It was released as a single by Polydor Records on January 5, 2024, served as the second opening theme for the 2023 Japanese anime series Frieren: Beyond Journey's End.

On December 22, 2023, following the sixteen episode "Long-Lived Friends" ended, it was announced that Yorushika would be in charge of the second opening theme for anime series Frieren: Beyond Journey's End, titled "Sunny", alongside the song's snippet via the anime's fourth trailer.

"[4] Ryōhei Ohara from Real Sound pointed out that the lyrics of "Sunny" are more abstract than Frieren's first opening theme, "Yūsha" by Yoasobi, but evaluated it as "a song that fits closely with the anime,"[5] while Eriko Ishii surprised "the raw emotion expressed in the powerful words that come out of the chorus," and praised Suis's vocals that is "not a voice that tells a story, but a voice that is determined to convey a strong message to the listener.

[8] Inspired by Haruki Murakami's 1985 novel Hard-Boiled Wonderland and the End of the World,[8] the story depicts a child and his father living in a town surrounded by impenetrable wall, showing different weathers.

[9] The special version of music video, featuring footages from the "First-Class Mage Exam" episode, was uploaded on March 29.