Background <p>Over recent decades, multifaceted nurse-led care models have been developed to reduce unplanned hospital transfers from long-term care facilities (LTCFs). In Switzerland, the INTERCARE model has demonstrated effectiveness, with core components including deployment of nurses in expanded roles (INTERCARE nurses), evidence-based communication tools, and advance care planning. However, resource-intensive implementation strategies such as 1:1 support meetings for model implementers pose challenges for scale-up, underscoring the need for more scalable implementation support. The INTERSCALE study compares two modes of delivering implementation support—an individualized and a collective-oriented approach—testing the hypothesis that the latter achieves non-inferior fidelity to the INTERCARE model and comparable reductions in unplanned hospital transfers at the LTCF level. Secondary aims are to compare implementation (acceptability, feasibility), economic (costs, cost-effectiveness), clinical (unplanned transfers), and organizational (staff absences, turnover) outcomes.</p> Methods <p>This non-inferiority, effectiveness–implementation hybrid type III trial uses a cluster-randomized controlled design, with LTCFs as the unit of randomization. Forty German-speaking LTCFs in Switzerland (≥20 long-term care beds; cantonal accreditation) will be randomized (1:1) after formal consent to either individualized or collective implementation support, without blinding of LTCFs or the research team. In the individualized arm (20 LTCFs), leadership receives 1:1 support meetings, and INTERCARE nurses receive 1:1 coaching, mirroring the original INTERCARE trial. In the collective arm (20 LTCFs), leadership support and INTERCARE nurse coaching are delivered in group formats involving several LTCFs/INTERCARE nurses together at two-monthly intervals. The primary outcome is LTCF-level fidelity to the INTERCARE core components, analyzed with a binomial generalized linear mixed model including a random LTCF effect. Non-inferiority of the collective mode will be concluded if the lower bound of its 95% confidence interval for fidelity is within 15% of the individualized mode. A 12-month cost-effectiveness analysis from a multi-stakeholder perspective (LTCFs and research group) will estimate the incremental cost-effectiveness ratio using differences in implementation costs and unplanned transfers between arms; secondary outcomes include unplanned transfers, staff turnover, and absences.</p> Discussion <p>This type III hybrid cluster trial addresses a key scaling challenge in implementation science by testing less resource-intensive implementation strategies for disseminating an evidence-based care model across LTCFs in routine practice.</p> Trial registration <p>Prospectively registered on June 25, 2024, at ClinicalTrials.gov nr. <a href="https://clinicaltrials.gov/study/NCT06473051">NCT06473051</a>.</p>