Hints Win Far More Agreement Than Explicit Collusion Deals: A Decoy-Controlled Sting for LLM Agents

For research question 3.2 of the AI Collusion Research Sprint, we tested whether a trusted “undercover” agent can expose or break up such collusion, and whether the way it approaches other agents matters. We built the test in Liar’s Bar, the bluffing card game Zeng & Rudzicz (2026) used to show that LLM agents readily accept secret collusion tools. A decoy, identical to the undercover agent but never approaching anyone, separates real favouritism from general leniency. All three studies were pre-registered, about 1,550 games in total, with small open models.

In Study 1, the agents mostly refused an explicit deal not to challenge each other but often accepted a casual hint, and only the hint changed how they played, slightly and not reliably. Key takeaway: agents said no to an open collusion offer but often said yes to a hint, so an evaluator who only makes open offers would wrongly conclude that agents refuse to collude.

In Study 2, we tried to replicate the original paper’s research based on secret channel. Our agents accepted the channel but used it to share cards and pick targets, rarely to protect each other. Key takeaway: the paper’s strong collusion effect did not carry over to our models.

In Study 3, we tried using undercover agent to break a pact of collusion between ai agents. We told two agents at the start of each game that they had secretly agreed to team up, and they did: they let each other’s claims through much more often than the decoy’s (76% against 52%). The undercover agent then tried four ways to split them up: a fair-play appeal, a warning that it was watching, a reminder that only one player can win, and a better offer. The agents often even revealed their pact during this, but none of the four approaches made them go measurably less easy on each other. Key takeaway: talking to colluding agents can reveal that they collude, but in our tests the 4 methods by undercover agent to break their collusion didn’t stop them from colluding.