Project Summary/Abstract Suicide is the 10th leading cause of death in the United States and has increased by 35% in the last two decades. Emergency Departments (EDs), often serving as the only clinical contact for individuals at risk for suicide, are a critical point for initiating suicide prevention interventions. Limited availability of psychiatric beds and challenges with linking to, or sustaining, post-ED care exacerbate the already heightened risk for suicidal behavior among discharged individuals. Best-practice guidelines for EDs recommend providing individuals at elevated suicide risk with brief interventions that include safety planning—emphasizing coping strategies to mitigate suicidal crises—as well as post-discharge contacts, however busy EDs often lack adequate resources to offer these interventions consistently or with fidelity. New approaches that can provide accessible, personalized, and resource-efficient continuity of care are urgently needed to prevent suicidal behavior during the high-risk post-ED period. Leveraging accessible technologies, we propose to develop and pilot an ED- initiated intervention package that incorporates an electronic safety plan (ESP) and adaptive text-based support to facilitate effective post-discharge coping and safety plan use, and ultimately reduce suicidal behavior in high-risk adults. Adults presenting to an ED for suicide-related concerns (N=120) will be randomized to ESP (n=40) or ESP + text-based support program (n=80) delivered for a month after discharge. The ESP + text-based support condition will include an embedded micro-randomized trial (MRT), with twice- daily randomizations over the month-long intervention, to optimize the frequency, timing, and content of messages and to inform the design of a just-in-time adaptive intervention (JITAI) for suicide prevention. Follow- ups will occur at 1 and 3 months, as well as twice-daily for the first month after discharge. The specific aims are to: (1) Develop and refine ESP and text-based support with stakeholder input; (2) Demonstrate feasibility, acceptability, and explore initial impact on mechanisms (coping self-efficacy, motivation for safety plan use) and distal outcomes (e.g., suicidal ideation severity), assessed at 1 and 3 months, for ESP with text-based support compared to ESP alone; and (3) Conduct a pilot MRT to inform the optimization of adaptive text-based support (JITAI). Specifically, through the series of micro-randomizations, we will explore if provision of (a) any coping message, (b) a specific type of coping message (untailored vs. ESP-tailored; with or without dynamic personalized feedback based on daily-level functioning), and (c) message timing (e.g., morning vs. evening) influence daily-level proximal mechanisms (e.g., safety plan use, coping behavior, self-efficacy) and distal outcomes (suicidal ideation). With potential for high public health impact, this proposal addresses a critical need for effective and scalable conti...