Tosefta is not a commentary on Shas.
Tosefta is one of the canonical texts of Torah She Ba’al Peh. It’s an additional tannaitic tradition (many of the beraitot in the Gemara are references to Tosefta that was redacted around the same time as the Mishna, and as a collection of such beraitot, it is generally arranged according to the same structure as the Mishna. (with the exception of Avot, Tamid, Middot, and Kinnim, every tractate in the Mishnah has a parallel tractate in the Tosefta, though the precise character of the content of the Tosefta tractate and its relationship to the material found in the Mishnah can vary radically).
Dayan Abramsky and the Grush Lieberman gave us the foremost commentaries on the Tosefta itself.