Was Kant a 'Kantian Constructivist'?

Both metaethicists and Kant scholars alike use the phrase 'Kantian constructivism' to refer to a kind of austere constructivism that holds that substantive ethical conclusions can be derived from the practical standpoint of rational agency as such. I argue that this widespread understanding of Kant is incompatible with Kant's claim that the Categorical Imperative is a synthetic a priori practical judgement. Taking this claim about the syntheticity of the Categorical Imperative seriously implies that moral judgements follow from extra-logical but necessary principles. These principles have to do not with the laws of practical thinking but the laws of practical thought about an object. I conclude that historical Kant was not what has come to be called a 'Kantian constructivist'.