“Originally, I was writing Hideo's Mysteries as a joke on Twitter.”
Kojima said Hideo’s Mysteries began as a joke on Twitter, a parody of guidebooks like Kono Mystery ga Sugoi. According to the episode’s editor Shiozawa, with Kojima present, a bookseller at Kinokuniya Umeda proposed turning it into a bookstore fair featuring twelve selected titles called the “Golden Dozen,” and several publishers later supported the project. Kojima thought bookstores would not be interested, but many set up a Hideo’s Mysteries corner and he expressed his gratitude. He said he chooses books from any publisher, had worried whether that would work, and in 2022 he turned down a rival offer out of loyalty to the project; that year’s edition included Brain Structure podcast stickers for purchases made in the section. In KOJI10 EP36, published in October 2026, Kojima announced his Hideo’s Mysteries book selection for 2026.
“Originally, I was writing Hideo's Mysteries as a joke on Twitter. […] I thought the bookstores wouldn't be interested in Hideo's Mysteries. But many of them have set up a corner for it in their store, so I'm very grateful. […] I was approached with an offer to do this mystery this year […] so I was like, alright then, and I turned the other offer down. […] we have partnered with this podcast and have created a Brain Structure podcast original sticker.”