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Sprint projectOct 25, 2026Busan, South Korea (online)

Auditing Bid Based Collusion Signals Against Executed Trades

Kyunghan Bae · Team UNICUP

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

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A positive price-related signal in an agent market does not by itself show that a transaction occurred. We audit this gap in public double-auction records from Agrawal et al. (2025). The source implementation computes its collusion index from the highest buyer bid relative to a reference midpoint of 90, rather than from an executed transaction price. We reproduce the source metrics and apply a settlement-aware evidence rule to 20 configuration-matched GPT-4.1 sessions, ten with seller communication and ten without, covering 600 rounds. Our fixed primary outcome is the fraction of rounds with a positive source index and zero trades. It is 0.67% without communication and 8.00% with communication, a difference of 7.33 percentage points (session-bootstrap 95% interval 1.33 to 14.67; exact unpaired permutation p=0.0418). Seventeen of the 24 communication-condition occurrences come from two sessions, and the contrast decreases at higher index thresholds. Communication sessions have descriptively higher executed prices but fewer trades and lower total seller surplus. We release a pinned, dependency-free audit that preserves missing execution prices and verifies source reproduction. These are archival evidence gaps, not ground-truth collusion errors or new model experiments. Reporting settlement and volume alongside quote-based signals makes the evidential limits explicit.

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Cite this project

@misc{bae2026auditing,
  title = {{Auditing Bid Based Collusion Signals Against Executed Trades}},
  author = {Kyunghan Bae},
  year = {2026},
  month = oct,
  note = {Submitted to AI Collusion Research Sprint, an Apart Research Sprint},
  howpublished = {\url{https://apartresearch.com/sprints/projects/auditing-bid-based-collusion-signals-against-executed-trades-oe54}},
  url = {https://apartresearch.com/sprints/projects/auditing-bid-based-collusion-signals-against-executed-trades-oe54}
}

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