California SB 53: The First US Frontier AI Safety Law

California's SB 53 — the Transparency in Frontier Artificial Intelligence Act — was signed September 29, 2025, making California the first US state to enact a frontier AI safety law. It took effect January 1, 2026. Here is what it requires, how it compares to the New York and Illinois laws that followed it, and why its federal deference mechanism may matter more than anything else in it.