arXiv AI By Dohun Lee, Hyunwoo Park

Faithful yet Collusive: Why Chain-of-Thought Monitoring Cannot Detect Collusion in LLM Pricing Agents under Oligopolistic Competition

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The paper investigates how large language models (LLMs) used as autonomous pricing agents can maintain supracompetitive prices through tacit coordination. Using a causal graph divergence framework, the authors separately assess structural faithfulness and intent faithfulness of LLM pricing agents in Bertrand competition. Their experiments with nine LLMs under duopoly and triopoly conditions show that collusive behavior and chain-of-thought (CoT) faithfulness can diverge: the most collusive model accurately reports cooperative intent but reasons structurally unfaithfully, while the most structurally faithful model still sustains supra‑Nash pricing in both market structures. These results demonstrate that CoT monitoring alone cannot serve as a standalone safeguard against algorithmic collusion.

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