Urban food systems must undergo a significant transformation if they are to avoid impeding the achievement of UN Sustainable Development Goals. One reconfiguration with claimed sustainability benefits is ICT-mediated food sharing - an umbrella term used to refer to technologically-augmented collective or collaborative practices around growing, cooking, eating and redistributing food - which some argue improves environmental efficiencies by reducing waste, providing opportunities to make or save money, building social networks and generally enhancing well-being. However, most sustainability claims for food sharing have not been evidenced by systematically collected and presented data. In this paper we document our response to this mismatch between claims and evidence through the development of the SHARECITY sustainability Impact assessment Toolkit (SHARE IT); a novel Sustainability Impact Assessment (SIA) framework which has been co-designed with food sharing initiatives to better indicate the impact of food-sharing initiatives in urban food systems. We demonstrate that while several SIA frameworks have been developed to evaluate food systems at the urban scale, they contain few measures that specifically account for impacts of the sharing that initiatives undertake. The main body of the paper focuses on the co-design process undertaken with food sharing initiatives based in Dublin and London. Attention is paid to how two core goals were achieved: 1) the identification of a coherent SIA framework containing appropriate indicators for the activities of food sharing initiatives; and 2) the development of an open access online toolkit for in order to make SIA reporting accessible for food sharing initiatives. In conclusion, the co-design process revealed a number of technical and conceptual challenges, but it also stimulated creative responses to these challenges.