Graduate student instructors (GSIs) play a critical role in higher education, serving as instructors of record for a substantial portion of courses at research universities. This study evaluates the structure and processes of Duke University Graduate School’s Teaching Triangles (TT) program, an interdisciplinary peer observation and feedback process designed to prepare graduate students for academic careers. Guided by the framework of communities of practice (CoP), this study explores how TT fosters pedagogical development, interdisciplinary learning, and professional preparation for GSIs. Utilizing a mixed-methods approach, the study analyzes 694 reflections from GSIs who participated in TT between Fall 2011 and Summer 2022. The findings indicate that TT effectively fosters a supportive environment for GSIs, promoting pedagogical self-reflection, confidence growth, and the adoption of improved teaching practices. Female GSIs, in particular, reported notable confidence gains, aligning with existing literature on gender and teaching self-efficacy. GSIs also valued the interdisciplinary nature of the program, which broadened their perspectives on teaching across various disciplines. Additionally, peer evaluation emerged as a key strength, allowing GSIs to experiment with new strategies in a non-hierarchical, low-stakes setting. This study provides recommendations for enhancing the TT program, such as modifying discussion formats to encourage timely feedback, expanding opportunities for long-term peer engagement, and developing a framework for continued evaluation. The findings emphasize the effectiveness of pedagogical training rooted in the CoP framework, demonstrating how peer observation programs can cultivate professional growth among graduate student instructors while fostering interdisciplinary collaboration in higher education.