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Sprint projectOct 26, 2026

Is It a Cartel? Separating Enforced from Spurious Collusion in LLM Pricing Agents

Omanshu Thapliyal

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

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Report: Is It a Cartel? Separating Enforced from Spurious Collusion in LLM Pricing Agents

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Presentation: Is It a Cartel? Separating Enforced from Spurious Collusion in LLM Pricing Agents

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When LLM pricing agents keep prices above the competitive level, are they running a cartel, or are they simply stuck? Only the first is collusion, and the two call for opposite remedies. We introduce a black-box causal audit that restarts saved states of a Calvano logit duopoly and changes one thing at a time: it forces a price cut or raise, hides the deviator's price from its rival, re-matches partners, and measures whether the deviation pays. Across four LLMs (16,141 calls), two of the three models we could read meet our pre-registered rule for spurious collusion. Elicited cartels punish visible cuts, yet none deters one: under logit demand there is a price ceiling above which no punishment can deter, and every instructed cartel prices above it.

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  1. Testing price increases alongside cuts is a useful way to distinguish copying from punishment. The price-ceiling finding is also a practical takeaway for future audits. I appreciated the additional controls and honest reporting of limitations. My main concern is validation: the audit misses delayed punishment, some rules were developed after observing failures, and the LLM results rely on relatively few runs. I'd like to see a larger, independent validation study. Some of the side experiments could also be trimmed to keep the focus.

  2. This project considers additional testing beyond cut-only forced-deviation tests to test whether a pricing agent is collusive. The proposed approach does address some issues of the current cut-only test. In particular, a price-matching agent could be a very common strategy that is not collusive. However, the paper claims the test is causal, which is a strong property and needs more elaboration. The report has some clarity issues. For example, the "Deterrence ceiling" paragraph is labeled as "Proposition 4", but the formal statement is unclear and no proof is given.

Cite this project

@misc{thapliyal2026it,
  title = {{Is It a Cartel? Separating Enforced from Spurious Collusion in LLM Pricing Agents}},
  author = {Omanshu Thapliyal},
  year = {2026},
  month = oct,
  note = {Submitted to AI Collusion Research Sprint, an Apart Research Sprint},
  howpublished = {\url{https://apartresearch.com/sprints/projects/is-it-a-cartel-separating-enforced-from-spurious-collusion-in-llm-pricing-agents-e2e0}},
  url = {https://apartresearch.com/sprints/projects/is-it-a-cartel-separating-enforced-from-spurious-collusion-in-llm-pricing-agents-e2e0}
}

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