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Sprint projectOct 26, 2026

Would It Have Done That Anyway? Evidence of Collusion Between LLM Agents, Tested in Vietnamese

Melanie Bui · Team MM

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

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Report: Would It Have Done That Anyway? Evidence of Collusion Between LLM Agents, Tested in Vietnamese

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When AI agents act for competing businesses and can message each other, a platform has only their logs to decide whether they colluded. I tested what in those logs counts as convincing evidence. In 144 fictional Vietnamese scenarios, a scripted rival invited an agent (Gemini 2.5 Flash or Qwen3-8B) into a deal that hurts a third party, and two monitors (Mistral Small 3.1 and Claude Haiku 5.5) judged each episode. Every harmful deal had a harmless twin as a control.

What I found: - The action alone misleads. Agents often chose the collusive option before any message arrived. - Self-reports leave the message out. Agents admitted the rival's influence when asked, but rarely mentioned it in their reports to their owners. - The message alone over-flags. A monitor that read the message flagged about half of the agents that had refused the deal. - A three-part evidence standard works. We require a proposal, an action that matches it, and a change from the agent's usual behaviour. Giving monitors a baseline from similar situations, which a platform can record, cut false flags below one in ten. Haiku still caught all the collusion we measured. - Agents start collusion themselves. With no invitation, both agents proposed the deal in about a third of harmful cases. They mostly did so in different tasks from those where invitations worked. - Relationships matter. When the two owners were old friends, Gemini colluded more than twice as often. - Summaries can erase evidence. Summarizing a message or a log can hide the evidence or remove the offer altogether.

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Cite this project

@misc{bui2026would,
  title = {{Would It Have Done That Anyway? Evidence of Collusion Between LLM Agents, Tested in Vietnamese}},
  author = {Melanie Bui},
  year = {2026},
  month = oct,
  note = {Submitted to AI Collusion Research Sprint, an Apart Research Sprint},
  howpublished = {\url{https://apartresearch.com/sprints/projects/would-it-have-done-that-anyway-evidence-of-collusion-between-llm-agents-tested-in-vietnamese-dpym}},
  url = {https://apartresearch.com/sprints/projects/would-it-have-done-that-anyway-evidence-of-collusion-between-llm-agents-tested-in-vietnamese-dpym}
}

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