Evaluating an Artificial Intelligence Mental Health Intervention (AIMHI) for Adults with Anxiety and Depression Symptoms: A Randomised Controlled Trial.

Intervention: The intervention is Cara Plus, an AI-enabled mental health smartphone application developed by Spectrum Wellness Limited (Spectrum.Life), intended to support the management of mild to moderate symptoms of anxiety and/or depression in adults. The app delivers support through secure, text-based chat with an AI companion. Users can choose a preferred personality style for the AI companion, which adjusts the conversational tone while keeping the underlying model unchanged. During chat, the AI companion responds to users and guides the conversation, offering short interactive evidence-based techniques (such as breathing exercises or guided meditations) when appropriate. The AI retains memory of prior sessions, enabling personalised support over time. There is no human involvement in the chat. If any message suggests possible risk or distress, the chat pauses and the user is prompted to contact appropriate crisis support services. Participants will install the app through a link to Google Play. They can initiate text conversations with the app at any time that is convenient for them. Participants will also receive reminder messages through the Prolific platform twice per week. Delivery of the Intervention: The intervention is delivered autonomously by the Cara Plus AI, which was developed with input from clinical psychologists, lived experience experts, and independent specialists to ensure a therapeutically sound and user-centred experience. The trial is led by Senior Lecturers in Clinical Psychology at the University of the Sunshine Coast, Australia. The Cara Plus application was developed and tested by Spectrum Wellness Limited, who will provide technical support throughout the trial. The intervention is delivered entirely online, on an individual basis, via the Cara Plus smartphone application on Android devices (operating system version 8 or higher). All study procedures, including recruitment, screening, consent, randomisation, assessments, and the intervention itself are conducted entirely online. Participants are recruited via the Prolific research platform, with recruitment occurring across Australia and the United States. Participants access the app using a direct download link and a unique login code provided at enrolment. Participants engage with the AI through interactive text chat (with optional voice-to-text), supplemented by in-app exercises. No face-to-face, group, or video-based components are included. The intervention is delivered over 8 weeks. Participants in the intervention group are advised to use the app for a minimum of 5 minutes per day, 5 days per week, amounting to a minimum of 200 minutes of application use across the study period. Assessments are conducted at four time points: baseline (Week 0), Week 2, Week 4, and Week 8. Adherence: Participants are provided with a clear usage goal at enrolment (minimum 5 minutes/day, 5 days/week for 8 weeks) and receive a brief schedule of assessments so they know when surveys are due. Adherence is supported through reminders sent twice a week via the Prolific platform, prompting participants to continue engaging with the app between assessment time points. App usage data, including frequency, duration, and engagement with specific features are collected by the development team and provided to the research team in de-identified form, enabling monitoring of engagement patterns. Participants in the intervention arm also receive a safety disclaimer at onboarding, and automated safety messaging is triggered if distress indicators are detected during app use. If distress or risk is detected, participants will be directed to the relevant crisis support services in their country: Australia: Lifeline on 13 11 14. In an emergency, participants will be advised to call 000. United States: Suicide and Crisis Lifeline by calling or texting 988. In an emergency, participants will be advised to call 911. The same progressive compensation structure applied to both groups (£8/hour at baseline to Week 2; £9/hour at Weeks 2–4; £10/hour at Weeks 4–8), providing a financial incentive to remain engaged with the study across the full 8-week period. Compensation will be provided through Prolific. The researcher sets the payment amount in British pounds. Participants receive the payment in their local currency, such as Australian dollars or United States dollars, using the applicable exchange rate on the date of conversion. Compensation for time spent using the app will be paid in addition to compensation for the time spent completing study assessments.