The pitchers are among 30 players on the PCL roster who will take on the International League All-Stars.
"The talent in our system continues to filter through to every level, where you can point to a handful of guys that you can imagine playing and having an impact in the Major Leagues," Astros general manager Jeff Luhnow said. "That's the way it's supposed to be, and these selections reinforce our belief in the quality of players we have."
Cosart, the club's No. 4 prospect according to MLB.com, was one of 13 players on the squad who were picked by managers, media members and online fan voting. He's got the third-best ERA in the league (2.98), the second-lowest opponent batting average (.205) and the fourth-highest strikeout total (90).
"Cosart is one of the top pitching prospects in baseball, and he's been consistently going out there and delivering good start after good start with dominating stuff," Luhnow said. "He's the type of guy whose success will translate pretty quick to the big leagues when he gets a chance to come up here.
"He's not a complete finished product yet, but he's getting closer every day. The selection is well deserved."
In 17 games -- 16 as a starter -- Cosart has paced the Redhawks with seven wins, 87 2/3 innings of work and 9.32 strikeouts per nine innings.
Stoffel is a reliever, and he was one of 17 players on the All-Star roster who were selected by the PCL office. The righty has a 1.89 ERA in 23 outings, striking out 27 in 33 1/3 innings and never allowing multiple runs in an appearance.
"He's a prospect we've been high on for a while," Luhnow said. "He has the stuff to put big league hitters away. We expect to see him in the bullpen up here next year."
It marks a second consecutive All-Star trip for the duo, as both were named Texas League Mid-Season All-Stars last year while pitching for Double-A Corpus Christi.
The Triple-A All-Star Game will be broadcast live on MLB Network, the American Forces Network and the Triple-A All-Star Game Radio Network at 8:05 p.m. CT on Wednesday, July 17.