Twenty-first-century education demands that students develop collaborative competencies alongside cognitive abilities, yet structured models for fostering and assessing collaborative skills remain limited, particularly those grounded in culturally responsive approaches. This study investigates how the Ethno-STEAM learning model—which integrates ethnomathematical principles with interdisciplinary STEAM learning through the LOCAL (Link with culture, Observe and organize, Connect with STEAM, Act through creation, and Learn and reflect) syntax—develops students' collaborative skills in authentic, culturally meaningful contexts. The LOCAL model positions each pedagogical stage to explicitly support specific collaboration indicators: active involvement, social interaction, decision-making, and evaluation. This study employed a mixed-methods design with 84 eighth-grade students across three junior high schools in Yogyakarta, Indonesia. Data were collected through self-assessment questionnaires, peer assessments, classroom observations, and structured interviews. Quantitative analyses included descriptive statistics and a paired samples t-test examining differences between self and peer assessments. Qualitative data underwent thematic analysis to explore students' collaborative reflections across LOCAL stages. Results indicated moderate to high levels of collaboration, with self-assessment social interaction scores highest (M = 3.27) and decision-making lowest (M = 3.04). A statistically significant difference emerged between self-assessment and peer-assessment scores (t(83) = 2.204, p = .030, d = 0.240), indicating positive self-awareness alongside need for multiple perspectives. Qualitative findings revealed that cultural context engagement strengthened active participation and that gender-based communication patterns influenced social interaction quality. The LOCAL model systematically scaffolds collaborative development while honoring cultural identity, offering educators a practical, pedagogically sound framework for integrating culturally grounded learning with 21st-century collaborative competencies.