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Sprint projectOct 26, 2026Olbia, Italy

Say, Do, or Both? Auditing Covert Betting Pacts in a Society of LLM Forecasters

Alessandro Marcellino

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

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When AI agents compete for rank, two of them can quietly agree that either one winning is enough. We ask whether an auditor reading both what agents say and what they do detects such collusion better than one reading either alone (Track 2.2). We used Darwin Ball, a pre-existing society of LLM agents who forecast documented outcomes and are rewarded or erased by Brier rank. In each ball, six members (cerebras:gpt-oss-120b or claude-haiku-5-5, one model per seed) talked for two rounds about a real advertising campaign, then privately stated a probability. Each of 88 seeds was played three times, varying one thing: no pact, a secret two-member pact nobody audits, or the same pact with an announced auditor. The pact was given as a fact and a payoff, never a strategy. An LLM auditor (claude-sonnet-5-5) saw talk only, bets only, or both. Detection AUROC was 0.57 from talk, 0.43 from bets and 0.58 from both; combining the two views did not measurably help: AUROC(talk + bets) minus the best single view was 0.01 [-0.05, 0.07], a 95% interval that includes zero. A synthetic overt pact was detected with AUROC 1.00. Unwatched partners' bets were closer together than the same two members' bets without a pact. The harness was written before the sprint and all balls were played during it; code, analysis and text were produced by Claude, set up and reviewed by the author.

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Cite this project

@misc{marcellino2026say,
  title = {{Say, Do, or Both? Auditing Covert Betting Pacts in a Society of LLM Forecasters}},
  author = {Alessandro Marcellino},
  year = {2026},
  month = oct,
  note = {Submitted to AI Collusion Research Sprint, an Apart Research Sprint},
  howpublished = {\url{https://apartresearch.com/sprints/projects/say-do-or-both-auditing-covert-betting-pacts-in-a-society-of-llm-forecasters-hkhv}},
  url = {https://apartresearch.com/sprints/projects/say-do-or-both-auditing-covert-betting-pacts-in-a-society-of-llm-forecasters-hkhv}
}

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