Charles Ellicott Commentary 2 Kings 17:13

"Yet Jehovah testified unto Israel, and unto Judah, by every prophet, and every seer, saying, Turn ye from your evil ways, and keep my commandments and my statutes, according to all the law which I commanded your fathers, and which I sent to you by my servants the prophets." — 2 Kings 17:13 (ASV)

Yet the Lord testified against Israel. — Rather, And Jehovah adjured Israel... The verb here means gave solemn warning, or charge. In 2 Kings 17:15 it is repeated, with a cognate noun as object: His testimonies which he testified against them; or, his charges (that is, precepts) which he had given them.

By all the prophets, and by all the seers. — The Hebrew text is, by the hand of all his prophets — namely, every seer. One or two manuscripts and the Targum have prophet, instead of his prophets. The Syriac has by the hand of all his servants the prophets, and all the seers. The Vulgate and Arabic also have both nouns plural.

Seers were people who, without belonging to the prophetic order, came forward in times of emergency under a sudden divine impulse. Thenius thinks Israel and Judah are mentioned together because the reference is to the time before the partition of the kingdom; more probably, it is because both apostatized, and prophets were sent to both.

And which I sent — that is, the law which I sent. But—since according to later Jewish ideas, the prophets did not bring the Law, but only interpreted it—it seems better to understand, with the Vulgate (et sicut misi), “and according to all that I sent to you (that is, enjoined upon you) by my servants the prophets.”