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In a context of territorialization of climate policies, carbon storage in soils is a new dimension that local authorities must take into account when drawing up their Climate Air Energy Territory Plan. In this article, we consider the co-construction of the soil carbon problem and its local public, taking the case of Aldo, a tool developed by ADEME, the French agency for ecological transition, for local authorities. Our analysis shows how the tool’s design crystallizes a public and a framing of the ‘soil carbon’ problem, which the designers define through the choice of data and scripts. Yet the actual use of Aldo by carbon engineers bears witness to an interest that goes beyond the anticipated public, beneath the implementation of the Climate Plan. We analyse the product of this encounter, as well as the implications of taking these users into account in the tool’s most recent developments.