The text message is the basis for one of the key arguments by the prosecution that Michelle Carter is responsible for the death of Conrad Roy, who killed himself using carbon monoxide from a generator in a Fairhaven parking lot in July 2014.
Carter was 17 at the time, and prosecutors said that the teenager sent her "scared" boyfriend text messages and phone calls, pressuring that he kill himself.
Prosecutors said both received similar texts from Carter after Roy died, saying she "heard him die", and that text messages between Mosolgo and Carter show Carter felt like she didn't have friends at school.
Roy was depressed after his parents' divorce, was physically and verbally abused by family members and had long thought of suicide, and it was Carter who urged him to get help, Cataldo said.
Carter was admitted to a hospital with her own mental health struggles for a period prior to Roy's suicide and was taking anti-depressant medications known to cloud the judgment and impulse control of adolescents, Cataldo said.
The defense team argues that Roy's death was a "tragic suicide", not a homicide.
Mr Cataldo said: "It was Conrad Roy's idea to take his own life, it was not Michelle Carter's idea".
Carter: So I guess you aren't gonna do it then.
In another text to Roy's mother Lynn, Carter wrote she wanted to organize a baseball tournament in his honor, and that the proceeds would go to suicide prevention.
Camdyn Roy, Conrad's 16-year-old sister who was 13 at the time of his death, told a similar story on the stand.
Carter waived her right to trial by jury on Monday, so the final decision will be up to a judge in a juvenile court, because she was a juvenile at the time of Roy's death.
But Carter's lawyer has argued that text messages are protected free speech.
The case could set a legal precedent if Carter is convicted; no law against assisted suicide or encouraging suicide exists in MA. "What I think is going to be significant, at least in the facts of this case, is that this was repeated, lengthy electronic communications between two young people". He was 18. Carter, now 20, is on trial this week on involuntary manslaughter charges. She faces up to 20 years in prison if convicted.
Roy's mother Lynn Roy testified that her son showed no signs he meant to harm himself, but acknowledged there was tension between Roy and his father.
"Usually, manslaughter charges involve direct action by the defendant ... some type of horrific unintentional killing where the behavior disregarded a risk, like firing a gun into a crowd", Medwed said. Her brother did not seem sad at the beach, she testified.
The defense consented to a non-jury trial, meaning Judge Lawrence Moniz will act as judge and jury. She is being tried as a youthful offender, a status that allows harsher punishments than typical juvenile cases and allows for court files to be open to public inspections.