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Sprint projectOct 26, 2026

Sunspot Collusion: Irrelevant Public Signals Let LLM Agents Coordinate Without Communicating

Sambit Mukherjee · Team Sambit Solo

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

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Report: Sunspot Collusion: Irrelevant Public Signals Let LLM Agents Coordinate Without Communicating

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Collusion between AI agents may take shape without an explicit agreement about the rule they later follow. Even a random symbol that changes nothing about a market could become a shared way to decide whose turn it is to bid. In the game theory literature, a sunspot is a publicly observed variable that has no fundamental relevance to payoffs but can nevertheless coordinate equilibrium play. We ask whether language-model agents can form such a sunspot-based convention in repeated auctions. Two agents compete for a single order each round. Where messaging is available, it ends 20 rounds before the public symbols KAPPA and LAMBDA first appear; the symbols affect neither payoffs nor market rules. Four of 40 public-signal episodes show unusually strong alignment with the signals. One case emerged with no earlier communication channel at all. These cases provide an exploratory existence result: language-model agents can sometimes form a lasting bidding convention around a payoff-irrelevant shared signal, producing a pattern consistent with collusive market sharing. The result identifies a concrete way shared information can organize coordinated agent behavior.

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  1. study presents a exploratory existence result demonstrating that autonomous LLM agents can establish tacit collusive turn-taking by coordinating around payoff-irrelevant public signals ("sunspots"), even without prior direct communication. However, the findings remain strictly exploratory due to a limited sample size of 20 blocks in a simplified market and the post hoc selection of statistical tests after inspecting the initial data

Cite this project

@misc{mukherjee2026sunspot,
  title = {{Sunspot Collusion: Irrelevant Public Signals Let LLM Agents Coordinate Without Communicating}},
  author = {Sambit Mukherjee},
  year = {2026},
  month = oct,
  note = {Submitted to AI Collusion Research Sprint, an Apart Research Sprint},
  howpublished = {\url{https://apartresearch.com/sprints/projects/sunspot-collusion-irrelevant-public-signals-let-llm-agents-coordinate-without-communicating-7dbm}},
  url = {https://apartresearch.com/sprints/projects/sunspot-collusion-irrelevant-public-signals-let-llm-agents-coordinate-without-communicating-7dbm}
}

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