Urban planners can address sustainability by leveraging urban potential for people and nature but face significant challenges in integrating diverse knowledge and facilitating collaboration. Geodesign, as a methodological approach, has the potential to support collaborative urban planning by managing the ‘boundaries’ between diverse knowledge holders. However, there is a paucity of empirical evidence and systematic assessment of its contribution to ‘boundary work’. The latter refers to the efforts to navigate the interface between diverse stakeholders engaged in co-producing knowledge for sustainability. This paper aims to evaluate how a geodesign process facilitates knowledge co-production through boundary work and to assess the scientific credibility, political saliency, and procedural legitimacy of its outputs in sustainable urban planning. We adapt and further develop recent conceptual frameworks of geodesign from a boundary work perspective and propose a systematic, adaptable, and replicable framework and protocol for assessing boundary work in future applications. We tested our approach in a case study of a collaborative geodesign process for ambitious urban transformations with nature-based solutions in the Skarpnäck district of Stockholm, Sweden, involving fourteen planning stakeholders in a two-day workshop. The findings indicate that all geodesign steps facilitated enhanced communication by promoting discussions and collective reasoning, although to varying degrees. Participants acknowledged contributions to knowledge co-production and decision-making by mediating between different perspectives and opinions. Data quality was identified as a critical factor affecting perceived credibility. Reservations were expressed regarding the translation function. Recommendations for future applications include co-designing the geodesign process, improving capacity and skills, and facilitating more integrated planning.