The Chilean Constitutional Convention of 2021-2022 represents an unusual case of a deliberative body with few agenda restrictions provided by other political bodies and fulfilled by non-partisan members. This lack of institutional constraints imposes a challenge to reach stable equilibria for collective action. As such, it is an ideal scenario to study the role of ideology to shape the emergence of collective action. In this study, we estimate the ideology of each of its members using the first month of roll calls. Then, the estimated ideology is used to explain members’ decisions to cosponsor bills focused on setting the rules of the game for the Convention.