<p>Outcome measurement hierarchy.</p>

Heterogeneity was substantial across most outcomes (<i>I2</i> > 75%). Benefits were observed across healthy (61 trials; <i>n</i> = 15,770), mental health (41 trials; <i>n</i> = 6,130), and medical samples (46 trials; <i>n</i> = 4,491), with population type not significantly moderating immediate post-intervention effects. Comparator type significantly moderated several analyses, with smaller effects against active than inactive controls.

Guidance did not significantly moderate immediate post-intervention effects, although it predicted larger short-term depression effects (<i>QM</i> = 4.17, <i>p</i> = .048). At short-term follow-up, effects remained statistically supported across all outcomes, while only depression, anxiety, and stress remained statistically supported at medium-term follow-up. GRADE certainty was moderate for most immediate post-intervention outcomes and low for quality of life.

Digital third-wave interventions produced small-to-moderate average benefits across symptom-focused, wellbeing-focused, and process outcomes. Effects were smaller against active comparators, indicating that pooled benefits should be interpreted in relation to comparator intensity. Future research should prioritise rigorous active controls, longer follow-up, and direct tests of whether changes in psychological flexibility mediate improvements in symptoms and wellbeing.</p></div>