Background: Evasive personality disorder (AvPD) and social phobia (SP) have substantial costs for patients and their families and great economic costs to the society. While psychotherapy can be an efficacious treatment, many patients drop out during treatment. Increased knowledge on how to decrease dropout from psychotherapy is warranted, including how to increase a patient’s readiness for psychotherapy. Methods: We describe a randomized controlled feasibility and superiority trial of 42 individuals with a clinical diagnosis of either SP or AvPD, who are to initiate psychotherapeutic treatment in Danish outpatient mental health services. They will be randomized in a 1:1 ratio to either assessment-as-usual and receive no further assessment or to a Modified Collaborative Assessment (MCA) provided as a pre-treatment intervention before psychotherapy initiation. MCA will include a battery of psychological tests designed to thoroughly assess the patients’ psychopathology. The tests are administered in collaboration with the patient, including detailed oral and written feedback. We hypothesize that the intervention is feasible regarding patient’s acceptance and adherence. We further hypothesize that patients randomized to MCA will reach higher levels of readiness for psychotherapy as assessed with the University of Rhode Island Change Assessment Scale (URICA). Discussion: This protocol assesses the feasibility, efficacy, acceptability, and safety of an intervention aimed at changing the readiness for participation in psychotherapy of patients with SP and AvPD. Results from this feasibility study could guide the development of future large-scale trials of MCA and procedures for MCA treatment fidelity assessment. Trial registration: NCT2021001.