BACKGROUND: In July 2017, a Dutch health insurer and primary care organization jointly implemented the All-In Contract (AIC), a population-based payment model for general practitioners (GPs). Affiliated GP-practices received a capitated payment per enrolled patient covering all GP care and multidisciplinary primary care for chronic conditions. Additionally, the care organization shared in savings and losses on total healthcare spending, contingent upon meeting quality targets. This study investigates the AIC's impact on spending, quality indicators, and provider experiences 2.5 years after implementation. METHODS: We employed a difference-in-differences approach comparing individual-level claims spending from enrollees of participating GP-practices (N = 16,425) with a control group (N = 212,251). Changes in indicators of chronic care management and patient satisfaction were investigated in a before-after analysis due to limited data availability. To contextualize the findings and explore provider experiences, focus groups were conducted with stakeholders involved in the development and/or implementation of the AIC. RESULTS: The AIC was associated with an insignificant 1.2% reduction of average quarterly total spending per enrollee (p = 0.476). We did find a - 10.2% decrease in primary care spending growth (p <
0.01), which was likely related to the indexation rate used for the capitation payment. Spending in other subcategories showed insignificant changes. Changes in patient satisfaction and chronic care management indicators were mixed and modest, but due to the lack of data from non-participating GPs, the extent to which these changes can be attributed to the AIC remains uncertain. The focus group participants reported improvements in provider flexibility in care provision, autonomy, and reduced administrative burdens. However, the focus group results may not fully capture the broader or more diverse experiences of all providers involved. CONCLUSIONS: In its first 2.5 years, the AIC had no significant effect on total healthcare spending growth. Trends in quality indicators suggest mixed results for patient satisfaction and chronic care management, while focus group results indicated improved provider experiences. To comprehensively evaluate population-based payment reforms, stakeholders should improve data collection strategies to enable causal assessment of population health, patient experiences, and provider well-being.