Humans Initiate, LLMs Follow: Convention Lock-In and Adaptive Coordination in Human-LLM Groups

Successful coordination in dynamic environments requires that agents flexibly alter coordination conventions when incentives change. We introduce the Letter Game, a dynamic coordination game in which coordination on an established convention is challenged by the introduction of new higher-payoff alternatives. We compare behavior in human-only, LLM-only, and mixed human–LLM groups. While humans frequently initiate transitions to a higher-payoff convention, LLM-only groups largely stick to precedent. Mixed human-LLM groups miscoordinate at first, but quickly adapt. A Bayesian latent-plan model attributes much of this difference to the initial weight assigned to transition plans. A follow-up prediction study supports this account; humans expect substantial switching by both humans and LLMs, while LLMs expect both to stick to precedent. Despite different initial strategies, mixed human-LLM groups can recover coordination as LLMs follow human-initiated transitions