Teacher agency plays a vital role in professional learning. The revealing practice indicates its fluidity in the process of collaboration. Research suggests that learning through interaction happens by engaging teachers. However, the role of teacher agency remains unclear at a systemic level. This article reports a yearlong ethnographic study of overlapping members from two teams collaborating on contemporary liberal arts curriculum across two different education systems in a Hong Kong secondary school. Discourse analysis of collaborative meetings demonstrated that the overlapping members performed various functions in the two settings. The connections and disconnections suggested that teacher agency was conditional based on the ecological boundary. By identifying their patterns of practice, individual teachers collectively demonstrated different capacities in the two settings within and over time. The findings imply that collaboration is an embedded activity and strongly influenced by the school environment. Overall, this study contributes to a reconceptualisation of professional learning.