Lawyers for a teenager found unfit to plead to animal cruelty charges, and receiving medication to quell homicidal fantasies, have applied for costs.

But the prosecution pledged to fight the attempt.

The application came after magistrate Peter Dingwall on Wednesday ruled the Canberra boy, who cannot be named, was mentally impaired.

A court can dismiss charges against a mentally ill person, and place them under the power of the ACT Civil and Administrative Tribunal, if deemed appropriate.

In December, Mr Dingwall dismissed a charge of aggravated animal cruelty causing death and remitted the 17-year-old to the care of ACAT.

The tribunal made a community care restriction order that places the boy under almost 24-hour scrutiny in out-of-home care, except when in his room, and prevents him from accessing the internet. Conversations, except those with his parents, are reported to youth justice staff and effectively he has zero contact with peers.

Mr Dingwall, in the ACT Children's Court, on Wednesday dismissed under mental health laws the four outstanding animal cruelty charges. Details of the alleged offences cannot be reported.

Defence lawyer Jacob Robertson used the dismissal to apply for $7324.50 in court costs.

But prosecutor Anthony Williamson pledged to vigorously contest the application.

''It's outrageous that someone who has clearly been proven to have committed these crimes has the audacity to apply for costs,'' Mr Williamson said.

Mr Dingwall reserved his decision and ordered cost submissions be made in writing by March.

The teen was released on bail late last year to live in a supported location after spending about eight months on remand after expressing homicidal fantasies after his arrest for the animal cruelty offences last year.

Two psychiatrists diagnosed him as having Asperger-type autism with probable youth psychopathic traits. Michael Inman