The front-line of the welfare state is increasingly not a letter, phone call or face-to-face visit, but an online user-interface. This ‘interface first’ bureaucracy is a fundamental reshaping of social security administration, but the design and operation of these interfaces is poorly understood. Drawing on interview data from senior civil servants, welfare benefits advisors and claimants on the UK’s flagship Universal Credit working-age benefit, this paper is a detailed analysis of the role played by interfaces in the modern welfare state.