This paper aims to assess the extent to which Brazilian companies adhere to the accuracy, balance, clarity, comparability, reliability, and timeliness criteria recommended by the Global Reporting Initiative (GRI) standards. A survey was conducted with experts in sustainability reporting with extensive experience in the Brazilian corporate context. The analytic hierarchy process was employed to assign weights to the criteria, and a grey fixed weight clustering model was proposed to analyze the collected data. Clarity and accuracy received the highest weights, which is concerning given the quality observed in sustainability reports. A lower emphasis on timeliness in sustainability reporting reflects many companies' publishing practices, prompting more efficient reporting. The majority of respondents perceive the quality of information in sustainability reports from Brazilian companies as falling within the medium maturity range, suggesting significant room for enhancement. The originality of this study lies both in the methodological approach, which combines multicriteria decision techniques capable of incorporating uncertainty related to data on sustainability reports, as well as in the results that provide information for improving sustainability reporting practices. The findings provide a foundation for companies and policymakers to enhance sustainability reporting, particularly in developing nations, promoting transparency, accountability, and international standard compliance.