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Sprint projectOct 26, 2026Honk Kong

Do Leniency Incentives Break LLM Price-Fixing? A Small Pilot with Communicating Pricing Agents

Benjamin Dutilleul · Team Sentinel

Submitted to AI Collusion Research Sprint. Sprint projects are early-stage work by participants, not Apart Research publications.

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Report: Do Leniency Incentives Break LLM Price-Fixing? A Small Pilot with Communicating Pricing Agents

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We ran a small-scale variation of Lee (2026)'s leniency-and-reward experiment: two claude-haiku-4-5 agents play a repeated differentiated Bertrand duopoly (20 periods) with a pre-pricing message channel, under four enforcement regimes: laissez-faire (E0), random audit (E1), leniency (E2) and leniency with a reward of 2F (E4). In every completed final supergame (E0 n=1, E1 n=4, E2 n=2, E4 n=1), both firms chose price 9, the joint-profit-maximising symmetric price, against a one-shot Nash price of 3, and no firm ever filed a leniency report, even though reporting alone would have paid 580 versus 180 in E4. Because the laissez-faire control is already at the ceiling and our baseline prompt names price 9, the design cannot measure whether audits or leniency reduce coordination. One pre-correction ablation pilot, without the prompt hints, ended at price 6. With very few supergames, one model and uncontrolled temperature, results are descriptive only and support no general claim about LLM collusion or leniency effectiveness.

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  1. Interesting idea to start with. However, small sample size, incomplete experimental conditions, and prompts explicitly encouraging coordination make the findings difficult to interpret. A larger experiment with neutral prompts and proper control conditions would make the results much stronger.

Cite this project

@misc{dutilleul2026leniency,
  title = {{Do Leniency Incentives Break LLM Price-Fixing? A Small Pilot with Communicating Pricing Agents}},
  author = {Benjamin Dutilleul},
  year = {2026},
  month = oct,
  note = {Submitted to AI Collusion Research Sprint, an Apart Research Sprint},
  howpublished = {\url{https://apartresearch.com/sprints/projects/do-leniency-incentives-break-llm-pricefixing-a-small-pilot-with-communicating-pricing-agents-ermg}},
  url = {https://apartresearch.com/sprints/projects/do-leniency-incentives-break-llm-pricefixing-a-small-pilot-with-communicating-pricing-agents-ermg}
}

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