Objective/Context: This article analyzes why the Petro administration’s pension reform was approved through ordinary legislative procedures, whereas other social reforms failed or required extra-legislative pressure. We argue that the outcome cannot be explained solely by the government’s ideology, democratization, or social pressure, nor by the legislative coalition-building tools identified in the literature on coalition presidentialism. Rather, it is explained by a specific configuration of political-institutional and sectoral factors. Methodology: The legislative process was reconstructed through process tracing, analyzing changes to the articles, public hearings, committee and plenary debates, media coverage, and technical documents. Counterfactual comparisons with government reforms were used to assess the plausibility of each causal condition. Conclusions: We identified a causal sequence of four necessary and jointly sufficient conditions explaining the success of the reform: (1) a prior technical-political consensus on the need to reform the pension system; (2) the structural weakness of the losers and opponents; (3) a pre-agreement that limited dissent and reduced vetoes, and (4) programmatic-clientelistic negotiations focused on Congress. Originality: The article distinguishes between the factors that explain reformist intentions and those that explain their actual approval in the legislative branch; it brings the literature on the expansion of social policy and executive-legislative relations into dialogue. Regarding the latter, it shows that its explanatory factors are, for the most part, invariant at the country-year level, with the relevant variation occurring at the sectoral level. It introduces technical-political consensus, pre-agreement, and the opposition's weakness as overlooked sectoral conditions. It also demonstrates the complementary role of programmatic and clientelistic negotiations in contexts of volatile parties and particularistic practices.