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 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.