Non-state actors are increasingly participating in international climate diplomacy. The tactics employed by diverse civil society agents to influence climate policymaking are radicalizing through the adoption of more confrontational language. Activist groups have been seeking opportunities to influence policymakers regarding the rules related to transparency, public participation and accountability in the Kyoto Protocols Clean Development Mechanism (CDM). By scrutinizing efforts of three environmental NGOs (ENGOs) - Climate Action Network, Center for International Environmental Law and Carbon Market Watch - the analysis concentrates on what tactical shifts have occurred in the framing positions and approaches of these activists during the 1997-2015 period. After several years of legal advocacy, expertise and/or critique in an effort to reform input legitimacy of CDM governance, the selected ENGOs have recently drifted away from narratives of green governmentality and ecological modernization and, instead, radicalized their rhetorical tactics by turning to a human rights perspective under the umbrella of climate justice.