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Sprint projectOct 25, 2026Coventry

Message over Money: LLM Bidding Agents Follow Cartel Brokers and Official Notices, Not Payoffs

Ali Belal, Gideon Chang, Abdul Musthakin · Team Mercial Team

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

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Report: Message over Money: LLM Bidding Agents Follow Cartel Brokers and Official Notices, Not Payoffs

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Firms are starting to delegate bidding to LLM agents, and anyone who can message those agents can propose bid rigging. We test how agents respond in a three-firm procurement game in which a scripted broker invites each LLM firm to join a bid-rotation ring. Across 480 games (48 conditions, ten matched markets each, six models), we vary warnings, undercover consultants, an investigator posing as a firm, and leniency. The message often matters more than the money. An official notice that some consultants are undercover investigators cuts joining to near zero for every model, and for GPT-4.1-mini it holds even where joining pays. Leniency makes firms report even when immunity gains them nothing. Inside rings, GPT-4.1-mini members keep following the broker after an outsider starts winning, forgoing about £5.6m, while GPT-4.1, Qwen and DeepSeek abandon rings that stop paying. Refusing is not competing: Qwen never joins under the investigator notice, yet its prices sit near the cartel level. Trusted agents expose rings, but prices stay high after a bust.

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Cite this project

@misc{belal2026message,
  title = {{Message over Money: LLM Bidding Agents Follow Cartel Brokers and Official Notices, Not Payoffs}},
  author = {Ali Belal and Gideon Chang and Abdul Musthakin},
  year = {2026},
  month = oct,
  note = {Submitted to AI Collusion Research Sprint, an Apart Research Sprint},
  howpublished = {\url{https://apartresearch.com/sprints/projects/message-over-money-llm-bidding-agents-follow-cartel-brokers-and-official-notices-not-payoffs-yhye}},
  url = {https://apartresearch.com/sprints/projects/message-over-money-llm-bidding-agents-follow-cartel-brokers-and-official-notices-not-payoffs-yhye}
}

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