Kuniyasu worked at in Kyoto in the early period, third of the six brothers, Kunitomo, Hisakuni, Kuniyasu, Kunikiyo, Arikuni and Kunitsuna, who made the school famous, and was called Tosaburo (藤三郎). The published sources count him, with his elder brothers Kunitomo and Hisakuni, among the of the Retired Emperor Go-Toba, and the sword references place his activity around the Jokyu era (1219 to 1222). His surviving signed works are , comparatively numerous for so early a smith; the single signed , recognized at session 31, is for that reason called "extremely precious". Fujishiro grades him Sai-jo .
His blades keep the figure of their day, slender, with high and a small , the curve settling faintly toward the point; one source reads the bearing as a gentle grace. On this figure he tempers a small-patterned in , mixing and over a base, with and fine in the . The intervals of the press close, for "many examples show the spacing of the drawn tight", and where the small run linked in series the judges go further, writing of one unsigned that by this linked tendency "even within the school it should be possible to specify Kuniyasu". The , "as with Kunitomo, characteristically clouds into "; gathers above the , where one source finds "this smith's habitual hand" plainly discernible; on another blade the forked, karimata-like at the reveals the characteristic features. The runs to a small , at times , the tip lightly brushed with .
The divides in two, and the published sources state the split outright: in his forging "there are two kinds, one with standing and one tightly knit". The tight register is packed , adhering in fine dust, slender woven through, at times a faint rising toward the ; the surface takes on the pear-skin texture the sources call "the school's characteristic ", and blades of this kind are read as "the manner closer to Hisakuni". The standing register shows with the grain raised, mingled in and conspicuous, and the published record ties it to his signed work: "among this smith's signed pieces those with prominently standing are comparatively many".
The two registers sort loosely by signature: most unsigned attributions are of the refined type and travel on judgments, a of Koson, a of Mitsutada, an of Koyu dated Genbun 3 (1738). The scholarship is candid about his softest quality: an early text judges of the shadowed dullness that often settles in his edge that "rather than a point of appreciation it is instead a defect", while later texts call the "one of this smith's notable features". His two-character signature was cut both large and small, and "in either case the character An shows its distinctive features", its cursive abbreviation always the , a major point in judging his . Honma notes of the attributed pieces that "among the there are occasionally wide ones like this sword", robust blades apart from his usual slender build.
Within he is the brother. The school's later course runs through Norikuni to Kuniyoshi and Yoshimitsu and settles into bright , while Kuniyasu holds to the compressed of the founding generation and its clouded . His follows Hisakuni, and his tightest approaches that brother closely; yet the linked picks him out alone within the school, and the pressed intervals and the he shares only with Kunitomo. Against the later generations the judgment entered on his signed is direct: "compared with Norikuni and Kuniyoshi the interior of the works well", fully showing, the text concludes, the depth of flavor of work.
Twenty-three designated works stand on record: three Important Cultural Properties, all signed , with five Bijutsuhin, two and twelve , fourteen blades in the and tiers together; nine pieces in the record are signed against eleven unsigned. Eleven blades carry recorded provenance, through the Owari Tokugawa house, one a gift of the shogun Tokugawa Tsunayoshi to Yoshimichi, fourth lord of Owari, and through the Tsugaru, Nanbu, Matsudaira, Mizoguchi and Hosokawa families and the Imperial Family. Of recorded whereabouts his blades rest with the Tokugawa Art Museum, the Tokyo National Museum, the Sano Art Museum and the itself. The three Important Cultural Properties are patrimony, preserved where they stand; some fifteen blades of recorded rank sit in the tiers that may lawfully change hands, and most are held rather than traded. The published record remarks that signed by Kuniyasu are encountered from time to time, and that is the honest measure of him: a of Go-Toba whose work a patient collector may still hope to meet, though each appearance of a example is an occasion.