The agroecological transition of agriculture not only requires changes in practices but also in ways of thinking and in their underlying values. Agroecology proposes broad scientific principles that need to be adapted to the singularities of each farm. This contextualization leads to the identification of agroecological practices that work locally and could serve as evidence-based practices to be transferred to local practitioners. This strategy was tested in a 4-year experiment conducted with dairy-sheep farmers in the South of France. The aim was to collectively engage in the agroecological transition. Our work focused on the creation of a diagnostic tool that was used to analyze local farms and identify virtuous practices to be promoted locally as evidence-based practices. But this strategy came up against the complexity and singularity of the situations to be transformed: not only was no evidence found but the construction of the diagnostic tool itself led to controversy concerning the models of agriculture to be promoted. The experiment then took a pragmatic turn: on-farm workshops were organized to enable farmers to benefit from the professional judgment of peer farmers who were willing to help their colleagues accomplish their agroecological transition. The diagnostic tool, which was designed to be prescriptive, was then used to judge the social acceptability of solutions that made sense in each farmer's own situation. Our experiment led us to conclude that managing the agro-ecological transition of farmers and their farming system is better conceived as a value-based process than as an evidence-based one.