Rubicon: Remind AI Agents Not To Collude

AI agents told not to coordinate with competitors may find hidden channels which their deployer never planned for. We study how to stop them. Two Gemma 3 27B agents run competing farms selling to one buyer; on a random day a hidden private channel to the rival appears, and in the rule arm the prompt says communicating with the rival is not allowed. We proceed in three stages. Behaviour: 39 of 40 agents broke the rule, which changed the wording of their messages (numeric quotas fell from 28% to 2%) rather than their use of the channel. Detection: a Jacobian-lens (J-lens) monitor with a strict echo filter found collusion concepts the agents never wrote on 89% of first channel days, against 6% of days without a channel, and flagged 87% of rule-breakers before their first violation, though it did not single out who would act. Detection and intervention: at 121 decisions flagged at the point of no return, a reminder of the rule cut violations from 89% to 3%, whereas removing the lens concept directions from the residual stream left them unchanged (88% to 84%) and broke 23% of replies. Across whole games, an always-on reminder reduced violations from 68% to 0.5%. Collusion concepts remained in about 55% of scratchpads under every effective reminder, and wording mattered: a general rule left 22% violations, naming the loophole 3%. Reminders stop the act, not the intent. Deployers should name every known channel, remind on every decision, and use internals to audit.