Someone googled:
“Whose decision is it to put someone in jail if you fail a drug test while on probation?”
Well, if you fail a U/A while on probation, your probation officer will file a motion to revoke which causes paperwork asking that a warrant issues to be sent to the court for the judge’s signature.
On some minor violations the court will actually issue a summons instead of a warrant (at least in Austin) but most judges around here will authorize the warrant for a positive specimen.