<p>Contemporary assessment research suggests that students should not be positioned as passive objects but as active agents. Nevertheless, student voices are rarely heard in assessment policy and practice. Even ‘student-centred’ practices are commonly built on compliance rather than ‘agency’, as they are often designed <i>for</i> students, not <i>with</i> them. In this study, we partner with students to co-design assessment and feedback in a Finnish secondary mathematics classroom. Our goal was to foster students’ agency and analyse whether this indeed happened. The co-designed assessment practices included, for example, a rubric, test items, and an e-portfolio. To capture the complexity of student agency, we present our findings in three parts, each emphasising different dimensions of agency. First, the assessment partnership process supported students’ agency over assessment artefacts as students took ownership over the materials they co-produced. Second, we explored agency with respect to social roles in assessment. While some students welcomed their redefined roles as active agents in assessment, others resisted. Finally, many students demonstrated a lack of agency regarding mathematical knowledge, highlighting our relative failure to reposition students as active users of mathematics. Our study provides empirical evidence about assessment partnerships in mathematics and conceptualises this democratic practice as fostering student agency.</p>