We trace the development and analyze the generalizability of the Classroom Assessment of Sociocultural Interactions (CASI), an observation system designed to measure cultural dimensions of classroom interactions. We establish CASI measurement properties by analyzing panoramic videos of 4th and 5th grade classrooms from the Measures of Effective Teaching project, and argue for its significance in terms of achievement opportunity for minoritized students and needed evidence regarding equitable teaching. We frame ten dimensions of sociocultural interactions within three domains: Life Applications (i.e., connections with what students know and do outside of school); Self in Group (i.e., interdependence to motivate learning and foster social identities); and Agency (i.e., how freedom and choice are managed). We demonstrate how measurement error is associated with raters, lessons, and lesson segments, and discuss implications for CASI refinement, as well as appropriate instrument uses to enrich learning opportunities for minoritized students across a variety of classroom settings.