Purpose Support implementation fidelity in intervention research with lesbian, gay, bisexual, transgender, queer, and sexual and gender diverse (LGBTQ+) populations, this study explores the systematic development of a fidelity process for AFFIRM, an evidence-based, affirmative cognitive behavioral therapy group intervention for LGBTQ+ youth and adults. Method As part of a clinical trial, the AFFIRM fidelity checklist was designed to assess clinician adherence. A total of 151 audio-recorded group sessions were coded by four trained raters. Results Adherence was high with a mean fidelity score of 84.13 ( SD = 12.50). Inter-rater reliability was 81%, suggesting substantial agreement. Qualitative thematic analysis of low-rated sessions identified deviations from the manual and difficulties in group facilitation, while high-rated sessions specified affirmative and effective clinical responses. Discussion Findings were integrated into clinical training and coaching. The fidelity process provides insights into the challenges of implementing social work interventions effectively with LGBTQ+ populations in community settings.