Sanenaga is recorded in the Kokon Meizukushi as a son of Osafune Mitsutada and the younger brother of Nagamitsu, placing his activity within the mainline Osafune succession during the late Kamakura period. Extant signed works appear with both two-character signatures and longer inscriptions, the latter group including dated examples from the nengō Shōdan, Kagen, Tokuji, and Enkyō. The question of a first and second generation remains open to further examination: the Kokon Meizukushi places the first generation around the Bun'ei–Kōan eras and treats works dated Shōdan and later as belonging to a second generation, though works bearing only the two-character signature show no dated inscriptions among them. Within the Osafune group of his time, Sanenaga occupies a distinctive position — his output occasionally approaches Nagamitsu in its more flamboyant expressions of chōji-midare, yet his characteristic manner is unmistakably his own, distinguished by a restrained temperament that sets him apart from his elder brother's more expansive style. Among his surviving works are tachi, katana, and the comparatively rare kodachi form, the latter known also from examples transmitted in the Saijō Matsudaira family.

The hallmark of Sanenaga's work, affirmed across the full body of designated examples, is a calm hamon of chū-suguha tonality into which ko-gunome, ko-chōji, and ko-notare are mixed, with ko-ashi and yō entering well. The nioiguchi is characteristically well tightened — a feature the NBTHK identifies as his principal distinguishing trait, described repeatedly as showing a shimarigokoro tendency that is bright and clear. His forging exhibits the refined quality expected of the main Osafune line: tightly packed ko-itame-hada with extremely fine ji-nie adhering thickly, delicate chikei, and vivid midare-utsuri standing out clearly. The bōshi takes on what is termed the "Sansaku-bōshi" — a shallow notare-komi turning back in ko-maru — a manner shared with Nagamitsu and Mitsutada that further affirms his position within the founding Osafune triumvirate. In addition to the predominant suguha-based mode, a secondary manner exists in which chōji mixed with gunome produces a comparatively flamboyant midare rendered in somewhat smaller patterns than Nagamitsu's, sometimes with a saka-gakari tendency and saka-ashi; these more archaic-feeling works are tentatively associated with the first generation.

The NBTHK consistently praises Sanenaga's works for the exquisitely fine forging characteristic of the main Osafune line and for the brightness and clarity of both ji and ha. The evaluative phrase kenzen — sound and well-preserved — recurs with notable frequency, underscoring the exceptional survival condition of his output. Blades are commended for retaining ample nikuoki and for a workmanship in which there is "not the slightest slackness" in either ground or temper. His finest pieces are described as displaying "this smith's full capabilities without reservation," and the gentle hamon often encountered among long-signature examples is singled out as "particularly fine." The provenance of his works reflects their historical esteem: examples have been transmitted in the Satsuma Shimazu, Kaga Maeda, Odawara Ōkubo, and Date families, with origami appraisals by successive Hon'ami masters — Kōmuro, Kōtsune, and Kōson — attesting to centuries of connoisseurial recognition. Sanenaga thus stands as a master of disciplined restraint within the Osafune tradition, his tightly governed nioiguchi and luminous steel embodying the refined aesthetic of the school's founding generation.