Cosart and Seaton, both right-handers, were among seven Minor League players placed on the 40-man roster, meaning they'll be in spring camp next year competing for a job. The Astros also added right-handed pitchers Jose Cisnero and Chia-Jen Lo, outfielder Robbie Grossman, left-hander Brett Oberholtzer and infielder Jonathan Villar.
"It's a big deal to be on the 40-man -- you're one of the guys," Cosart said. "You want to be on the 25-man roster, though. You want to get to the big leagues and win games in the big leagues. That's ultimate goal -- to get on the 25-man roster and be there for a long time."
The Astros' 40-man roster stands at 36 after the Astros outrighted infielder Scott Moore to Triple-A Oklahoma City and left-handed reliever Mickey Storey was claimed off waivers by the Yankees. Moore indicated he's likely to become a free agent.
The moves leave the Astros with four available slots to sign free agents or add someone from the Rule 5 Draft during which Houston will have the first pick. Among the players the Astros didn't protect are pitchers Josh Zeid and Jason Stoffel and outfielder Marc Krauss.
"We're at the point now as an organization we are starting to build some depth, so you are risking losing some guys by leaving them off the 40-man," Astros general manager Jeff Luhnow said. "We felt good about the guys we added."
Major League Baseball's deadline to protect players was Tuesday at 10:59 p.m. CT. Players first signed at age 18 must be added to 40-man rosters within five years or they become eligible to be drafted by other organizations through the Rule 5 process. Players signed at 19 years old are to be protected within four years.
Clubs pay $50,000 to select a player in the Rule 5 Draft, to be held on Dec. 6 at the conclusion of the Winter Meetings in Nashville, Tenn. If that player doesn't stay on the 25-man roster for the full season, he must be offered back to his former team for $25,000.
"I think anybody that comes to big league camp has a shot to make the team," Luhnow said. "The guys that had more time at Triple-A are more likely to establish themselves in Spring Training. These are all players that have a Major League future with the Astros."
Cosart, 22, is a hard-thrower who's the team's top pitching prospect. He went 6-7 with a 3.30 ERA in 21 combined appearances (20 starts) at Double-A Corpus Christi and Oklahoma City last season and appears poised to compete for a spot in the rotation behind Bud Norris, Lucas Harrell and Jordan Lyles.
He was acquired by the Astros from Philadelphia on July 29, 2011, as part of the trade involving Hunter Pence. The Phillies drafted him in the 38th round out of Clear Creek High School in League City, Texas, in 2008.
"It's just another stepping-stone," he said. "You have to work hard and put in the time, especially for high school kids. You're not as advanced, you're really raw and when you come out of high school you have to go level by level and step by step.
"It's a grind, but that's my goal and I don't look at it as much more than that -- I'm coming to Spring Training. I've put my time in the Minor Leagues, and [manager Bo] Porter told me the best 25 guys are going to get on that plane when we break Spring Training."
Seaton, 23, went a combined 8-9 with a 3.93 ERA and 117 strikeouts in 29 starts between Oklahoma City (four) and Corpus Christi (25). A product of Second Baptist High School in Houston who was drafted in the third round in 2008, he spent the majority of the 2012 season with Corpus Christi, going 8-8 with a 4.07 ERA and just 31 walks.
"Man, it's such an exciting opportunity," Seaton said. "When I was told I was going to be on the 40-man roster, I was in awe and taking it all in and thanking Jesus. This is a great opportunity here, especially growing up in Houston. You're literally one step away from being in the big games for the team I grew up watching."
Cisnero, 23, was among the organization's leaders in victories in 2012, posting a 13-7 record in 28 combined starts at Corpus Christi and Oklahoma City with a 3.70 ERA. Cisnero, who was twice named Texas League Player of the Week, had one of the top strikeouts-per-innings ratios among starting pitchers with 148 strikeouts in 148 1/3 innings of work.
Grossman, 23, was acquired by the Astros from Pittsburgh on July 24 as part of the trade involving Wandy Rodriguez. In 131 combined games at Double-A Altoona and Corpus Christi, he hit .266 with 28 doubles, six triples, 10 home runs, 47 RBIs, 77 walks and 13 stolen bases while posting a .376 on-base percentage.
Lo, 26, is coming off a solid Arizona Fall League season, during which he appeared in 14 games and went 1-0 with three saves and a 2.40 ERA for the Mesa Solar Sox. He missed the first two months of the regular season while recovering from Tommy John surgery, but returned to post an 0.90 ERA in 19 appearances combined for the Astros' Gulf Coast League affiliate and Class A Lancaster.
Oberholtzer, 23, split his 2012 season between Corpus Christi and Oklahoma City, combining to post a 10-10 record with a 4.37 ERA in 28 starts. He began his 2012 season with Corpus Christi and earned Texas League All-Star honors before his promotion to Oklahoma City, where he went 3-5 with a 3.31 ERA in 10 starts.
Villar, 21, hit .261 with 54 runs, 11 homers, 50 RBIs and 39 stolen bases in 86 games with Corpus Christi in 2012. The switch-hitting shortstop was a 2012 Texas League Mid-Season All-Star and led the league in steals despite missing the final 54 games of the season due to a broken hand suffered punching a door.
Moore, 29, hit .259 with nine homers and 26 RBIs in 201 at-bats in a part-time role with the Astros last year. He admitted he was disappointed by the Astros' decision to remove him from the 40-man roster.
"I'm a little bit surprised by the decision, and I'm not sure if I would come back yet," he said.