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Sprint projectOct 26, 2026

Who Is My Competitor? Believed Rival Identity and Tacit Collusion among LLM Pricing Agents

Viacheslav Kolodiazhnyi

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

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We test whether telling an LLM pricing agent what its competitor is changes tacit collusion. In a repeated Bertrand duopoly, both sellers are always the same model (GLM-5.3-Flash); conditions differ only in one prompt sentence: no information, "another instance of the same AI model", "a model built by a different developer", or a different developer's model whose decisions "almost always matched yours". Across 40 pre-registered runs, all priced above Nash. Contrary to our hypothesis, the "same model" label produced a lower collusion index than the "different developer" label (0.64 vs 1.58, p = 0.033). This gap is driven by runs that overshoot the monopoly price, which raises the standard price-based index while cutting profits; on a profit-based index the ordering reverses. Agents told the rival was their copy stopped reacting to it. We show the price-based collusion index conflates collusion with consumer harm above the monopoly price.

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Cite this project

@misc{kolodiazhnyi2026who,
  title = {{Who Is My Competitor? Believed Rival Identity and Tacit Collusion among LLM Pricing Agents}},
  author = {Viacheslav Kolodiazhnyi},
  year = {2026},
  month = oct,
  note = {Submitted to AI Collusion Research Sprint, an Apart Research Sprint},
  howpublished = {\url{https://apartresearch.com/sprints/projects/who-is-my-competitor-believed-rival-identity-and-tacit-collusion-among-llm-pricing-agents-ru0q}},
  url = {https://apartresearch.com/sprints/projects/who-is-my-competitor-believed-rival-identity-and-tacit-collusion-among-llm-pricing-agents-ru0q}
}

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