There is increasing recognition that public sectors in Western countries alone cannot tackle complex societal challenges. However, detailed research on the role of public agents in implementing collaborative approaches remains limited. This article examines how policy entrepreneurship strategies contribute to collaborative public sector innovation (CPSI) by shaping policy design processes in transformative innovation policy. Focusing on the Shared-Agendas initiative in Catalonia, the study explores how public sector actors operate across the policy process to advance place-based approaches to sustainability transitions. By framing CPSI as a policy design challenge, the findings show that policy entrepreneurs influence these processes by reframing problems by taking a systemic perspective, co-constructing shared visions, enabling experimental implementation, and institutionalizing learning practices. The analysis contributes to a deeper understanding of how agency drives collaborative and adaptive policymaking within complex governance settings. Although based on an in-depth examination of an extreme case, the strategies identified may be transferable to other contexts where public administrations seek to foster CPSI through cross-sectoral, place-based innovation efforts.