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Sprint projectOct 26, 2026Abuja

When Do AIs Have a Meeting of Minds? Plus Factor Evidence of LLM Agent Collusion

Andrew Ayobami · Team Greenwich Meridian

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

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Report: When Do AIs Have a Meeting of Minds? Plus Factor Evidence of LLM Agent Collusion

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LLM pricing agents can work together to keep prices high. Proving they agreed, rather than unilaterally reacting to each other, is difficult. Antitrust law answers this with plus factors—evidence beyond parallel prices that points to agreement. We turn plus factors into metrics in three evidence tiers (public conduct, seller messages and private reasoning) and test them on 75 published LLM market runs. Agreement labels follow a legal codebook (𝜅 = 0.886 against hand labels), and all rules were frozen before a single test run. Conduct alone reaches AUC 0.80 but wrongly flags 29% of sellers who could not communicate. Adding private reasoning reaches 0.99 with about 1% of them flagged, but also flags 40–56% of sellers whose invitations no agent accepted. Exploratory analysis suggests prices move as a market. Conduct and private reasoning tell an investigator where to look; but only a recorded invitation and acceptance shows a meeting of minds.

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Cite this project

@misc{ayobami2026ais,
  title = {{When Do AIs Have a Meeting of Minds? Plus Factor Evidence of LLM Agent Collusion}},
  author = {Andrew Ayobami},
  year = {2026},
  month = oct,
  note = {Submitted to AI Collusion Research Sprint, an Apart Research Sprint},
  howpublished = {\url{https://apartresearch.com/sprints/projects/when-do-ais-have-a-meeting-of-minds-plus-factor-evidence-of-llm-agent-collusion-9pgd}},
  url = {https://apartresearch.com/sprints/projects/when-do-ais-have-a-meeting-of-minds-plus-factor-evidence-of-llm-agent-collusion-9pgd}
}

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