Kuniyasu was the third of the celebrated six brothers — Kunitomo, Hisakuni, Kuniyasu, Kunikiyo, Arikuni, and Kunitsuna — who brought the Yamashiro tradition to its early zenith. Known by the common name Tosaburou, he is recorded alongside his elder brothers Kunitomo and Hisakuni among the , the roster of smiths appointed to serve Retired Emperor Go-Toba. The literary record of swordsmiths residing at in Yamashiro Province extends to the early- setsuwa collection Uji Shui Monogatari, attesting to the school's deep roots in the capital. Kuniyasu's active period falls around the Jokyu era (1219-1222), placing him squarely in the formative decades of -period sword production, when the school established the refined aesthetic sensibility that would define Yamashiro workmanship for generations.
Kuniyasu's technical signature is anchored in a tightly forged that produces the prized (pear-skin texture) characteristic of the finest work. His divides broadly into two modes: one exhibiting an immaculately compact surface with dense, fine and brilliant , and another showing a more conspicuously standing grain () with larger patterning and prominent . Both modes share the luminous steel quality for which the school is celebrated. The is characteristically a -based temper mixing , , and , with tightly spaced undulations. A defining diagnostic feature — and the principal point of for this smith — is the tendency of the toward , a moist, softly diffused quality that distinguishes his work from the crisper edges of Hisakuni and others in the group. Brilliant adheres thickly, accompanied by , , and, in the finest examples, small appearing above the . The bosshi is typically straight, turning back in . His signed works bear a distinctive cursive rendering of the character (安), which remains consistent across examples despite other calligraphic variations. Extant signed works are exclusively , though a single signed has also been confirmed, making that form exceptionally rare within his oeuvre.
Kuniyasu occupies a position of considerable importance within the canon of early swordsmithing. His blades preserve the slender, deeply curved silhouette of the period — a graceful, (feminine) bearing with and pronounced — representing the classical ideal of Yamashiro form. The finest examples display a luminous clarity in both and that examiners consistently describe as fully manifesting the aesthetic virtues of workmanship. One in the Imperial collection () retains its original with bold signature, standing as an important reference point for authentication. Many surviving works are and , yet Kuniyasu's distinctive hand — the linked tendency, the in the , and the vigorous internal activity within the tempered edge — renders attribution possible even without signature. His work demonstrates the full expressive range of the aesthetic: from the serene refinement of -like surfaces to the dynamic interplay of -based activities along the cutting edge, embodying the technical and artistic mastery that defined the golden age of Yamashiro sword production.