Jett inched out Alpha Des Obeaux in a thrilling finish to the Irish Daily Star Chase at Punchestown.
The pair took the last fence together and battled it out on the run-in, with the Jessica Harrington-trained Jett (22/1) getting the verdict by a short head in the hands of Robbie Power.
Alpha Des Obeaux made a brave bid to lead all the way in the Grade Three contest, but was always being shadowed by Balko Des Flos and Jett.
There was little change in the order until Balko Des Flos was dropped by the other two after jumping the penultimate obstacle.
It was nip and tuck all the way to the line, with the outsider of six winning the day.
"He's a funny horse. He loves small fields, loves that ground and finally the trainer has got him over the right distance," said Harrington.
"That's the best round of jumping he's ever put in, because he wasn't under pressure and I think he's got a bit stronger this summer."
Jett could head to Wetherby next for the Charlie Hall Chase.
"He's not a horse to run in handicaps, so we'll look at Grade Two and Grade Three chases over three miles," said Harrington.
"He'll have to go to England now and maybe he could go to Wetherby (for the Charlie Hall)."
Harrington and Power had earlier teamed up with Conron (7/1) to win the Ladbrokes Where The Nation Plays Handicap Hurdle.
Power got a superb leap out of the five-year-old at the last to collar Ellie Mac and go on to score by four and a quarter lengths from Le Vagabond. Ellie Mac was third.
"He's a course specialist and loves it here. He won his maiden hurdle here, he's been second at the festival and he won on this day last year," said Harrington.
"He'll probably go to the November meeting now at Cheltenham at around that trip.
"He's been a great servant, is still only five and will be a lovely chaser next year."
Jan Maat won an incident-packed race for the Ladbrokes Acca Boosty Buck House Novice Chase.
Rachael Blackmore steered the Henry de Bromhead-trained gelding to a smooth victory over the only other finisher Cool Colonnade after Daly Tiger and Swordsman exited in dramatic circumstances early in the race.
Daly Tiger led to the fourth fence, going away from the stands, but he just got over the obstacle as his saddle had slipped forward giving Mark Walsh no option but to pull him up.
Swordsman came down at the same jump, leaving Jan Maat as the only one of the three Gigginstown House Stud-owned horses left in the four-horse affair.
Jan Maat (2/1) gamely stuck to his task and was seven and a half lengths too strong for Cool Colonnade.
De Bromhead said: "Obviously it was made a bit easier, but he jumped great and it's lovely to win a Grade Three.
"He's done really well and deserved to win one. He came up against Wicklow Brave a couple of times, and hopefully he'll keep progressing over fences.
"The Craddockstown is a possibility, but we'll see what everyone wants to do. He has been busy and on the go since the summer. Ground would also be important to him."
Swordsman's fall proved an unfortunate one for his rider Jack Kennedy, who was left with a broken collarbone that is expected to sideline him for around six weeks.
Blackmore, meanwhile, was completing a double after Noel Meade's Beacon Edge (4/6 favourite) benefited from the final-flight fall of The Very Man in the Bangor-on-Dee Annual Members Maiden Hurdle.
Meade said: "I do think he's a very nice horse and he might even be better over further, although he did show us plenty of boot last year. He did stay well in those bumpers we ran him in.
Emma Sweeney, 22, a primary school teacher in Dunshaughlin, enjoyed her first winner under rules on her first ride on the track over fences on the Gavin Cromwell-trained Allduckordinner (4/1), owned and bred by her mother, Una.
Kennedy suffers broken collarbone
Jack Kennedy faces around six weeks on the sidelines after breaking his collarbone in a fall from Swordsman in the Ladbrokes Acca Boosty Buck House Novice Chase at Punchestown.
Kennedy sustained the injury when he was unshipped from the Gordon Elliott-trained gelding at the fourth fence in the Grade Three contest won by Jan Maat.
He was stood down for the day and further examination revealed the extent of the damage.
Dr Jennifer Pugh, senior medical officer for the Irish Horseracing Regulatory Board, posted on the IHRB Twitter feed: "Following a fall today @punchestownrace, Jack Kennedy has a fractured collarbone which will rule him out for approximately 6 weeks."
Kennedy enjoyed another fine season last term, primarily for Elliott in the colours of Gigginstown House Stud, picking up six Grade One wins, three of them on top-class mare Apple's Jade.