The Public Defender’s Office has gained a prominent position within the organization of the justice system and the public policies for access to justice established in Brazil by the 1988 Constitution. It was designed as a permanent institution, essential to the jurisdictional function of the State, with the responsibilities of providing legal assistance, promoting human rights, and defending - at all levels, both judicial and administrative - the individual and collective rights of individuals and social groups in situations of vulnerability, as a full and free legal aid. Based on a case study of the vaccination policy for the prison population in the state of São Paulo during the Covid-19 pandemic, this paper identifies and characterizes dimensions of technical and political institutional capacities mobilized by the Specialized Center for the Incarceration Situation of the Public Defender’s Office in its participation in the judicial regulation of this public policy. The methodological approach is qualitative, drawing on data from documentary sources and semi-structured interviews with public defenders who coordinate the center, as well as a professional from its multidisciplinary support team. As results, the article identifies technical capacities such as available resources, internal coordination, and the monitoring and evaluation of actions. In turn, political capacities are associated with interaction with civil society, the relationship with the High Administration of the São Paulo Public Defender’s Office, and the advocacy directed at institutions responsible for the contested public policy. We argue that a combination of these dimensions of capacity is necessary for the performance of the Public Defender’s Office in order to enhance the quality and impact of its actions. We also highlight some challenges for the improvement of its institutional policies.