Recent work on consultants and academics in public policy has highlighted their transformational role. The paper traces how, in the absence of an explicit government strategy, external advisors establish different organizational arrangements to build Behavioral Insights in public agencies as a new form of administrative expertise. This variation shows the importance of the politico‐administrative context within which external advisors exert influence. The focus on professional expertise adds to existing understandings of ideational compatibility in contemporary policy advisory systems. Inspired by the sociology of professions, expertise is conceptualized as professionally constructed sets of diagnosis, inference, and treatment. The paper compares four Danish governmental agencies since 2010, revealing the central roles external advisors play in facilitating new policy ideas and diffusing new forms of expertise. This has implications for how we think of administrative expertise in contemporary bureaucracies, and the role of external advisors in fostering new forms of expertise.