["Health Expectations, Volume 29, Issue 4, August 2026. ", "\nABSTRACT\n\nBackground\nExperience‐based co‐design (EBCD) seeks to bring together people with lived experience of healthcare with other interest holders to collaboratively generate creative solutions.\n\n\nObjective\nA cluster randomised controlled trial is planned to assess the feasibility, acceptability and effectiveness of a co‐designed multicomponent intervention to improve communication in aged care services.\n\n\nDesign\nThis paper describes a nested process evaluation design for the EBCD and implementation of the intervention with the aim of understanding the role of co‐design and other factors in achieving impact.\n\n\nMethod\nPrimary data sources will include surveys, interviews, focus groups, field notes and observations of co‐design activities. Secondary sources will include data collected and materials produced by the researchers developing the intervention.\n\n\nParticipants\nAged care recipients; family members and significant others; direct care workers; managers; and other healthcare professionals will be engaged as co‐designers of the intervention, and as research participants in the evaluation of its co‐design and implementation.\n\n\nMain Variables Studied\nEvaluation findings will compare experiences of co‐design, within and across informant groups, with an explanatory theoretical model of change described in previous research employing EBCD. Additionally, the role of the co‐design and other contextual factors in the intervention's impact will be organised using the RE‐AIM (Reach, Efficacy, Adoption, Implementation and Maintenance) framework.\n\n\nDiscussion\nThe involvement of people who use and deliver health services in the development of complex interventions is increasingly recommended as a means to improve impact and health outcomes. However, research evidence supporting the mechanisms of impact of co‐design and related approaches in healthcare is limited.\n\n\nConclusion\nNested process evaluations of co‐design are needed to support the continued theoretical and methodological development and resourcing of co‐design in healthcare.\n\n\nPatient or Public Contribution\nA lived experience advisory group (LEAG) of older people receiving aged care services and family or other informal supporters of those receiving aged care services has provided input to the evaluation design described in this manuscript, and a member of this group has co‐authored this article. LEAG members will continue to contribute to the conduct of the evaluation as well as the analysis and dissemination of the evaluation findings.\n"]