Background <p>Cancer survivors with chronic conditions (e.g., complex cancer survivors) are a growing population that faces greater cancer-specific and cardiovascular disease (CVD) related morbidity and mortality. A shared care model, which involves primary care and cancer clinicians, is the optimal care delivery model for complex cancer survivors. Yet, there is little guidance on how to effectively implement and support complex cancer survivors’ continued engagement with primary care during the cancer treatment phase.</p> Methods <p>This protocol describes a multi-phase, mixed methods, stakeholder-engaged research process that tailors and tests a multi-level health system intervention to improve primary care provider and cancer team ‘shared care’ for complex cancer survivors. This study employs a design-for-dissemination, theory-guided perspective, blending implementation science and care delivery conceptual frameworks: Exploration, Planning, Implementation, and Sustainment (EPIS) and Cancer Multi-team System (cMTS). These frameworks guide our understanding and strategies deployed on the multi-level factors of implementing shared care in a health system. Primary Care Connect (PCC) is a health system intervention designed to align complex cancer survivors, healthcare team members, and health system implementation actors’ understandings, capacities, and practices to promote the adoption of shared care delivery models for complex cancer survivors. A hybrid type 3 effectiveness-implementation study design will test the effectiveness of PCC (<i>n</i> = 266 patients) in a patient-level randomized controlled trial on primary care connection, chronic disease management, and patient-reported outcomes. PCC implementation (e.g., feasibility, acceptability, and appropriateness) will be evaluated using a mixed methods approach to inform sustainable usage of the intervention. A concurrent learning evaluation will assess PCC’s adaptations over time and convene health system stakeholders to explore intervention properties to inform adoption and sustainment considerations.</p> Discussion <p>The overarching goal of this protocol is to examine whether cancer survivors becoming “lost in transition” is a preventable problem. PCC tests whether the challenge of transitioning patients back to primary care can be avoided altogether by re-engineering care coordination with primary care at the outset of cancer treatment. Study findings are poised to have a profound impact on the adoption of shared care delivery models throughout the U.S. to optimally mitigate complex cancer survivors’ CVD risks.</p> Trial registration <p>Registered with ClinicalTrials.gov on February 19, 2025: NCT06834204.</p>