A Path of Least Resistance: How Language Model Agents Collude When the Honest Route Disappears

Teams of language-model agents increasingly share workspaces and communication channels, which makes collusion possible: coordination on an outcome that the rules forbid. Prior work shows that collusion can arise from private channels, market structure, reward design and pressure, and that a single agent exploits shortcuts more often on harder tasks. These studies change the channel, the market or the reward; none isolates the cost of the honest alternative for a team that must also decide whether to share a shortcut. We position this work at that gap. In a controlled testbed we vary a single factor, the cost of being honest: the honest route to the same reward ranges from an answer written in a public file to no honest route at all, while the shortcut stays fixed, a leaked answer file found by one member of some teams, and no agent is prompted to collude. Across 600 runs of five agentic tasks and six models, collusion behaves as a step rather than a slope: it stays low while any honest route exists, and once none remains teammates are five times as likely to use the leak, a rise concentrated in three of six models. An overseer that reads the conversation catches shared leaks nearly five times as often as silent cheating.