Cultural-historical activity theory (CHAT) approaches to intervention aim for transformative agency, that is, collective actions that expand and bring about new possibilities for activity. In this article, we draw on CHAT as a resource for organizing design research that promotes teachers' agency in designing new science curriculum materials. We describe how CHAT informed our efforts to structure a collaborative design space in which teachers and other participants sought to develop new curriculum materials intended to help realize a new vision for science education. Specifically, we describe the tools and routines we deployed to support the design process, and we analyze the ways in which teachers took up elements of our design process as well as how they adapted, resisted, and suggested alternative tools and strategies to help develop new curriculum materials. In so doing, we illustrate ways in which CHAT can serve as a guide both for organizing collaborative design processes and for analyzing their efficacy.