After graduating from Nihon University, Kumagai became a member of the Biprogy badminton team on April 1, 2024.

They finished as runner-up after losing in the final against the world's 42nd-ranked pair from France, Lucas Corvée and Ronan Labar.

In June 2024, Kumagai competed in the 74th All Japan Business Corporation Championships as part of the Biprogy team.

In the semi-final, they defeated Kazuki Shibata and Naoki Yamada, representatives of the Japan national team, in two straight games, and in the final, they defeated the 2021 world champions Takuro Hoki and Yugo Kobayashi, also in two straight games (21–8, 21–18).

[5] Kumagai is a member of the 2025 Japan National Team, playing men's doubles with Nishi and coached by Lee Wan Wah and Hiroyuki Endo.