Catching collusion in useful-work verification

Systems that pay for access with useful work often have participants check each other's work, which invites collusion between workers and checkers. That includes AI agents that share a model. We ask which single restriction removes the most collusion at the least cost (question 1.5), in a peer-verified useful-work turnstile that guards a network relay. Our seeded simulation has 200 workers, 12 verifier nodes, 50 audited epochs and 30 seeds per condition. A cartel controls a share of both roles, against five switchable defences: commit-reveal assignment of workers and of verifier panels, blinding, hidden canaries, and planted faults (honeypots). The controls behave as expected. With no cartel, no wrong answer is accepted. With a 20% cartel and no defences, 20.1% ± 0.5% are. The restrictions interact strongly. Redundancy alone makes collusion worse (49.2% of wrong answers accepted), because colluders steer partners onto the same items. Commit-reveal alone does nothing. Together they reach 0.75%. Canaries are the cheapest single restriction (7.8% at 1.18 answers per item), and the only one that removes a cartel that lies only when unanimous. On the verifier side, random panels are the best single restriction (0.06%, at no added cost). Blinding or planted faults alone do nothing, but together they expose every colluding verifier. A cautious cartel that lies on 5% of its chances causes 0.13% wrong answers and is almost never caught. Audits bound the damage, but they don't remove patient colluders.