The "Stress Testing Accountability and Transparency Act" aims to enhance the transparency and accountability of the Federal Reserve's financial stress testing processes. It mandates the Federal Reserve Board to issue a rule within 90 days detailing all models, assumptions, and methodologies used to determine the stress capital buffer requirement for bank holding companies, with any changes requiring notice and comment rulemaking. The Board must also ensure there is no double-counting of capital requirements for the same risks. Additionally, the bill requires the Board to issue a rule establishing the specific scenarios for stress tests at least 30 days before they are conducted, starting in the first calendar year after enactment. A significant provision prohibits the Board from subjecting nonbank financial companies to climate-related stress tests under its existing authority. Finally, the bill directs the Government Accountability Office to conduct a study and report to Congress every three years on the effectiveness of the Board's stress tests in evaluating the safety and soundness of nonbank financial institutions and the stability of the U.S. financial system.