Costello, the nation's No. 40 overall prospect, had narrowed his choices to Michigan, Stanford and USC in recent weeks, and had been close to making a commitment since the beginning of the month, finally feeling comfortable enough with his final choice to do it Thursday.
This is a significant recruiting win for Stanford, as the Cardinal elected not to take a scholarship quarterback in the 2015 class after missing on top targets Ricky Town and Brandon Wimbush. In the 2016 class, the Cardinal focused all their attention at the position on Costello and did not make an offer to any other signal-caller.
Costello broke onto the scene as a sophomore, throwing for 1,478 yards and 13 touchdowns for Santa Margarita Catholic High School in Coto de Caza, California. His first offer came the following April as Florida State came calling. Costello then raised his game as a junior, passing for 3,123 yards and 23 touchdowns.
Grabbing a commitment from Costello means David Shaw and staff don't have to regroup and begin recruiting other quarterbacks in earnest, and it should give the Cardinal an in-state recruiting bump. Costello is the biggest name at the position in state, and is good friends with ESPN Junior 300 wide receiver and teammate Dylan Crawford, who also holds a Stanford offer.
Cardinal coaches also earn some recruiting bragging rights with the commitment, as Costello's mother attended USC and the 6-foot-4, 216-pound quarterback has said he grew up very familiar with the Trojans' program. Stanford also beat out Michigan, where coach Jim Harbaugh undoubtedly used his success with the Cardinal as a recruiting pitch for the Wolverines.
Costello is commitment No. 2 in the 2016 class for Stanford, joining defensive tackle Bo Peek.
After missing out on Malik Henry, Shea Patterson and Costello, USC could go after another quarterback, although the need is lessened after the Trojans signed two ESPN 300 signal-callers in the 2015 class and appear to be in relative good standing with 2017 quarterbacks Tate Martell and Tua Tagovailoa.
Michigan's next quarterback target is likely Brandon Peters, an ESPN 300 quarterback who already holds an offer from the Wolverines.