This article reports from an empirical study on the affordances and constraints for using staged socio-political debates for authentic summative assessment of scientific literacy. The article focuses on conditions for student participation and what purposes emerge in student interaction in a socio-political debate. As part of the research project, a socio-political debate was designed for assessing student competences of scientific literacy in classroom practices. The debate centred on a fictive case about a lake where a decline in the yield of fish had been established. The students were assigned the task of participating in the debate from appointed roles as different stakeholders. Data were collected with video recordings of the enacted student debates. Student participation was analysed with the theoretical framework of communities of practice. The results show that multiple conflicting purposes of the socio-political debate as an assessment task emerged. The emergent purposes were (1) putting scientific knowledge on display versus staying true to one's role, (2) putting scientific knowledge on display versus expressing social responsibility, (3) putting scientific knowledge on display versus winning the debate, and (4) using sources tactically versus using sources critically. As these purposes emerged in classroom practice, tensions between different ways of enacting participation in the debates became manifest. Based on these findings, this article discusses the affordances and constraints for using a socio-political debate for classroom-based assessment of scientific literacy and argumentation in terms of validity, reliability and affordability.