I'm aware he was in France at the time, and that he would go on to lead the Democratic-Republicans. But was he opposed to the idea of the Constitution? Or was he like James Madison, a Pro-Federalist at the Convention but opposed to the Federalist Party?
Thinking Thomas Jefferson was an anti-Federalist is a common misconception, probably resulting from his opposition to the later Federalist Party, which was a separate faction from the earlier Federalists. Thomas Jefferson supported the Constitution, but thought a Bill of Rights should have been added. He himself spoke of where he stood:
Therefore, I protest to you, I am not of the party of Federalists. But I am much farther from that of the anti-Federalists.
From a letter to Francis Hopkinson Paris, Mar. 13, 1789.