Adoption of sustainability assessment tools in agricultural practice is often disappointing. One of the critical success factors for adoption is the tool development process. Because scientific attention to these development processes and insights about them are rather limited, we aimed to foster the scientific debate on this topic. This was done by reflecting on the development process of a Flemish sustainability assessment tool, MOTIFS. MOTIFS was developed with the aim of becoming widely adopted by farmers and farm advisors, but this result was not achieved. Our reflection process showed success factors favoring and barriers hindering tool adoption. These were grouped into three clusters of lessons learned for sound tool development: (1) institutional embeddedness, (2) ownership, and (3) tool functions. This clustering allowed us to formulate actions for researchers on the following aspects: (1) learning from stakeholders and end users, (2) providing coaching for appropriate tool use, and (3) structuring development of different tool types and exploring spin-offs from existing tools. We hope these normative results evoke other researchers to feed a debate on understanding tool development.