The Fiscal Sponsorship Transparency Act of 2026 requires certain tax-exempt charitable organizations to report detailed information about their fiscal sponsorship arrangements, which are agreements where organizations receive and manage funds on behalf of other parties. Organizations must disclose the names of non-exempt parties involved, the amounts transferred, descriptions of activities funded, the responsible officer, and dates the arrangements began and ended. The bill also creates new penalties for "improper conduit arrangements" where organizations solicit contributions for specific non-exempt individuals or entities without maintaining genuine control over how the funds are used, imposing a 20 percent tax on organizations and 5 percent on responsible managers, with additional 100 percent and 50 percent penalties respectively if the violations are not corrected. The law specifically excludes private foundations and donor-advised funds from the reporting requirements and applies to tax years beginning after December 31, 2027. The Treasury Secretary is directed to issue regulations clarifying which arrangements must be reported and what constitutes proper discretion and control over funds.