A federal appeals court largely upheld a California law on Tuesday, making it illegal, absent parental permission, for social media companies to provide children with "addictive feeds" that the state fears could damage their mental health.

The 9th U.S. Circuit Court of Appeals rejected most claims by the technology trade group NetChoice, which said California's Protecting Our Kids from Social Media Addiction Act was overbroad and vague and violated the First Amendment.

Addictive feeds are algorithms that select personalised media for users based on those users' online behaviour.

NetChoice, whose 41 members include Google, Facebook and Instagram parent Meta Platforms, Netflix, and Elon Musk's X, said the law signed by Governor Gavin Newsom last September unconstitutionally limited members' ability to speak to children through the algorithms.

Writing for a three-judge panel, Circuit Judge Ryan Nelson said the issue of which algorithm-based feeds were "expressive" for First Amendment purposes was fact-intensive, and NetChoice did not show the California law's alleged unconstitutional applications predominated.

Nelson also found NetChoice premature in challenging a requirement that platforms take steps to verify users' ages, rather than simply limit feeds to users it knows are children, because the requirement doesn't take effect until 2027.

The court blocked a requirement that accounts' default settings prevent children from seeking how many likes and other comments their posts receive. It said that the requirement was not the least restrictive way to protect children's mental health.

Paul Taske, co-director of the NetChoice Litigation Center, said the group is "largely disappointed" with the decision.

"California's law usurps the role of parents and gives the government more power over how legal speech is shared online," Taske said. NetChoice has filed many lawsuits challenging state-level internet restrictions.

Spokespeople for California Attorney General Rob Bonta, who defended the state's law, did not immediately respond to requests for comment.

The appeals court returned the case to U.S. District Judge Edward Davila in San Jose, California, who enjoined other parts of the law last December 31.

"For the most part, the district court got it right," Nelson wrote.

The case is NetChoice LLC v Bonta, 9th U.S. Circuit Court of Appeals, No. 25-146.