Participants randomized to the immediate treatment group will begin using the MI-Coach: ED app immediately for a four-week intervention period. During this time, participants will complete interactive modules designed to enhance motivation and readiness for treatment, focusing on topics such as ambivalence, self-acceptance, compassionate motivation, and behavior change strategies. Weekly brief assessments of motivation and readiness for change will be completed throughout the initial four-week period.

Participants randomized to the delayed treatment group will remain on the waitlist without access to the MI-Coach: ED app for the first four weeks. During this period, participants will complete weekly brief assessments of motivation and readiness for change to evaluate changes while waiting for access to the intervention. No additional interventions or activities will be provided during this initial four-week period.

This arm includes all participants who engage with the MI-Coach: ED app during the eight weeks following the initial four-week period. Participants from the immediate treatment group (Arm 1) will continue to use the app for the full 12 weeks of the study, including an eight-week extended intervention period and follow-up assessments at 8 and 12 weeks to evaluate the app's sustained effects. Participants transitioning from the delayed treatment group (Arm 2) will also use the app for an eight-week intervention period after their initial four-week waitlist phase, ensuring both groups have the same opportunity to complete the full intervention. Weekly assessments of motivation and readiness for change will continue for all participants during their respective intervention periods, along with follow-up assessments to compare immediate and delayed access effects, as well as the app's long-term impact on motivation and clinical outcomes.

The MI-Coach: ED mobile app is a self-guided intervention designed to enhance motivation and readiness for treatment in adults with eating disorders. The app includes seven sequential modules based on motivational interviewing (MI) principles, addressing ambivalence, self-acceptance, compassionate motivation, behavior change goals, sustain and change talk, commitment language, and relapse prevention. Each module incorporates interactive content, including educational videos, articles, and exercises, tailored to support motivation across a range of eating disorder diagnoses. The app is designed to be user-friendly and scalable, providing an accessible, cost-effective solution for individuals on waitlists for formal eating disorder treatment.

Overall motivation for treatment will be assessed using the Readiness and Motivation Questionnaire (RMQ), a validated measure evaluating readiness to change eating disorder symptoms and behaviors. The RMQ provides a total motivation score (ranging from 0 to 100) and includes subscales for precontemplation, action, internality, and confidence. Higher scores indicate greater readiness for treatment. Changes from baseline to 4 weeks will be evaluated using linear mixed modeling (LMM), accounting for fixed effects (treatment group, time) and random effects (individual variability). A significant change from baseline will be defined as a statistically significant fixed effect of time (p < 0.05), with a greater increase in motivation scores observed in the treatment group compared to the delayed treatment control group.

Weekly motivation will be assessed using the Motivational Stages of Change for Adolescents Recovering from an Eating Disorder (MSCARED), a single-item self-report measure. Participants select their current stage of change from six stages: precontemplation, contemplation, preparation, action, maintenance, and recovery. Rate of change will be calculated using LMM, incorporating interaction terms between treatment group and time to evaluate differences in the trajectory of weekly motivation. A significant result will be defined as a statistically significant interaction term (p < 0.05), indicating a slower decline or greater increase in motivation over time in the treatment group compared to the delayed treatment control group.

Confidence to engage in treatment will be assessed using the confidence subscale of the RMQ, which measures participants' belief in their ability to address eating disorder behaviors. Scores range from 0 to 100, with higher scores indicating greater confidence. Changes from baseline to 4 weeks will be analyzed using LMM, incorporating fixed effects for treatment group and time, and random effects for individual differences. A significant change from baseline will be defined as a statistically significant fixed effect of time (p < 0.05), with confidence scores in the treatment group showing stability or improvement compared to the delayed treatment control group.

Attrition will be measured as the percentage of participants who do not complete study activities (e.g., assessments or app usage) within the first 4 weeks. Descriptive statistics, including means, percentages, and ranges, will be calculated to summarize attrition rates for both groups. Simple t-tests will be used to compare mean attrition rates between the treatment and delayed treatment control groups, with a significant difference defined as a p-value < 0.05, indicating lower attrition in the treatment group.

Anxiety symptoms will be assessed using the GAD-7, a validated self-report measure with scores ranging from 0 to 21 (higher scores indicate greater severity of anxiety symptoms). Descriptive statistics, including means and standard deviations, will summarize anxiety scores at baseline, 4 weeks, and follow-up time points. Exploratory analyses will use simple t-tests to compare changes in mean anxiety scores between the treatment and delayed treatment control groups at 4 weeks. Changes over time will also be analyzed using linear mixed modeling (LMM) to evaluate group differences in trajectories. Statistical significance will be defined as p < 0.05 for group comparisons or time effects.