This article analyses the use of a locally developed assessment tool designed to generate aggregated data to evaluate the work of a psychiatric and addiction clinic. The use of tools, methods and interventions in the Swedish social services is usually based on recommendations in national guidelines established by the National Board of Health and Welfare (NBHW). Thus, a locally produced and systematically used assessment tool provides an interesting deviant case for discussing knowledge production and use from the perspective of evidence-based practice. The assessment tool was characterised by the specific psychiatric and addiction clinic context, where local needs and prerequisites were prioritised over the recommendations found in NBHW guidelines. The empirics comprise interviews with 12 professionals who used the tool, where experience of using the tool was analysed using a thematic analysis. The findings can be summarised in three main conclusions. First, tinkering of tools and methods is not necessarily associated with limited practice applicability or relevance. Second, professionals are more likely to appreciate a tool if that tool is designed with a treatment and conversation rationality in mind. Third, rather than perceived as more valid than other types of knowledge, NBHW-recommended tools are associated with a certain shape or style – but a shape or style that is permeated by legitimacy.