The present paper explores the role of design in supporting public administrations in setting a NetZero city strategy at the local level. The following intervention presents the codesign process we designed for the Municipality of Prato to lead and facilitate in order to support the city elaborating the Climate City Contract draft, the document in which the city presents its commitment to climate neutrality through a concrete portfolio of actions. Based on four pillars (energy efficiency, sustainable mobility, circular economy, and agriculture and urban land use-forestry), the process aimed at identifying systemic barriers and preparing the ground for the codesign of a portfolio of actions to achieve carbon neutrality by 2030. The strategy to support the process was based on two main phases (system understanding and codesign of a portfolio) transversal to another level of investigation consisting, in turn, of two phases: the front-end (the process of facilitation at the local level) and the back-end (the process of systemic interconnection).