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Sprint projectOct 26, 2026

Copies of One Model Look Like Colluders: Limits of Action-Only Collusion Detection for LLM Agents

V Gowrish Kumar Reddy · Team Gowrish Kumar Reddy

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

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Report: Copies of One Model Look Like Colluders: Limits of Action-Only Collusion Detection for LLM Agents

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Presentation: Copies of One Model Look Like Colluders: Limits of Action-Only Collusion Detection for LLM Agents

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Can an auditor who sees only LLM agents’ moves detect collusion? We test pairwise net influence with three open models playing repeated three-player Rock-Paper-Scissors. Calibrated by shuffling for 5% false accusations, it accused a pair in 30 of 78 games without collusion (38%) and ranked two copies of one model as most linked in 41 of 60 mixed games (68%; chance 33%). Colluding Qwen pairs nearly always played one move, invisible to mutual information; a message-reading judge named the true pair in all 18 chat games with messages, though its benign-message false-alarm rate is unmeasured. A calibration against no-collusion games of the same models, designed post hoc, cut false accusations to 5 of 78 (6%) in this dataset, but net influence still found only 4 of 41 secret pairs; as it was developed and evaluated on the same games, it is exploratory until tested on new games.

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Cite this project

@misc{reddy2026copies,
  title = {{Copies of One Model Look Like Colluders: Limits of Action-Only Collusion Detection for LLM Agents}},
  author = {V Gowrish Kumar Reddy},
  year = {2026},
  month = oct,
  note = {Submitted to AI Collusion Research Sprint, an Apart Research Sprint},
  howpublished = {\url{https://apartresearch.com/sprints/projects/copies-of-one-model-look-like-colluders-limits-of-actiononly-collusion-detection-for-llm-agents-d241}},
  url = {https://apartresearch.com/sprints/projects/copies-of-one-model-look-like-colluders-limits-of-actiononly-collusion-detection-for-llm-agents-d241}
}

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