Right-hander Luis Castillo and the Seattle Mariners are in agreement on a five-year, $108 million contract extension that will keep the prized trade-deadline acquisition part of the ascendant team's core for the foreseeable future, sources familiar with the deal told ESPN on Saturday.

The contract, which includes a vesting option that could take its total value to $133 million, buys out four years of free agency for the 29-year-old, who has been dominant in eight starts since joining Seattle before the Aug. 2 trade deadline. In 54 innings, Castillo has struck out 64, walked 13 and allowed just five home runs with a 2.83 ERA -- even better than the 2.86 ERA he posted in 14 starts with the Cincinnati Reds and a key element in helping push the Mariners to the precipice of their first playoff berth in 21 years.

A two-time All-Star, Castillo will be paid on an annual basis among the upper echelon of pitchers. The deal includes a $7 million signing bonus, a $10 million salary for next season -- when he was due to make around $12 million in his final arbitration season before reaching free agency -- and annual salaries of $22.75 million from 2024 to '27.

The contract also includes a variation on the sixth-year option. If Castillo undergoes elbow surgery, including Tommy John surgery, to address an issue with the ulnar collateral ligament in his right elbow in the 2025, 2026 or 2027 seasons that causes him to miss at least 130 days, the Mariners receive a club option for $5 million in 2028.

His 2028 option for $25 million vests if he throws at least 180 innings in 2027, and Castillo will receive a full no-trade clause for the 2023 to '25 seasons with a $1 million assignment bonus if he's moved after that.

The Castillo deal continues a watershed year of spending for the Mariners, who in the winter signed American League Cy Young winner Robbie Ray to a five-year, $115 million free agent deal and star rookie Julio Rodriguez to a deal that guarantees him at least $209.3 million -- and almost certainly will be worth more, depending on his performance and its effect on potential options.

Since Castillo's first start for Seattle on Aug. 3, the Mariners have the third-best starter ERA in the major leagues at 3.04 and second best in the AL behind division rival Houston. Among Castillo, Ray, 25-year-old Logan Gilbert and 24-year-old George Kirby, the Mariners' plethora of starters -- backed by an equally excellent bullpen, which over that same time period has the fourth-best ERA in the big leagues at 2.63 -- makes them a potentially scary playoff team.

Castillo is expected to start Game 1 of a potential playoff series. At 81-67, the Mariners occupy the third and final wild-card spot in the AL and hold a four-game lead on the next-closest team, Baltimore. If that holds, Seattle would play a best-of-three wild- card series in Cleveland, presently in first place in the AL Central.

Seattle sought an impact arm at the deadline and paid a hefty price for Castillo, giving Cincinnati its two top prospects in shortstops Noelvi Marte and Edwin Arroyo along with starter Levi Stoudt and reliever Andrew Moore.

Castillo had been traded four previous times before landing in Cincinnati, where his explosive two-seam fastball and solid slider and changeup led him to the front of the Reds' rotation. Over six seasons with Cincinnati, Castillo went 47-54 with a 3.62 ERA and nearly a 3-to-1 strikeout-to-walk ratio.