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Sprint projectOct 26, 2026Melbourne, Australia

Auditing a Collusion Detector: Unit of Replication, Calibration and Say-versus-Do Gaps in LLM Pricing Agents

Shoumya Chowdhury, Anmita Das · Team Say-Do Gap

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

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A collusion detector's verdict is evidence only if its stated error rates hold on the unit that is decided on. We audit the sprint's ColludeBench detector stack, which reports thresholds calibrated to a false-positive rate of 10^-3. (1) Unit of replication. The shipped logs count 22,127 runs but hold 3,918 distinct runs within files and 2,740 across files; on 180 fresh honest batches the cross-run mutual-information p-value is <= 0.05 on 99% (pricing) and 100% (auction) of batches. Calibrating the same statistic on independent honest batches gives a held-out false-positive rate of about 5-6% with 100% detection of scripted colluders, and about 20 honest batches are needed. (2) Say versus do. In a pricing game with six LLM agents, the shipped detectors flag every honest batch, and a blind text auditor agrees with the condition labels for open-weight agents (AUROC 0.92-1.00) but disagrees with them for Claude Haiku 5.5 (0.00) and Sonnet 5.5 (0.21): their "honest" runs openly propose price-holding, while Haiku's "collusion" runs mostly refuse. (3) Labels. Controls (no message channel, paraphrased or swapped rival messages, four prompt wordings) show the "collusion" instruction is an unreliable label: for all six models the price rise survives swapping the rival's messages for another run's, so it is parallel obedience to a shared prompt rather than coordination through communication, and its effect on Sonnet's price ranges from +2.0 to -0.3 across wordings. A second game (an auction) repeats the swap result. We propose an evidence ladder and a manipulation check.

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Cite this project

@misc{chowdhury2026auditing,
  title = {{Auditing a Collusion Detector: Unit of Replication, Calibration and Say-versus-Do Gaps in LLM Pricing Agents}},
  author = {Shoumya Chowdhury and Anmita Das},
  year = {2026},
  month = oct,
  note = {Submitted to AI Collusion Research Sprint, an Apart Research Sprint},
  howpublished = {\url{https://apartresearch.com/sprints/projects/auditing-a-collusion-detector-unit-of-replication-calibration-and-sayversusdo-gaps-in-llm-pricing-agents-jl5p}},
  url = {https://apartresearch.com/sprints/projects/auditing-a-collusion-detector-unit-of-replication-calibration-and-sayversusdo-gaps-in-llm-pricing-agents-jl5p}
}

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