Drawing on rhetorical genre studies and recent work in activity system theory, this study focuses on the collaborative development of a new written form, a municipal plan for protecting and managing natural areas. The author advances a twofold claim: (a) that the written plan is developed in the absence of a stable textual model and (b) that the text, as part of the context, functions, in turn, as a mediational tool for solving the rhetorical problem of audience resistance. Findings show that as participants reconfigure the project into successive cycles of activity, they create corresponding zones of proximal development. This study contributes to our understanding of the dynamics of the text-context relationship and to recent elaborations of genre as an activity system that help explain the relationship between genre and learning.