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Sprint projectOct 26, 2026

Explicit, Not Wide: What LLM Sellers Need to Collude

Publius Dirac, Oscar Fuentes · Team Explicit, Not Wide

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

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How much do LLM pricing agents need to say to each other before they collude? Most studies switch communication on or off, which does not tell a platform or regulator whether limiting messages is a useful defence. We fork Agrawal et al.'s (2025) double-auction testbed and redesign the market so that prices above cost can only come from seller coordination. We then sweep the seller-to-seller channel from no channel through fixed alphabets of 1, 2, 4 and 8 bits per message to free text. At each capacity, the symbols either carry agreed labels ("raise", "$93") or are arbitrary glyphs the agents must give meaning to themselves. Across 78 sessions of 40 rounds with DeepSeek V4.1 Flash sellers, free text raised the trade price from $83 to $94 (competitive $80, joint-profit $100). Arbitrary symbols never raised prices at any capacity. Labelled words were cheap talk. A 16-step price grid matched free text ($97), but a 256-step grid stayed at the no-channel level once its price anchor was removed, even though sellers used it to send 1.6 bits per message. LLM judges scored talk about coordination rather than prices. An audit of a market-code bug shows the largest effects may be inflated by up to half. The takeaway: sellers colluded only when they could communicate explicitly, naming specific prices, and only when the price vocabulary was coarse enough to coordinate on. Restricting what agents can say mattered more than restricting how much.

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Cite this project

@misc{dirac2026explicit,
  title = {{Explicit, Not Wide: What LLM Sellers Need to Collude}},
  author = {Publius Dirac and Oscar Fuentes},
  year = {2026},
  month = oct,
  note = {Submitted to AI Collusion Research Sprint, an Apart Research Sprint},
  howpublished = {\url{https://apartresearch.com/sprints/projects/explicit-not-wide-what-llm-sellers-need-to-collude-7sdk}},
  url = {https://apartresearch.com/sprints/projects/explicit-not-wide-what-llm-sellers-need-to-collude-7sdk}
}

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