This article investigates how the approval of Constitutional Amendment no. 97/2017, which prohibited proportional coalitions, reduced party fragmentation in Brazilian municipal chambers. The hypothesis raised is that the previous competitive context had an impact on the intensity of the reduction in the effective number of parties (ENP) between the 2016 and 2020 elections. Data from legislative electoral disputes at the municipal level in both elections were used, along with statistical techniques such as cluster analysis and ordinary least squares regression. The results indicate that, on average, there was a reduction in the ENP in Brazilian municipal chambers after the prohibition of proportional coalitions. However, the intensity of this reduction was greater in municipalities where parties were more intensely allied in the legislative competition. These data demonstrate that the configuration of the political competition market in previous elections is relevant for analyzing the degree of fragmentation observed in future elections conducted under new rules.