LOS ANGELES — TikTok agreed to settle a landmark social media addiction lawsuit just before the trial kicked off, the plaintiff’s attorneys confirmed.

The social video platform was one of three companies — along with Meta’s Instagram and Google’s YouTube — facing claims that their platforms deliberately addict and harm children. A fourth company named in the lawsuit, Snapchat parent company Snap Inc., settled the case last week for an undisclosed sum.

Details of the settlement with TikTok were not disclosed, and the company did not immediately respond to a request for comment.

At the core of the case is a 19-year-old identified only by the initials “KGM,” whose case could determine how thousands of other, similar lawsuits against social media companies will play out. She and two other plaintiffs have been selected for bellwether trials — essentially test cases for both sides to see how their arguments play out before a jury and what damages, if any, may be awarded, said Clay Calvert, a nonresident senior fellow of technology policy studies at the American Enterprise Institute.

Joseph VanZandt, co-lead counsel for the plaintiff, said in a statement Tuesday that TikTok remains a defendant in the other personal injury cases, and that the trial will proceed as scheduled against Meta and YouTube.

Jury selection starts this week in the Los Angeles County Superior Court. It’s the first time the companies will argue their case before a jury, and the outcome could have profound effects on their businesses and how they will handle children using their platforms. The selection process is expected to take at least a few days, with 75 potential jurors questioned each day through at least Thursday. A fourth company named in the lawsuit, Snapchat parent company Snap Inc., settled the case last week for an undisclosed sum.

“This was only the first case — there are hundreds of parents and school districts in the social media addiction trials that start today, and sadly, new families every day who are speaking out and bringing Big Tech to court for its deliberately harmful products,” said Sacha Haworth, executive director of the nonprofit Tech Oversight Project.

KGM claims that her use of social media from an early age addicted her to the technology and exacerbated depression and suicidal thoughts. Importantly, the lawsuit claims that this was done through deliberate design choices made by companies that sought to make their platforms more addictive to children to boost profits. This argument, if successful, could sidestep the companies’ First Amendment shield and Section 230, which protects tech companies from liability for material posted on their platforms.