The game was first announced as "Emio" on July 10, 2024 through an uncharacteristically vague marketing campaign on Nintendo's social media accounts, and was promoted with the hashtag "#WhoIsEmio?" It was widely speculated that Emio could have been a new horror game, as the Australian upload of the game's teaser trailer contained a content rating with warnings for themes of suicide and violence, while some theorized that it was a collaboration with the Polish developer Bloober Team. Just two days prior, Bloober Team president Piotr Babieno revealed that they were developing a new horror title codenamed "Project M" exclusively for the Nintendo Switch and the unannounced Nintendo Switch 2. He described it as a crucial long-term project for the company and that they were "working on it in cooperation with the world's best game creators for Nintendo platforms". However, this rumor would turn out to be unfounded when the game was fully revealed on July 17 as a new installment in the Famicom Detective Club series.