MIAMI — In the end, after an unfathomable 3½-month descent that suggested they were more bottom feeder than World Series contender, the New York Mets’ assignment Sunday was simple: Beat the Miami Marlins and October baseball would follow.

The Mets, in fitting fashion for a frustrating season, fumbled the opportunity, falling 4-0 at LoanDepot Park to crush their playoff hopes on the last day of the regular season.

With the Cincinnati Reds losing to the Milwaukee Brewers on Sunday, a win would’ve sent the Mets backpedaling into the postseason but into it nonetheless as the National League’s third and final wild-card club. They understood the assignment required to avoid a historic collapse.

It still didn’t matter.

The Mets finished tied with the Reds in the standings at 83-79, but the Reds were awarded a ticket to the tournament for the first time since 2020 because they held the head-to-head tiebreaker. The Reds will start a three-game wild-card series against the Dodgers in Los Angeles on Tuesday night. The Mets went home Sunday scratching their heads.

“It’s hard to describe,” Mets manager Carlos Mendoza said. “I just got done addressing the team and there’s no word to describe what we’re going through. It’s pain. It’s frustration. You name it. We came in with a lot of expectations and here we are going home. Not only did we fall short, we didn’t even get into October.”

Said shortstop Francisco Lindor: “It was something that was on us, on me, on the players to get it done and we didn’t execute. We didn’t do the job.”

Reaching October seemed like a foregone conclusion all summer. Coming off an unexpected run to the National League Championship Series last season, a finish fueled by vibes and clutch moments, the Mets reloaded over the offseason by giving Juan Soto the richest contract in North American sports history, re-signing Pete Alonso, and adding complimentary players.

On June 12, through 69 games, they held the best record in baseball. Their pitching staff had the best ERA in baseball despite not having a bona fide ace and with Sean Manaea on the injured list. Soto, their $765 million investment, was off to a relatively slow start that generated external noise that the Mets dismissed. The thinking was the Mets were not even at their best yet. But that ended up being the peak.

The Mets went 38-55 the rest of the way, a record only better than four other clubs over that span. Ultimately, the results were reversed from 2024 when the team overcame a dreadful start to seal a postseason spot on the regular season’s final day.

“It’s beyond frustration,” said Alonso, who confirmed he will opt out of his contract and become a free agent again this offseason. “It’s straight-up disappointing. And there’s no other way to sugarcoat it. Just the way it is. Super-talented team and the reality is we fell short. I mean, we didn’t even get to October.”

A rash of pitching injuries, beginning with Kodai Senga straining his hamstring on June 12, combined with a rotation that couldn’t pitch deep into games enough, an overworked bullpen and baffling offensive inconsistency cooked up the disaster.

“We had rough moments with our rotation,” Soto said. “They got hurt. They were going up and down. They were unbelievable at the beginning and then they started getting hurt and that’s when everything started going down.”

The rotation’s shortcomings were evident this weekend down to Game 162. On Friday, the Mets gave the ball to Brandon Sproat, one of the three rookies who made their major league debuts over the season’s final six weeks and were given rotation spots in the heat of a playoff race. He surrendered four runs over 4⅔ innings in a loss to the Marlins that resulted in the Mets ceding control of their playoff destiny.

On Saturday, Clay Holmes, a reliever converted to starter this season, took the mound with a pitch limit of 70 and registered the best start of his career: six innings of one-hit ball on 78 pitches. Then on Sunday, the Mets picked Manaea — who signed a three-year, $75 million contract last winter — to start but weren’t willing to give him much of a leash. As a result, the left-hander was pulled at the first sign of trouble, after 1⅔ scoreless innings, to begin a game of bullpen roulette.

Manaea finished the season with a 5.64 ERA in 60⅔ innings over 15 outings after sitting out the season’s first three months because of oblique and elbow injuries.

“I think it starts with me,” Manaea said. “Being hurt half the season is not ideal. Not being out there with the boys but then to come back and perform like I did, it’s a lot of expectations for me, not just from myself but from the organization and everybody. … From my side, it’s a complete failure.”

The Marlins scored the game’s only four runs in the fourth inning off veteran relievers Brooks Raley, Ryne Stanek and Tyler Rogers. The Mets’ best chances to cut the deficit surfaced in the fifth and eighth innings.

In the fifth, with the bases loaded against Marlins starter Edward Cabrera, Alonso smashed a 116 mph line drive — his hardest hit ball of the season — to the left-center field gap that had an expected batting average of .780. But left fielder Javier Sanoja, shaded to the gap, tracked it down and snagged it to end the inning, leaving Alonso in shock down the first-base line.

“You don’t want to be in my mind for that one,” Alonso said. “I thought for sure it was a double and we were going to get rolling there.”

Said Mendoza: “I thought it was in the gap and we’re right back in the game. And before you know it, the guy’s making a play and it takes a toll on the guys. There was our chance right there and we didn’t capitalize.”

Three innings later, Francisco Alvarez, playing with a UCL sprain in his right thumb and a left pinkie fracture, struck out to leave runners stranded on first and second. Frustrated, Alvarez snapped his bat over his knee. With their final shot, knowing the Reds had lost and a win would erase the frustrating summer, Lindor grounded into a double play to end the Mets’ season 0-70 when trailing after eight innings.

One such comeback — whether in April, June or on Sunday — would’ve made the difference and given the Mets the chance to change how the 2025 season will be remembered. But that didn’t happen and 2025 joined 2007 and 2008 on the franchise’s list of collapses cemented with losses to the Marlins on the last day of the regular season.

“It’s a failure,” Soto said. “Anytime you don’t make it to the playoffs or win a championship, it’s a failure. That’s how we’re going to look at it, and that’s how we’re going to go through things in the offseason.”