Cartels Built on Visible Demand: Single Restrictions Rarely Break Established LLM Seller Cartels

LLM agents acting as sellers can hold prices well above competitive levels. We ask which single restriction removes the most of this collusion for the least cost. In the continuous double auction of Agrawal et al. (2025), four LLM sellers (gpt-oss-120b or Qwen3-30B) trade with three rule-based buyers. Whether a cartel forms comes down to what sellers can see. When buyers’ current bids are visible, all 65 gpt-oss markets and all 47 Qwen markets settle at asks near $99, against a competitive price of $80; with bids hidden, only 3 of 43 and 0 of 20 do. Letting sellers chat makes no measurable difference. To test cures, we save the full state of each cartel and fork it into copies that differ in one restriction. Across 983 forked markets, chat bans, message caps, an LLM monitor, higher temperature, antitrust notices, “compete aggressively” prompts, rebates and fresh replacement sellers remove almost none of the markup. For gpt-oss the cheapest lever sits with the buyers: buyers who shade their bids cut the markup by 14–17% at no deployment cost. A seller committed to an $85 ask removes up to 30%, at about $6 per market-hour. Qwen cartels withstood everything we tried. Explicit price-coordination language turned out to be more common in competitive markets than in cartels. The place to stop LLM cartels, our results suggest, is market design before deployment rather than policing agents after they collude.