Co-design rationales as living documents for reporting the exchange of experiential, professional and scientific perspectives in mental health care innovation.

Co-design has become a prominent approach in mental health care innovation, reflecting an increasing recognition of epistemic pluralism as much-needed for participatory change. However, despite its growing adoption, co-design studies frequently offer limited insight into how these exact collaborations influenced the resulting artefacts or change. Drawing on over a decade of co-design practice in mental health care, this essay identifies lessons related to epistemic pluralism, dissensus, serendipity, contextual constraints, participatory approaches, paradigmatic differences, and power relations. We make the argument that these challenges arise from a broader reporting gap: the lack of systematic accounts detailing how various ways of knowing contribute throughout the co-design process. To address this issue, the concept of co-design rationales is introduced, defined as a framework for documenting the development of design decisions through co-learning among stakeholders with experiential, professional, and scientific expertise. Building on established design rationale literature, we develop an architecture that is operationalized through a co-design rationale canvas, enabling the iterative documentation of both the design process and the collaborative production of knowledge. Co-design rationales, as living documents, updated across iterations, discussed with stakeholders, and used to support shared decision-making, clarify who contributed, how decisions were negotiated, which forms of knowledge informed design choices, and how these choices shaped the final artefact or change. Beyond this, we postulate that reciprocity serves as the relational condition, or hallmark, of co-design, enabling epistemic pluralism and collective learning. Without co-design rationales, we may know that stakeholders participated, but not how their ways of knowing shaped the outcome or where reciprocity broke down.