Transnational policy discourses shape teacher professionalism through discursive patterns in policy initiatives from the global policy actor, OECD. The policy idea of 'the teacher assistant' has emerged through discourses on teacher professionalism, spurring ambiguities regarding what the policy idea is and ought to be in Sweden. The aim of this article is to critically examine the construction and legitimation of the policy idea of 'the teacher assistant', in relation to teachers and the educational institution, through the lens of discursive institutionalism and strands in Curriculum Theory. The focus is a critical understanding of the interplay between ideas, discourses, actors and institutional context. The analysis of policy documents shows how the policy idea is constructed and legitimised through actors' coordinative and communicative discourses at the national level, influenced by the OECD at the transnational level. The policy idea is intertwined with ideas and discourses on teachers' professional development through a national professional programme and institutional conditions for goal attainment in schools. Tensions emerge regarding underlying assumptions about teacher quality, highlighting ideas of what teachers are and ought to be, within comprehensive reform strategies. Tensions entail emerging ideas of standardisation and differentiation and ambiguities regarding the policy idea of 'the teacher assistant'.