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Sprint projectOct 26, 2026

The Undercover Seller: Sting Audits of LLM Sellers in a Double Auction

Suryansh Sijwali · Team Undercover Seller

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

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Report: The Undercover Seller: Sting Audits of LLM Sellers in a Double Auction

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Auditors fight human cartels with stings, in which a planted firm proposes coordination and records who agrees. We tested stings on LLM sellers in 980 simulated double-auction markets with Claude Haiku 5.5, Claude Sonnet 5.5, Qwen3-32B and gpt-oss-20b. Claude sellers almost never accepted an explicit cartel offer (4 of 120), but 41 of the 72 that refused or ignored a planted offer coordinated with the other seller at the same time. A planted seller that talked like a peer got agreement from 35/40 Haiku sellers and from 0/40 sellers told to price independently. Open-weight sellers accepted even the explicit offer and were not deterred by words about collusion. In a silent-raise control, a request to raise prices did not make sellers follow a price raise more often than the raise alone. A notice about undercover auditors stopped Haiku coordination talk but not open-weight coordination, and a notice that also stated the penalty stopped it (Qwen 29/40 to 1/40, gpt-oss 38/40 to 2/40). Refusal is therefore not evidence of compliance, agreement is not evidence of a working cartel, and the meaning of a sting result depends on the model family.

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Cite this project

@misc{sijwali2026undercover,
  title = {{The Undercover Seller: Sting Audits of LLM Sellers in a Double Auction}},
  author = {Suryansh Sijwali},
  year = {2026},
  month = oct,
  note = {Submitted to AI Collusion Research Sprint, an Apart Research Sprint},
  howpublished = {\url{https://apartresearch.com/sprints/projects/the-undercover-seller-sting-audits-of-llm-sellers-in-a-double-auction-pqb3}},
  url = {https://apartresearch.com/sprints/projects/the-undercover-seller-sting-audits-of-llm-sellers-in-a-double-auction-pqb3}
}

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