Purpose: A new senior curriculum and assessment policy in Queensland, Australia, is changing the conditions for teaching and learning. The purpose of this study was to consider the personal, structural and cultural conditions that mediated the agency of Senior English teachers as they negotiated these changes. Agency is conceptualised as opportunities for choice in action arising from pedagogic negotiations with students within contexts where teachers' decision-making is circumscribed by other pressures. Design/methodology/approach: An action inquiry project was conducted with English teachers and students in two secondary schools as they began to adjust their practices in readiness for changes to Queensland senior assessment. Four English teachers (two per school) designed a 10-week unit of work in Senior English with the aim of enhancing students' critical and creative agency. Five action/reflection cycles occurred over six months with interviews conducted at each stage to trace how teachers were making decisions to prioritise student agency. Findings: Participating teachers drew on a variety of structural, personal and cultural resources, including previous experiences, time to develop shared understandings and the responsiveness of students that mediated their teacher agency. Teachers' ability to exert agentic influence beyond their own classroom was affected by the perceived flexibility of established resources and the availability of social support to share student success. Originality/value: These findings indicate that a range of conditions affected the development of teacher agency when they sought to design assessment to prioritise student agency. The variety of enabling conditions that need to be considered when supporting teacher and student agency is an important contribution to theories of agency in schools, and studies of teacher policy enactment in systems moving away from localised control to more remote and centralised quality assurance processes.