Seixas grew up in Lyon, the son of two competitive karate athletes, and started racing at eight with Lyon Sprint Évolution. He moved to Vélo Club Villefranche Beaujolais in 2021 and took a French cadet title on the road that year, then another in cyclo-cross in 2022. Cyclo-cross stayed on the winter programme through his junior years, and he added the French junior title in the discipline.
His 2024 season with the AG2R Citroën under-19 squad ran through the junior calendar. He won Liège-Bastogne-Liège Juniors in April ahead of Daan Dijkman, took the French junior time trial title and bronze in the European junior time trial. At the world championships in Zurich he won the junior time trial by six seconds from Jasper Schoofs, the day before his eighteenth birthday, then finished seventh in the road race.
Decathlon AG2R La Mondiale moved him straight into the WorldTour squad for 2025 rather than routing him through an under-23 team. He opened with fifth at the Grand Prix La Marseillaise and made his WorldTour debut at the UAE Tour. Second place at Paris-Camembert in April was his first professional podium, and he won the points classification at the Tour of the Alps later the same month.
He finished eighth overall at the Critérium du Dauphiné in June, the youngest rider to place inside the top ten of a WorldTour stage race. In August he won the Tour de l'Avenir, taking the race lead on the closing time trial ahead of Jarno Widar and Jørgen Nordhagen, and becoming the first French winner since David Gaudu in 2016. He did not ride a Grand Tour that year.
The autumn brought bronze in the European road race behind Tadej Pogačar and Remco Evenepoel, silver in the mixed team relay at the Kigali world championships, and thirteenth in the elite road race there. He closed the season seventh at Il Lombardia, his first Monument and the youngest top-ten finish in a men's Monument in more than a hundred years. He said afterwards that he had been aiming for the podium.









