This strategy will primarily include self-directed family access to the online JASPER modules developed for this project. The online modules follow a stepped hierarchy beginning with foundational concepts (e.g., engagement states) and systematically adding new concepts and strategies each week (e.g., balancing imitation and modeling, play routines). Access to the modules will be paired with a weekly brief (20 minute) check-in call provided by a community interventionist. The call is designed to create a space for the caregiver to ask questions about the week's content and to provide basic troubleshooting for implementation.

Slower responders to phase 1 ONLINE intervention will intensify to receive COACH in phase 2.

Families randomized to add coaching will receive access to the ONLINE intervention program as described above. However, every second check in call will be shifted to a 45-minute live coaching session with their community interventionist. The coaching session will begin with a brief check-in and review of the target content for the session. This will be followed by support to set up the environment. The child will then join the interaction and the interventionist will provide real time coaching supports for the caregiver to practice the intervention strategies with their child. The session will end with a short discussion to debrief on the practice session.

Slower responders to phase 1 ONLINE + COACH intervention will intensify in phase 2 where check in calls are modified to include video feedback and review.

The ONLINE intervention applies the JASPER (Joint Attention, Symbolic Play, Engagement, and Regulation: Kasari et al., 2021) social communication intervention for young children with autism. The JASPER approach has been tested in a caregiver-mediated format where caregivers are taught to use the JASPER strategies with their children during play and home routine (e.g., Kasari et al., 2010; Kasari et al., 2014). The JASPER intervention is being adapted in this study to include online web based materials.

Caregivers' ratings of their strategy use, confidence, and interest in using the strategies with their child. Each item is rating on a scale of 0-5 with higher score indicating greater acceptability.

Using the Caregiver Involvement Scale, interventionists will rate caregivers' comfort, confidence, and accurate usage of the strategies with their child using seven questions rated from 1 (not comfortable) to 5 (very comfortable). Higher scores indicate greater adoption.

Interventionists' JASPER coaching will be measured to assess the local providers' ability to accurately deliver the intervention across families. The items are rated from 0 (strategy is missing) to 5 (high quality, consistent and appropriate strategy use) and are summed to obtain a total coaching fidelity percentage score. High scores indicate greater coaching fidelity.

Total JASPER Implementation Fidelity (percentage). Each item is rated on a score of 0 (skill is not present) through 5 (high quality, consistent use of the skill/strategy). Higher scores indicate greater use of JASPER strategies.

Nonverbal and Spoken spontaneous initiations of joint attention coded by independent raters from the Short Play and Communication Evaluation (SPACE)