Understanding service users’ experience of using digital health interventions (DHIs) is essential for facilitating engagement. The Actissist app is a DHI for psychosis based on Cognitive Behavioural Therapy (CBT) principles, co-produced in collaboration with individuals with lived experience of psychosis. This qualitative study aimed to explore participants’ experiences with the Actissist app by analysing exit interviews from those who received the intervention during the Actissist 2.0 trial. Qualitative interviews were conducted with 21 participants allocated to the Actissist group in the Actissist 2.0 randomised controlled trial. Qualitative framework analysis was conducted on the interview data using a predefined sampling framework. Four themes were identified. Two themes were established a priori: (1) user interaction with the app; and (2) perceived mechanism of change. Two themes were data-driven: (3) benefits of using the app; and (4) perceived barriers to app use. Participants viewed the Actissist app as acceptable and beneficial. Participants perceived two elements, a sense of support and normalisation and increased awareness of mental health, as the key that enabled Actissist to positively impact their mental health. Future developments of DHIs should increase the level of human support and explore the potential of adaptive sampling methods and generative AI technology to improve personalisation in frequency of prompts and content of the intervention. ISRCTN76986679, Registration Date: 07/02/2018. The online version contains supplementary material available at 10.1186/s12888-025-07071-0.