This study examines the evolution of structural and effective features of a Teacher Professional Development Program (T-PDP) in Chile over a 5-year period, drawing on Desimone's and Kennedy's frameworks for effective T-PDPs. The program, called ARPA, was designed to enhance mathematical problem-solving skills in in-service teachers, has undergone continuous adaptation based on both theoretical models and participant feedback. Through content analysis of workshop materials and an in-depth interview with a key program member, this research explores how ARPA has integrated active learning, a content-specific focus, and teacher collaboration, while aligning with national educational standards. The findings highlight the program's commitment to continuous improvement, adapting its structure and content to meet emerging educational challenges. This study provides valuable insights for designing and implementing T-PDPs, emphasising the importance of a robust theoretical foundation combined with a flexible, responsive approach to evolving educational needs.