TY - JOUR AU - N. Vakili AU - C. Cassidy AU - S. Chawla AU - J. A. Curran AU - S. Munro AU - D. Phillips AU - N. S. H. Pho AU - R. Walls AU - A. Wood AU - L. Wozney A1 - AB - BACKGROUND: The transition from child to adult mental health services is a vulnerable period marked by service disengagement, care gaps, and worsening mental health outcomes. Although planned, developmentally appropriate transition processes can improve functioning, youths report insufficient preparation, limited continuity of care, and unmet expectations for support. Existing transition supports remain underevaluated and require further adaptation for mental health contexts. Youth consistently report needing clearer information, concrete support, and sustained connection. Digital tools, particularly SMS text messaging, which is widely used, accessible, and acceptable to youth, offer a promising way to deliver timely transition supports. Yet most digital mental health tools are developed without meaningful youth involvement, highlighting the need for participatory approaches to ensure relevance, usability, and uptake. OBJECTIVE: This study aimed to co-design and refine prototypes for a transition-focused SMS text messaging intervention by engaging youth with lived experience in a participatory co-design activity (design jam) to identify priority content, key functionality, and implementation enablers to support the transition from child to adult mental health services. METHODS: We conducted a 3-hour mixed methods, participatory design jam to co-design transition-focused SMS text messaging prototypes, recruiting youth aged 16-26 years in Canada who had recently transitioned to, or were approaching transitioning to, adult mental health services. Data sources included workshop artifacts, observational field notes, and audio recordings from structured activities involving evidence review, brainstorming, rapid prototyping, brief team pitches, and evaluation. Rapid qualitative analysis, integrating open coding, content analysis of visual prototypes, and the rapid identification of themes from audio recordings, was used to identify priority content, key functionality, and implementation enablers. Findings were refined through a member-checking debrief with youth participants. RESULTS: Seven youths aged 19-24 years participated in the design jam. Across two teams, participants generated 54 content ideas and 50 feature ideas. Two distinct prototypes were developed: one emphasizing long-term affirmation, self-advocacy, self-care, and profile-based customization, and the other prioritizing shorter-term informational support, navigation resources, and flexible message frequency. Youth across both groups highlighted the importance of interactive and visually engaging elements. Analysis revealed 3 thematic tensions shaping youth design preferences: balancing autonomy with ongoing support (roaming/reconnecting), balancing personalization with the need for simplicity (customization/convention), and balancing knowledge delivery with motivation for action (learning/living). Participants rated the design jam positively. CONCLUSIONS: Youth meaningfully contributed to co-designing an SMS text messaging intervention to support transition from child to adult mental health services, generating concrete content, functionality, and implementation priorities. Their prototypes highlighted the need to balance autonomy with support, personalization with simplicity, and information with motivational guidance. These findings demonstrate the value of participatory co-design in developing youth-centered digital transition supports and underscore the importance of evaluating such prototypes in real-world settings to determine feasibility and impact. AD - Centre for Research in Family Health, IWK Health, Halifax, NS, Canada.; School of Nursing, Dalhousie University, Halifax, NS, Canada.; Faculty of Computer Science, Dalhousie University, Halifax, NS, Canada.; Strengthening Transitions in Care Lab, IWK Health, Halifax, NS, Canada.; Mental Health and Addictions, IWK Health, Halifax, NS, Canada.; Mental Health and Addictions, Nova Scotia Health, Halifax, NS, Canada. AN - 42059873 BT - JMIR Form Res C5 - HIT & Telehealth; Healthcare Disparities DA - 05/2026 DO - 10.2196/91834 DP - NLM ET - 20260529 JF - JMIR Form Res LA - eng N2 - BACKGROUND: The transition from child to adult mental health services is a vulnerable period marked by service disengagement, care gaps, and worsening mental health outcomes. Although planned, developmentally appropriate transition processes can improve functioning, youths report insufficient preparation, limited continuity of care, and unmet expectations for support. Existing transition supports remain underevaluated and require further adaptation for mental health contexts. Youth consistently report needing clearer information, concrete support, and sustained connection. Digital tools, particularly SMS text messaging, which is widely used, accessible, and acceptable to youth, offer a promising way to deliver timely transition supports. Yet most digital mental health tools are developed without meaningful youth involvement, highlighting the need for participatory approaches to ensure relevance, usability, and uptake. OBJECTIVE: This study aimed to co-design and refine prototypes for a transition-focused SMS text messaging intervention by engaging youth with lived experience in a participatory co-design activity (design jam) to identify priority content, key functionality, and implementation enablers to support the transition from child to adult mental health services. METHODS: We conducted a 3-hour mixed methods, participatory design jam to co-design transition-focused SMS text messaging prototypes, recruiting youth aged 16-26 years in Canada who had recently transitioned to, or were approaching transitioning to, adult mental health services. Data sources included workshop artifacts, observational field notes, and audio recordings from structured activities involving evidence review, brainstorming, rapid prototyping, brief team pitches, and evaluation. Rapid qualitative analysis, integrating open coding, content analysis of visual prototypes, and the rapid identification of themes from audio recordings, was used to identify priority content, key functionality, and implementation enablers. Findings were refined through a member-checking debrief with youth participants. RESULTS: Seven youths aged 19-24 years participated in the design jam. Across two teams, participants generated 54 content ideas and 50 feature ideas. Two distinct prototypes were developed: one emphasizing long-term affirmation, self-advocacy, self-care, and profile-based customization, and the other prioritizing shorter-term informational support, navigation resources, and flexible message frequency. Youth across both groups highlighted the importance of interactive and visually engaging elements. Analysis revealed 3 thematic tensions shaping youth design preferences: balancing autonomy with ongoing support (roaming/reconnecting), balancing personalization with the need for simplicity (customization/convention), and balancing knowledge delivery with motivation for action (learning/living). Participants rated the design jam positively. CONCLUSIONS: Youth meaningfully contributed to co-designing an SMS text messaging intervention to support transition from child to adult mental health services, generating concrete content, functionality, and implementation priorities. Their prototypes highlighted the need to balance autonomy with support, personalization with simplicity, and information with motivational guidance. These findings demonstrate the value of participatory co-design in developing youth-centered digital transition supports and underscore the importance of evaluating such prototypes in real-world settings to determine feasibility and impact. PY - 2026 SN - 2561-326x SP - e91834 ST - Co-Designing a Text Messaging Intervention for Youth Transitioning From Child to Adult Mental Health Services: Participatory Design Jam Study T1 - Co-Designing a Text Messaging Intervention for Youth Transitioning From Child to Adult Mental Health Services: Participatory Design Jam Study T2 - JMIR Form Res TI - Co-Designing a Text Messaging Intervention for Youth Transitioning From Child to Adult Mental Health Services: Participatory Design Jam Study U1 - HIT & Telehealth; Healthcare Disparities U3 - 10.2196/91834 VL - 10 VO - 2561-326x Y1 - 2026 ER -