["Health Expectations, Volume 29, Issue 4, August 2026. ", "\nABSTRACT\n\nBackground\nPolicy models increasingly guide policy decision‐making. Incorporating public and stakeholder perspectives into modelling processes can produce evidence that is more relevant, appropriate and acceptable to the public. However, public and stakeholder involvement (PSI) in modelling studies is an emerging practice in the field of health inequalities, with little guidance available to support researchers.\n\n\nObjective\nWe present a methodological reflection on a PSI process supporting development of a policy model estimating the health impacts of major income supplementation policies. We include a multi‐layered systems map, co‐developed with public and stakeholder contributors, which guided modelling strategy development.\n\n\nMethods\nWe engaged 32 people with lived experience of income insecurity and over 60 stakeholders with expertise in socio‐economic determinants of health. Perspectives were gathered through three participatory systems mapping workshops and five policy prioritisation workshops. Workshop contributions were analysed using descriptive thematic analysis. Insights were fed back to the research team, who made changes or additions to the modelling strategy.\n\n\nResults\nWe experienced successes and challenges incorporating public and stakeholder perspectives into model development, specifically: engaging people with lived experience of income insecurity; ensuring PSI settings support effective engagement; and appropriately remunerating and safeguarding public contributors. We learned the importance of breaking down methodological silos within research teams to translate rich qualitative insights into actionable modelling outputs.\n\n\nConclusion\nIn data‐driven modelling studies, PSI requires carefully considered design, strong collaboration between team members, and appropriate resourcing to create enjoyable, productive engagement spaces where contributors can safely share experiences and expertise.\n\n\nPatient or Public Involvement and Engagement (PPIE) Contribution\nPSI was central to this work. Our project partners at The Poverty Alliance, a third sector organisation, facilitated involvement of people with lived experience of income insecurity using their internal trauma‐informed approaches throughout all stages.\n"]