[This paper is part of the Focused Collection on Curriculum Development: Theory into Design.] Common models of curricular development in physics education research (PER) have typically involved a hierarchical relationship between researchers and students, where researchers lead the design and testing of curriculum for students. We draw from work in students as partners and related fields in order to codesign curriculum in partnership with students. Such work has the potential to disrupt typical hierarchical relationships and interactions between students and faculty by involving students in the process of making curricular decisions. We invited undergraduate students to participate in a partnership to codesign a set of curricular materials for topics in quantum mechanics that students often struggle with. Four undergraduate students, one PER graduate student, and one PER faculty member met for a series of codesign meetings. We collected videotapes of the meetings, written artifacts, and meeting reflections. This paper presents a fine-grained analysis of one interaction in which researchers attempted to create space for students to contribute to decision making about how the collaboration should proceed. Through analyzing the complex dynamics of how participants negotiated decision-making space, including characterizing the types of decisions that were made, we describe how access to those decisions were opened up or cut off, and how those decisions contested or reaffirmed participants' roles. Working towards partnership is a complex and messy process: attempts to open up space for some forms of decision making closed off access to other forms of decision making. In some ways, the interactions between the participants also reified the traditional student and faculty roles that the partnership had intended to disrupt. Through closely analyzing these dynamics, we aim to self-critically reflect on the challenges and tensions that emerge in codesign partnerships.