Ko-Bungo (古豊後) names the opening chapter of the Bungo school, the work made in the province of Bungo in Kyushu from the close of the period into the era, before the school descended into the prolific Takada production of later centuries. Its central figure is Yukihira, signing Bungo no Yukihira , whom the place among the Kyushu koten- (the Kyushu classical group) as the maker of the highest technical level, with a comparatively large number of surviving works. The phase is rooted in the temple smiths of Mount Hiko (Hikosan): tradition makes Yukihira a disciple of the monk Sadahide, whose own signatures (Bungo no Sō Sadahide ) appear on slender of closely related manner, while the sources disagree over whether Sadahide was teacher or pupil. The entries also record Yukihira as one of the serving the retired Emperor Go-Toba, the smith Chōen transmitted variously to Buzen, Yamato, or Bungo, and the grandson Chikushi Masatsune, marking out the early Kyushu circle from which the school grew.
The style the describe is archaic and quiet. The is slender with high and pronounced , a small , and curvature that settles toward the point, a form said not to descend beyond the early . The is tending to , soft and (viscous and clinging) in feel, with thick , , and a whitish cast or suggestion. The runs to with and , worked in , the repeatedly turning (moist and soft); , , and appear, and a drops the temper just above the , a habit the entries flag as a fingerprint shared with old Kyushu work such as Ko-Hōki and Ko-Naminohira. Carving is distinctive: the Ko-Bungo blades carry , , , Jizō Bosatsu, and the "pine-eating crane" (matsukui-zuru), motifs the state are not seen in works before Yukihira and were later transmitted to Bungo. This restrained, -laden, carving-rich manner stands apart from the standardized later Bungo Takada output.
For , the isolate firm anchors: the soft steel with whitish tone, or with , the base , and the signature placement Yukihira followed against the convention of his day, a practice the entries link only to Munechika and certain descendants. The dated of Genkyū 2 (1205) fixes the working period. The Kanchi-in text warns that crude are genuine and skilled ones suspect, since Yukihira "could not write well," and that forgeries arose already in . Provenance runs deep: a Yukihira identified with the Minamoto treasure "Usugoromo / Usumidori" and bestowed by Tsunayoshi per the Tokugawa Jikki, two Imperial Household Yukihira , and Sadahide blades transmitted through the Mōri and tied to Emperor Go-Mizunoo, with by Kōon, Kōchū, and Kōon recurring across the group.