SummaryAim. The fidelity of an intervention can be defined as the degree to which a program is implemented following the program model, i.e. a set of well-defined procedures for such intervention. This article aims to describe the steps taken to develop an assessment tool of fidelity to the original protocol of intervention for Critical Time Intervention -Task Shifting (CTI-TS). Methods. The development of an instrument to assess fidelity includes some key stages, such as def-The development of an instrument to assess fidelity includes some key stages, such as definition of the intervention model, definition of the key components of the intervention, preparation of a preliminary scale, development of fidelity assessment sheets, development of a manual on the application of fidelity scale, training fidelity assessor and testing the preliminary scale by carrying out a pilot study. results. Based on the experts' opinion, 18 key components of the intervention were defined. From these components, a fidelity scale consisting of 20 items was built, divided into three sections, each evaluating one of the types of fidelity: compliance, context and competence. Conclusion. The CTI-TS had its fidelity assessment method developed in accordance with the recommendations in the current literature. The protocol compliance is essential for the results, whether positive or not, can be credited to the intervention itself.