Attorneys for Michelle Carter, sentenced last summer to 15 months in jail for pressuring her boyfriend into taking his own life, filed papers with the state’s highest court on Friday appealing her conviction for involuntary manslaughter.

The legal brief argues that prosecutors did not prove Carter was responsible for the suicide of 18-year-old Conrad Roy III and that Lawrence Moniz, the juvenile court judge in Carter’s bench trial, “wrongly convicted Carter for a wanton and reckless failure to act,’’ though her indictment cited a “wanton and reckless affirmative act.’’

It also contends, among other claims, that Carter’s conviction for actions not spelled out in state law as being illegal violates her right to due process and that convicting her based on communications with Roy infringes on her First Amendment free-speech rights.

Carter’s trial focused on text messages she sent Roy in 2014, when she was 17, urging him to kill himself. She remains free while her appeal is pending.

Carter’s lawyers filed a notice of their intent to appeal last August, and in March the Supreme Judicial Court agreed to hear her appeal.

The brief filed Friday contends that prosecutors showed “cherry-picked’’ text messages to the grand jury that indicted Carter, presenting her as having “engaged in ‘a systematic campaign of coercion’ against Roy’’ that exploited his vulnerabilities.

In one text exchange days before Roy’s suicide, Carter wrote to him: “I still don’t think you want to do this so you’ll have to prove me wrong . . . hang yourself, jump off a building, stab yourself. I don’t know. There’s lots of ways.’’

At trial, though, according to the filing, the judge found that prosecutors didn’t prove that Carter caused Roy’s death, only that she made no effort to save him after she allegedly encouraged him to get back into a pickup truck that was filling with carbon monoxide.

But Shoshana E. Stern, a Bristol assistant district attorney, argued in a response to Carter’s legal team earlier this year that the trial judge found Carter guilty on both theories of involuntary manslaughter.

“While he discussed the ‘failure to act’ theory at some length, he also found that ‘instructing Mr. Roy to get back in his truck constituted wanton and reckless conduct by Ms. Carter,’ ’’ Stern wrote in court papers.

This is the first suicide-related case in legal history, Carter’s attorneys contend, in which a defendant was convicted for involuntary manslaughter based on words alone, even though she was not present. Carter was about 50 miles away as Roy sat dying in his truck.

“No state has interpreted its common law, or enacted an assisted-suicide statute, to criminalize such speech,’’ Carter’s attorneys wrote, “and no defendant has ever been convicted for encouraging suicide where the defendant neither physically participated nor provided the means.’’

Jeremy C. Fox can be reached at jeremy.fox@globe.com. Follow him on Twitter @jeremycfox.