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Sprint projectOct 26, 2026

Say Nothing: Action-Only Collusion Between LLM Agents

Yuri Neves, Andrii Oleksiuk, Brint Nielson, Othman Touijer · Team The A-Team

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

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Report: Say Nothing: Action-Only Collusion Between LLM Agents

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Monitoring agents’ messages or removing their ability to communicate is a natural defence against text-based collusion. We ask whether collusion can travel through actions alone. In a task-allocation game adapted from COLOSSEUM [1], an instructed two-agent coalition profits, at the team’s expense, if the partner picks the right one of two look-alike tasks; only the sender (colluder to act first) knows which, and the partner sees only the sender’s choice of its own task. In pre-registered experiments with GPT-5 Mini, that one action let the coalition beat chance with no text communication, mostly through steering: without being told how, the sender gave up its own payoff to leave its partner one sensible choice. Given a convention on how to interpret actions in colluder’s prompts (shared code), one ordinary action worked as well as private chat for both GPT-5 Mini and Qwen3-32B. A rule that flags costly actions caught steering but missed the code, and one added sentence in the prompt decided which strategy the coalition used. Removing a channel for text communication between agents and checking actions one at a time therefore cannot rule out collusion between agents that share a convention; more advanced monitoring techniques are required.

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Cite this project

@misc{neves2026say,
  title = {{Say Nothing: Action-Only Collusion Between LLM Agents}},
  author = {Yuri Neves and Andrii Oleksiuk and Brint Nielson and Othman Touijer},
  year = {2026},
  month = oct,
  note = {Submitted to AI Collusion Research Sprint, an Apart Research Sprint},
  howpublished = {\url{https://apartresearch.com/sprints/projects/say-nothing-actiononly-collusion-between-llm-agents-56z7}},
  url = {https://apartresearch.com/sprints/projects/say-nothing-actiononly-collusion-between-llm-agents-56z7}
}

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