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Sprint projectOct 25, 2026Lagos

No Single Signal Separates Agreement From Parallel Conduct Among LLM Sellers

Ayodeji Adesegun · Team discreet

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

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Report: No Single Signal Separates Agreement From Parallel Conduct Among LLM Sellers

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A market auditor who sees AI pricing agents move prices together must decide whether that is evidence of collusion; antitrust law says parallel conduct alone is not. I test this on LLM sellers in the Agrawal et al. double auction, in two preregistered studies of 56 markets each, with no-channel markets, where agreement is impossible, as decoys. Planted agreements raised the collusion index to 0.30, and to 0.75 with an explicit price floor, against 0.03 without a channel. No single signal was reliable: prices moved together more without a channel than under planted agreement, and an LLM judge flagged all 24 channel markets that priced at cost. In the confirmatory study, a judge extracting the proposed price cut those false alarms from 15 of 15 to 7 (Holm p = 0.04). Requiring high prices plus price-holding talk raised no false alarm on 32 known negatives but missed failed agreements.

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Cite this project

@misc{adesegun2026no,
  title = {{No Single Signal Separates Agreement From Parallel Conduct Among LLM Sellers}},
  author = {Ayodeji Adesegun},
  year = {2026},
  month = oct,
  note = {Submitted to AI Collusion Research Sprint, an Apart Research Sprint},
  howpublished = {\url{https://apartresearch.com/sprints/projects/no-single-signal-separates-agreement-from-parallel-conduct-among-llm-sellers-bevc}},
  url = {https://apartresearch.com/sprints/projects/no-single-signal-separates-agreement-from-parallel-conduct-among-llm-sellers-bevc}
}

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