Teachers are rarely included as partners in school reforms or in planning teacher supports to enact new initiatives. The authors explored co-design as a professional learning (PL) framework to support localized enactment of a high school language arts project-based learning (PBL) course. They examined aspects that facilitated or limited progress toward PBL goals within the co-design PL. Results suggest that productive PL requires teachers to be positioned as key members from the onset who feel they have agency in adapting and enacting curriculum. When teachers are not part of the decision-making, it is difficult to agree on the focus and invest in the shared work. Co-design facilitated the professionalization of teachers when teachers had agency, partners developed trust through vulnerability, and partners experienced small successes together. The authors share the Partnership Planning Tool that can be used to identify the complex interplay between partners to support positive change.