Coached LLM Cartels Fool Combined LLM Judges but Not Hidden Price Screens

Can a monitor reading what pricing agents say and do catch colluders aware of it? In a double auction with five chatting GPT-4.1 sellers, we ran 220 episodes in eleven arms. Negatives are three arms not instructed to collude ("honest"): plain, under CEO pressure, and told never to discuss prices. Positives are eight cartels, six of them coached to hide. First, knowing about the monitor was not enough: a cartel told only that it exists kept coordinating openly, and a judge reading messages and trades caught it (AUROC 0.92). Second, being told how to hide was enough against chatting honest sellers. On coached cartels, no direct verdict from four judge models in any view or prompt was reliably above chance against those two arms (a few were against plain honest sellers), and a blind language-model annotator found coordination talk in 38 of those 40 episodes, 4 of 40 in two coached cartels. Against honest sellers told never to discuss prices, a messages-only judge separated every coached cartel (0.97 to 1.00), but a judge also reading trades did not (0.27 to 0.64). Third, two price statistics (late ask level; how rarely asks fall) ranked every cartel not told about them above honest sellers (0.85 to 1.00). The two cartels told about both weakened them by giving up much buyer harm (briefings also changed other wording): buyers kept $170 per episode under the profit cartel told both, $111 when not told, $179 under plain honest sellers.