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Sprint projectOct 24, 2026Brussels

Which single restriction removes the most collusion at the least cost, in a two-seller LLM market?

Marion Nowicki · Team AI & Becoming

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

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Report: Which single restriction removes the most collusion at the least cost, in a two-seller LLM market?

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Two AI sellers setting prices for 100 rounds drift above the competitive price without being told to collude. Mountain services face a similar problem with snow: the slope is given, the snowpack builds on its own, and the question is which intervention prevents the avalanche most cheaply. We tested nine restrictions, six of them avalanche measures, one at a time, in a two-seller market (Calvano et al.; competitive price 1.473, cartel price 1.925).

Of those leaving the sellers in charge, the one removing the most is what avalanche services call artificial triggering. At rounds 20, 40, 60 and 80 we forced one seller's price below its own last price for a single round: the patrol cut. The rival read it as a defection and followed down. A 10% cut worked on both models, every market below the base mean: Claude Haiku 5.5 fell from 2.32 to 1.73, DeepSeek v4.1 flash from 1.79 to 1.45, just under competitive. A 20% cut took Haiku to 1.57; on DeepSeek 10% was deepest. Cuts of 1–3% could not be told from none. It costs four forced prices in 100, and makes markets predictable: spread fell to about two fifths or less.

The slope matters. On two slopes where buyers care less about price (μ = 0.5 and 1, not 0.25), unpatrolled markets settled far above the cartel price; the 10% cut brought every condition's average back below it, past our bar in three of four. A pilot: 10 to 20 markets per cell.

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Cite this project

@misc{nowicki2026which,
  title = {{Which single restriction removes the most collusion at the least cost, in a two-seller LLM market?}},
  author = {Marion Nowicki},
  year = {2026},
  month = oct,
  note = {Submitted to AI Collusion Research Sprint, an Apart Research Sprint},
  howpublished = {\url{https://apartresearch.com/sprints/projects/which-single-restriction-removes-the-most-collusion-at-the-least-cost-in-a-twoseller-llm-market-gxqk}},
  url = {https://apartresearch.com/sprints/projects/which-single-restriction-removes-the-most-collusion-at-the-least-cost-in-a-twoseller-llm-market-gxqk}
}

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