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. Humans and LLMs have different propensities to initiate convention change in this paradigm, yet mixed groups can recover coordination as LLMs follow human-initiated transitions. These results show that evaluating human–LLM coordination requires measuring not only whether agents can coordinate, but whether they can revise conventions together when the environment changes.