...

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