A woman who encouraged her suicidal boyfriend to kill himself in dozens of text messages and told him to “get back in” a truck filled with toxic gas has been jailed for 15 months for involuntary manslaughter.

Michelle Carter, now 20, was convicted in June by a judge who said her final instruction to Conrad Roy III caused his death. Carter was 17 when 18-year-old Mr Roy was found dead of carbon monoxide poisoning in July 2014.

Juvenile Court Judge Lawrence Moniz gave Carter a two-and-a-half-year jail sentence but said she had to serve only 15 months of that.

He also sentenced her to five years of probation. He granted a defence motion that would keep Carter out of jail until her appeals in Massachusetts courts are exhausted.

The judge called the case, which has garnered international attention, “a tragedy for two families”.

Carter’s lawyer, Joseph Cataldo, had asked the judge to spare his client any jail time and instead give her five years of probation and require her to receive mental health counselling. He said Carter was struggling with mental health issues of her own – bulimia, anorexia and depression – during the time she urged Mr Roy to kill himself.

“Miss Carter will have to live with the consequences of this for the rest of her life,” Mr Cataldo said. “This was a horrible circumstance that she completely regrets.”

Prosecutor Maryclare Flynn called probation “just not reasonable punishment” for her role in Mr Roy’s death. Prosecutors asked the judge to send Carter to state prison for seven to 12 years.

In dozens of text messages, Carter had urged Mr Roy to follow through on his talk of taking his own life. “The time is right and you are ready… just do it babe,” Carter wrote in a text the day he killed himself.

The sensational trial was closely watched on social media, in part because of the insistent tone of Carter’s text messages.

“You can’t think about it. You just have to do it. You said you were gonna do it. Like I don’t get why you aren’t,” Carter wrote in one text.

Mr Cataldo argued that Mr Roy was determined to kill himself and nothing Carter did could change that. He said Carter initially tried to talk Mr Roy out of it and urged him to get professional help, but eventually went along with his plan. Mr Cataldo also argued that Carter’s words amounted to free speech protected by the First Amendment.

In convicting Carter, the judge focused his ruling on Carter telling Mr Roy to “get back in” after he climbed out of his truck as it was filling with carbon monoxide and told her he was afraid.

The judge said those words constituted “wanton and reckless conduct” under the manslaughter statute.