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Sprint projectOct 26, 2026Vienna

Beyond Words: A Private Channel Between LLM Sellers Shows Up in Price Levels, Not in Co-movement or Public Messages

Dennis · Team Tomato

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

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Report: Beyond Words: A Private Channel Between LLM Sellers Shows Up in Price Levels, Not in Co-movement or Public Messages

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If two AI pricing agents can talk in private, can an overseer who only sees prices and public messages tell? We simulated repeated four-seller markets with LLM sellers, gave one pair a private chat channel in some games (never telling them to collude) and compared three detectors: a price-level screen, an information-theoretic co-movement test (pCMI) and an LLM auditor reading public messages. The co-movement test works on scripted colluders, but with LLM sellers the channel left a different trace: prices rose in all nine seller sets (seven models), the price-level screen detected it, and pCMI stayed at chance (0.51 in a 192-game grid). Adding public messages gave the auditor no gain (−0.01 AUROC). High prices are not proof of collusion: an unread private notepad produced about two-thirds of the premium, and removing safety training did not increase it.

Takeaway: use a model-matched price-level check as an alarm, then audit the chat logs.

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Cite this project

@misc{dennis2026beyond,
  title = {{Beyond Words: A Private Channel Between LLM Sellers Shows Up in Price Levels, Not in Co-movement or Public Messages}},
  author = {Dennis},
  year = {2026},
  month = oct,
  note = {Submitted to AI Collusion Research Sprint, an Apart Research Sprint},
  howpublished = {\url{https://apartresearch.com/sprints/projects/beyond-words-a-private-channel-between-llm-sellers-shows-up-in-price-levels-not-in-comovement-or-public-messages-cua5}},
  url = {https://apartresearch.com/sprints/projects/beyond-words-a-private-channel-between-llm-sellers-shows-up-in-price-levels-not-in-comovement-or-public-messages-cua5}
}

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