The idea that feedback improves outcome in therapy is now supported by several studies and meta-analyses. The primary aim of this study will be to test the effectiveness of client-reported feedback through the DeePsy platform (a Czech feedback system). Furthermore, it is hypothesized that several mechanisms mediate the effect of feedback on outcomes; we will test the mediating effect of these variables. The experimental group (with feedback) and the control group (without feedback) will be compared through a within-therapist between-client design: each client will be randomly assigned to the experimental or control condition. In addition, the two conditions above will be compared to a no-feedback archival comparison group. Therapists will be trained prior to the experiment. The findings on feedback mechanisms will have important consequences for implementing feedback in routine care and for training psychotherapists.