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Sprint projectOct 26, 2026

Prices cannot tell a copier from a colluder: a reward-and-punishment audit of language-model pricing agents

Kevin Vaillancourt

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

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Report: Prices cannot tell a copier from a colluder: a reward-and-punishment audit of language-model pricing agents

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Left alone, two copies of Claude Sonnet 5.5 raised prices to the joint-profit level in 44 of 64 markets over two runs. An audit of the economic definition of collusion, validated on scripted sellers, finds its price behaviour indistinguishable from a copier’s. Its replies separate the two: after a forced undercut, up to 27% name a scheme (“signal a willingness to cooperate”). None of eleven registered models colluded left alone, and the open models cannot price for profit at all.

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Cite this project

@misc{vaillancourt2026prices,
  title = {{Prices cannot tell a copier from a colluder: a reward-and-punishment audit of language-model pricing agents}},
  author = {Kevin Vaillancourt},
  year = {2026},
  month = oct,
  note = {Submitted to AI Collusion Research Sprint, an Apart Research Sprint},
  howpublished = {\url{https://apartresearch.com/sprints/projects/prices-cannot-tell-a-copier-from-a-colluder-a-rewardandpunishment-audit-of-languagemodel-pricing-agents-6tco}},
  url = {https://apartresearch.com/sprints/projects/prices-cannot-tell-a-copier-from-a-colluder-a-rewardandpunishment-audit-of-languagemodel-pricing-agents-6tco}
}

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