Gov. Gavin Newsom on Sunday vetoed a California artificial intelligence safety bill, blocking the most ambitious proposal in the nation aimed at curtailing the growth of the new technology.

The first-of-its-kind bill, S.B. 1047, required safety testing of large A.I. systems, or models, before their release to the public. It also gave the state’s attorney general the right to sue companies over serious harm caused by their technologies, like death or property damage. And it mandated a kill switch to turn off A.I. systems in case of potential biowarfare, mass casualties or property damage.

Mr. Newsom said that the bill was flawed because it focused too much on regulating the biggest A.I. systems, known as frontier models, without considering potential risks and harms from the technology. He said that legislators should go back to rewrite it for the next session.

“I do not believe this is the best approach to protecting the public from real threats posed by the technology,” Mr. Newsom said in a statement. “Instead, the bill applies stringent standards to even the most basic functions — so long as a large system deploys it.”

The decision to kill the bill is expected to set off fierce criticism from some tech experts and academics who have pushed for the legislation. Governor Newsom, a Democrat, had faced strong pressure to veto the bill, which became embroiled in a fierce national debate over how to regulate A.I. A flurry of lobbyists descended on his office in recent weeks, some promoting the technology’s potential for great benefits. Others warned of its potential to cause irreparable harm to humanity.

California was poised to become a standard-bearer for regulating a technology that has exploded into public consciousness with the release of chatbots and realistic image and video generators in recent years. In the absence of federal legislation, California’s Legislature took an aggressive approach to reining in the technology with its proposal, which both houses passed nearly unanimously.

While lawmakers and regulators globally have sounded the alarm over the technology, few have taken action. Congress has held hearings, but no legislation has made meaningful progress. The European Union passed the A.I. Act, which restricts the use of riskier technology like facial recognition software.

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