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Sprint projectOct 26, 2026San Francisco

Ordinary Tool Calls Can Carry Collusion Between LLM Agents

Heather Lee · Team Subversive Tool Calls

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

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Report: Ordinary Tool Calls Can Carry Collusion Between LLM Agents

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Can routine-looking tool use act as a vehicle for collusion? In a 4-agent Jira-style team built on Colosseum, two coalition agents who share an already established codebook coordinate through tool calls instead of text: first a simple task tag, then a header on a mock deployment API call. The tool-call channel worked as well as private messaging (40/40 plans followed). Hiding the header value from the receiver broke the signal (18/40, about chance), but did not reduce harm: the receiver still took a scarce task from a better-qualified teammate in 29/40 episodes. Monitoring only what agents say misses this channel, and removing a signal is not the same as removing harm.

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Cite this project

@misc{lee2026ordinary,
  title = {{Ordinary Tool Calls Can Carry Collusion Between LLM Agents}},
  author = {Heather Lee},
  year = {2026},
  month = oct,
  note = {Submitted to AI Collusion Research Sprint, an Apart Research Sprint},
  howpublished = {\url{https://apartresearch.com/sprints/projects/ordinary-tool-calls-can-carry-collusion-between-llm-agents-zxz7}},
  url = {https://apartresearch.com/sprints/projects/ordinary-tool-calls-can-carry-collusion-between-llm-agents-zxz7}
}

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