Left-hander Garrett Crochet and the Boston Red Sox are in agreement on a six-year, $170 million contract extension, sources told ESPN on Monday, keeping the 25-year-old ace with the organization that traded for him this winter to lead its rotation.

The deal starts in 2026 and includes an opt-out after the 2030 season, when Crochet would be 31. It guarantees him the most money ever for a player with four-plus years of service, trumping the five-year, $137.5 million contract Jacob deGrom signed with the New York Mets in 2019.

Though the sides had been discussing an extension for months, they hit roadblocks because of the difficulty in valuing Crochet. He has thrown only 224 innings in his career, spending 2020 and 2021 as a reliever, sitting out 2022 after Tommy John surgery, returning to the bullpen in 2023 and transitioning to the rotation with the Chicago White Sox last year. Because of his lack of bulk numbers, Crochet will make only $3.8 million this year after a breakout season in which he struck out 209 and walked 33 in 146 innings.

Boston rewarded him like a frontline starter nevertheless, enticing him with ace-level money that does not include any deferrals and keeping him from reaching free agency after the 2026 season.

The Red Sox saw enough from Crochet this spring to put to rest that possibility. Should he opt out after 2030, Crochet still could receive another nine-figure deal. Left-hander Max Fried, who was 31 on Opening Day this year, received an eight-year, $218 million free agent contract from the New York Yankees. And at 32, left-hander Blake Snell signed with the Los Angeles Dodgers for five years and $182 million.

With a fastball that reaches 100 mph and a dastardly cutter he added to his repertoire last season, Crochet is among the best left-handed pitchers in baseball -- a factor in Boston paying heavily for him in dollars and players.

Giving up catcher Kyle Teel, outfielder Braden Montgomery, infielder Chase Meidroth and right-hander Wikelman Gonzalez in a trade for Crochet during the winter meetings illustrated the Red Sox were ready to transition from years of mediocrity to contention. With a solid major league core and the emergence of prospects Kristian Campbell -- who is now starting at second base for Boston -- outfielder Roman Anthony and shortstop Marcelo Mayer, the Red Sox spent the winter aggressively pursuing upgrades.

The signing of third baseman Alex Bregman to a three-year, $120 million contract and right-hander Walker Buehler to a one-year, $21.05 million deal added veteran big leaguers, though Crochet was the prize of the winter.

No pitcher who received a nine-figure contract had done so with fewer than 800 innings. That Crochet did with not even one-third of that speaks to the impression the 6-foot-6, 245-pound fireballer made on the Red Sox during his short time with the team.

Chosen No. 11 by the White Sox out of Tennessee in the 2020 draft, Crochet skipped straight to the major leagues, throwing six scoreless innings down the stretch of the COVID-shortened season. He remained in the big leagues the next year, serving as a high-leverage reliever for a White Sox team that won the American League Central Division.

Elbow reconstruction slowed Crochet's ascent and kept him out for all of 2022 and all but 12 innings in 2023. Chicago's decision to move him into the rotation proved prophetic, as Crochet made the AL All-Star team and was perhaps the most sought-after player at the trade deadline.

With a planned shutdown to limit his innings, Crochet let teams know that he would pitch only for a contender if given a contract extension. No team obliged the request, and Crochet spent the final three months of the season throwing no more than four innings per start.

Chicago put him back on the trade block over the winter and struck a blockbuster with Boston, which expressed interest in extending Crochet and eventually came to terms on a deal that can max out at $180 million with escalators.