Disability benefits function by demarcating categories of need (the administrative category of disability) and determine eligibility using assessments of functioning. In the UK, these assessments are the Work Capability Assessment and PIP assessment. Inherently technical and abstruse processes, these assessments have been opportune sites for welfare reform in recent years. Disability benefits have also been a central point of contention between disability studies and sociology. Sociology has traditionally favoured an ‘incomes approach’ and called for more adequate financial support from the state. Early figures in the disabled people’s movement rejected this position, and aligned with an oppression paradigm, argued for a more radical economic and social inclusion. We contend that this divide, set out in the Fundamental Principles of Disability, remains relevant for researching welfare reform today. This article treats benefits assessments as epistemic practices—interactional processes wherein claimants, their personal health professionals and commercial assessment providers come together in the production of knowledge about disability. Data include 50 in-depth interviews with benefit claimants and a discourse analysis of official texts directed at claimants, personal health professionals and commercial assessment providers. We outline a phenomenon we term ‘epistemic sabotage’, whereby the knowledge claims of claimants and their health professionals are systemically disqualified.