For those who have paid only fleeting attention to this nightmarish campaign, the Mets conveniently crammed all of their shortcomings into one awful inning Friday night.

The Readers’ Digest version of a season unhinged featured Matt Harvey reverting to shell-of-his-former-self form, followed by a meltdown from a bullpen withering beneath the stress of overuse.

For the Mets, the result was a 12-7 defeat, one that unspooled during a seven-run sixth inning in which 12 Pirates batted.

The culprit: a pitching staff that has devolved into the Mets’ biggest weakness and began the day with the second-worst ERA in the National League (4.75)

“One of the things we’ve been very lucky and very fortunate to see here in the last couple of years is the consistency of what you’re going to get,” manager Terry Collins said. “And right now, it seems like when we start a game, we’re not sure what we’re going to get. That’s not a real good feeling.”

In the lineup because Francisco Cervelli was a late scratch with an illness, backup catcher Elias Diaz knocked in a career-high six runs. He hit a three-run homer off reliever Paul Sewald, who was tagged for a career-high five runs in one-third of an inning. Diaz also lashed a three-run double off Harvey, who squandered a bounce-back night from the offense.

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“If you score seven runs, we should win the ballgame,” said Harvey, who failed to pitch six innings for the seventh time in 11 starts.

Lucas Duda hit a pair of mammoth homers that traveled a combined 846 feet. He has six homers in his last nine games.

Michael Conforto hit his 14th homer, a two-run shot to the opposite field. His blast came in the Mets’ five-run fifth inning, enough to open a 7-4 lead.

But nothing is a given, especially with a pitching staff that has crumbled beneath the weight of injuries and ineffectiveness. Even signs of recent improvement increasingly have looked like a mirage.

Matt Harvey gets ready to exit mound during the sixth inning against the Pirates at Citi Field on Friday, June 2, 2017. Photo Credit: Kathleen Malone-Van Dyke

Just five days earlier, Harvey turned in one of his best starts of the season, beating the Pirates by holding them to one run in six innings. On Friday night, he regressed in his five innings-plus, allowing six runs, five hits and four walks with five strikeouts.

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Harvey loaded the bases with nobody out in the first but allowed only one run. He didn’t get away with loading the bases in the fourth, though. That’s when Diaz struck with his three-run double.

Harvey took a three-run lead into the sixth but promptly allowed a solo homer by Josh Bell and walked Andrew McCutchen, ending his night.

In came Sewald, who had allowed only three runs in his previous 20 innings, lowering his ERA to 2.21. In a burned-out bullpen, he had emerged as a rare burst of light. The Pirates made it their business to extinguish that flame.

“Today was tough,” said Sewald, who faced eight hitters but retired only one, left to sort through it because the bullpen again was shorthanded thanks to recent use.

Included in the fiasco was Diaz’s three-run shot and a bases-loaded walk to Bell that forced in another run, one of the five on Sewald’s ledger.

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Soon the carnage was complete, with the Mets (23-30) forced to stomach another night sabotaged by the lousy pitching that has been their undoing. They have lost 14 of their last 21 games in falling 11 games behind the NL East-leading Nationals and nine games out of a wild-card spot.

“Right now, these are the guys that have to get it done,” Collins said. “They have track records of getting it done. They’ve got to step up.”