BackgroundEvidence-based practices (EBPs) are shown to improve a variety of outcomes for autistic children. However, EBPs often are mis-implemented or not implemented in community-based settings where many autistic children receive usual care services. A blended implementation process and capacity-building implementation strategy, developed to facilitate the adoption and implementation of EBPs for autism spectrum disorder (ASD) in community-based settings, is the Autism Community Toolkit: Systems to Measure and Adopt Research-based Treatments (ACT SMART Toolkit). Based on an adapted Exploration, Adoption decision, Preparation, Implementation, Sustainment (EPIS) Framework, the multi-phased ACT SMART Toolkit is comprised of (a) implementation facilitation, (b) agency-based implementation teams, and (c) a web-based interface. In this instrumental case study, we developed and utilized a method to evaluate fidelity to the ACT SMART Toolkit. This study responds to the need for implementation strategy fidelity evaluation methods and may provide evidence supporting the use of the ACT SMART Toolkit.MethodsWe used an instrumental case study approach to assess fidelity to the ACT SMART Toolkit during its pilot study with six ASD community agencies located in southern California. We assessed adherence, dose, and implementation team responsiveness for each phase and activity of the toolkit at both an aggregate and individual agency level.ResultsOverall, we found that adherence, dose, and implementation team responsiveness to the ACT SMART Toolkit were high, with some variability by EPIS phase and specific activity as well as by ASD community agency. At the aggregate level, adherence and dose were rated notably lowest during the preparation phase of the toolkit, which is a more activity-intensive phase of the toolkit.ConclusionsThis evaluation of fidelity to the ACT SMART Toolkit, utilizing an instrumental case study design, demonstrated the potential for the strategy to be used with fidelity in ASD community-based agencies. Findings related to the variability of implementation strategy fidelity in the present study may also inform future adaptations to the toolkit and point to broader trends of how implementation strategy fidelity may vary by content and context.Supplementary InformationThe online version contains supplementary material available at 10.1186/s43058-023-00434-2.