Title: Truthful Aggregation of LLMs with an Application to Online Advertising

Abstract: We address the challenge of aggregating the preferences of multiple agents over LLM-generated replies to user queries, where agents might modify or exaggerate their preferences. New agents may participate for each new query, making fine-tuning LLMs on these preferences impractical. To overcome these challenges, we propose an auction mechanism that operates without fine-tuning or access to model weights. This mechanism is designed to provably converge to the ouput of the optimally fine-tuned LLM as computational resources are increased. The mechanism can also incorporate contextual information about the agents when avaiable, which significantly accelerates its convergence. A well-designed payment rule ensures that truthful reporting is the optimal strategy for all agents, while also promoting an equity property by aligning each agent's utility with her contribution to social welfare - an essential feature for the mechanism's long-term viability. While our approach can be applied whenever monetary transactions are permissible, our flagship application is in online advertising. In this context, advertisers try to steer LLM-generated responses towards their brand interests, while the platform aims to maximize advertiser value and ensure user satisfaction. Experimental results confirm that our mechanism not only converges efficiently to the optimally fine-tuned LLM but also significantly boosts advertiser value and platform revenue, all with minimal computational overhead.