On June 1, a short, emotional video popped up on TikTok with news about Sarah Mandel, a 42-year-old psychologist, wife and mother of two young daughters.

“If you’re reading these words right now,” the message on the screen said, “then I have died.”

She had asked for help creating the video while in a weakened state after learning that she had little time left.

“Only a year ago, I never in a million years would have thought I would want my death announced on social media,” she said in the video. But in the previous year, she had found encouragement from the comments on TikTok after posting videos about the course and treatment of the advanced, metastatic breast cancer she had been diagnosed with in 2017.

And so there she was, delivering the news to the world and a cosmic message to her daughters, Sophie, 10, and Siena, 6.

“I love you and I am so proud of you,” she told them. “I may be somewhere beyond our concepts of infinity. That’s how much I love you.”

Ms. Mandel’s final video, a series of pictures and clips, showed her very much alive: blowing out a candle on a 42nd birthday cupcake; stretching in her hospital room; dancing with her family; looking thrilled after opening a box filled with copies of her newly published book; and singing along with a band in a plaza after leaving a chemotherapy session.