PROJECT SUMMARY Emergency department (ED) visits for suicidal ideation and attempts in adolescents are rising, underscoring a need for effective interventions preventing post-ED recurrence in youth suicidal crises. Parents are tasked with implementing post-discharge suicide prevention recommendations (e.g., lethal means restriction, attending to suicide warning signs, providing support), highlighting opportunities for parent-facing interventions to improve post-ED youth outcomes. Leveraging accessible text messaging, together with a Just -In-Time Adaptive Intervention (JITAI) framework designed to tailor support based on individuals’ unique and changing needs, our goal is to examine the effectiveness of an adaptive text-based intervention for parents of suicidal youth transitioning from ED care. We build on results from our NIMH-funded pilot of a text-based intervention with two components, provided to parents for 6 weeks after ED discharge: (1) adolescent-centered (A-C) texts, encouraging parental engagement in suicide prevention activities to promote youth safety; and (2) parent- centered (P-C) texts, intended to enhance parents’ own well-being. In addition to feasibility and acceptability, promising pilot results showed that the intervention improved key mechanisms of action and was associated with lower post-ED youth suicide attempts. We expand on this pilot work to determine the effectiveness of the text-based intervention in a full-scale multi-site trial and to investigate its implementation potential. In this hybrid type 1 effectiveness-implementation trial, 420 parents of adolescents presenting with suicide risk across two EDs will be randomized to standard ED services alone or paired with the 6-week text-based intervention integrating A-C and P-C components. An embedded full-scale MRT in the intervention arm, with twice-daily micro-randomizations to deliver (vs. not deliver) P-C messages, will inform a JITAI providing tailored parent- directed support. The specific aims are to: (1) Compare standard ED services with and without the text-based intervention on the primary outcome of youth suicidal behavior (actual, interrupted, aborted attempts) 12 weeks after ED discharge and on two secondary outcomes of time-to-suicidal behavior and time-to-return ED visits within 24 weeks; (2) Using the embedded MRT, determine if delivering vs. not delivering P-C messages impacts proximal (within hours) outcomes of parental stress (primary) as well as parental positive and negative affect (secondary); and (3) To support uptake in EDs, engage stakeholders to assess implementation barriers and facilitators and produce an implementation package. Secondary aims are to examine if mechanisms of action (parents’ engagement in suicide prevention activities, self-efficacy) mediate the intervention’s impact on youth suicidal behavior, and to investigate time-varying parent and adolescent moderators (e.g., affect, functioning) of micro-randomizations to identify wh...