In-service teachers implementing a science, technology, engineering, and math (STEM) curriculum centered on computational thinking (CT) work with unique content and pedagogical experiences. Understanding how curriculum design and teacher professional development affect curriculum implementation can help researchers understand the critical aspects of supporting the teacher in "just right" ways to learn and teach an embedded CT curriculum. We qualitatively analyzed 22 teachers' discourse through a case study approach. We identified how CT is afforded and constrained through curriculum design and teacher professional development support. Teachers expressed that the supports that were critical to their confidence and perceived ability to teach CT were (a) a program that provides "just right" support, (b) a program that provides options and individualization, (c) an internal personal identity that embraces continual innovation and learning, and (d) an educational system that is encouraging and supportive of the effort and creativity it requires to implement innovative and intensive embedded CT programs. These findings can inform how to support teachers to integrate CT within existing STEM curricula as a core scientific and mathematical practice and to foster student interest in computer science.