LOS ANGELES — A California state court trial over allegations that Meta and Snap knowingly designed addictive features harming teenage users entered a critical phase on Friday, with plaintiffs' lawyers presenting internal company research to the jury. The case is the first of thousands of consolidated claims to reach trial.

The proceeding, part of coordinated litigation before the Los Angeles Superior Court, centres on bellwether plaintiffs who allege that engagement-maximising design choices contributed to anxiety, depression and self-harm among minors. Attorneys for the families argued that both companies prioritised screen time over documented risks to young users.

Lawyers for Meta Platforms and Snap Inc. rejected the claims, arguing that their products include parental controls and that the companies are shielded by Section 230 of the Communications Decency Act. Judge Carolyn Kuhl has allowed several design-defect claims to proceed despite those objections, narrowing the companies' immunity defence.

The outcome carries weight far beyond the individual plaintiffs. More than 1,800 personal-injury suits and separate actions brought by dozens of US state attorneys general hinge on how juries weigh evidence that platform features were engineered to be compulsive.

A plaintiff verdict would strengthen the broader coordinated litigation and pressure the firms toward settlement. Tidjane Deme, a plaintiffs' attorney involved in the coordinated cases, said the trial would test whether design choices themselves, rather than user-posted content, could expose platforms to liability for the first time at this scale.