For the most part, Zac Thornton has taken advantage of chances afforded by a lost Mets season and run with the opportunity.

Saturday, his gait slowed to a stumble.

The lefty, who has been one of the real curiosities of the second half while pitching far better than his pedigree, became victim to a laser show from Giants batters in a 9-5 loss in front of 30,046 at Citi Field.

Thornton’s 12th major league start was his worst, serving up four home runs in five innings — after letting up a total of seven in his initial 61 ⅓ innings — from Giants hitters who clearly looked to pounce early in the count.

Rafael Devers and Christian Koss needed to see just seven pitches from Thornton to launch four home runs, the unlikely pair of bash brothers (Koss, the No. 9 hitter, entering with four career home runs) teaming to knock out Thornton after he was charged with six runs on seven hits and a walk with six strikeouts.

New York Mets pitcher Zac Thornton (49) gives up a home run to San Francisco Giants first baseman Rafael Devers (16) in the first inning at Citi Field on Saturday, Sept. 5, 2026. Robert Sabo for NY Post

Every time Thornton gets the ball serves as an audition for next season. It is possible he enters this offseason as a depth option whom the Mets pencil in to the Triple-A Syracuse rotation next April. It is possible Thornton — who had not allowed more than four earned runs in a start until Saturday — pitches his way, in September and in camp, to a No. 5 job likely behind Nolan McLean, Christian Scott, Sean Manaea and a winter addition.

A 2023 fifth-round pick who pitched serviceably but not superbly with Syracuse, Thornton does not have the velocity to put away many batters.

He has to mix, think and locate to be successful, which he largely has during an impressive major league run in which he owns a 3.53 ERA.

But Devers and Koss were guessing early and guessing correctly, launching homers off four different pitches.

New York Mets pitcher Zac Thornton (49) gives up a two-run home run to San Francisco Giants third baseman Christian Koss (50) during the third inning. Robert Sabo for NY Post

Devers, the second batter of the game, pulled a third-pitch cutter to right for the first run; Koss turned on a second-pitch fastball that was about shoulder-high — far out of the zone — and swatted it to left for a two-run shot in the third inning; Koss added another in the fifth, keeping a first-pitch sweeper just fair and tucking it inside the left field foul pole; before Devers, two batters later, got a first-pitch slider on the outside edge and went with it for an opposite-field shot that made it 6-2.

Late Giants home runs against Jefry Yan and Justin Hagenman added to the deficit.

The Mets’ offense did not do enough against Anthony Molina and the Giants’ bullpen, managing just a two-run homer from A.J. Ewing in the second, a bases-loaded walk from Brett Baty in the seventh and a two-run double from Baty in the ninth.