The science-diplomacy interface between the IPCC and the UNFCCC is sometimes presented as a model of mutual understanding and learning. In practice, however, the relationship between the two institutions is more complex and controversial. This article analyses both collaboration and contestation about what constitutes policy-relevant and useable scientific knowledge using the concept of the ‘trading zone’ from Science and Technology Studies (STS). Based on ethnographic work carried out between 2021 and 2023, the paper explores the various ways in which the main findings of the IPCC are made relevant to the UNFCCC, before examining how these findings are debated within international climate negotiations. It highlights the significant efforts required to align IPCC assessments with the needs of the UNFCCC and emphasises the fragile nature of the exchanges between the two organisations.