<p>As healthcare systems around the world face the demographic challenge of aging populations, innovative care models such as hospital-at-home offer alternatives to traditional hospitalization. This study examines the implementation of a cross-sectoral admission avoidance hospital-at-home model in Denmark, developed collaboratively by a hospital emergency department, three municipalities, and general practitioners. The model provides acute medical care to elderly patients in their own homes, aiming to reduce hospital stays and associated risks. Employing an action research approach, the study investigates both the practical implementation process and the role of iterative learning in enabling systemic change. The methodology emphasizes co-creation, reflection, and problem-solving among stakeholders, while policy implementation theory provides a lens to understand the dynamic interplay between leadership, frontline staff behavior, and inter-organizational collaboration. The findings highlight the importance of trust, role adaption, and iterative adjustments in facilitating effective cross-sectoral collaboration. Practical tools such as structured task descriptions and reflective meetings supported alignment of diverse practices, while challenges, including logistical constraints and resistance to new roles, were addressed through real-time problem solving. By combining action research and policy implementation theory, the study provides practical guidance for Hospital-at-home implementation and contributes theoretical insights into mechanisms of systemic change, cross-sector collaboration, and governance. These findings advance both HaH literature and understanding of how complex healthcare interventions can be successfully implemented and adapted in practice.</p>