This agreement, which includes all documents that it explicitly incorporates, is the complete agreement between the parties. It supersedes all other promises, representations, understandings, and agreements between the parties concerning the subject matter of this plea agreement that are made at any time before the guilty plea is entered in court. Thus, no oral or written promises made by the government to defendant or to the attorney for defendant at any time before defendant pleads guilty are binding except to the extent they have been explicitly incorporated into this agreement.
It has previously been the rule in the Sixth Circuit that "an integration clause normally prevents a criminal defendant, who has entered into a plea agreement, from asserting that the government made oral promises to him not contained in the plea agreement itself." This certainly makes sense as it is the whole reason for reducing agreements to writing in the first place. In Quesada, the Court of Appeals now extends this rule to apply to prior written proffer agreements.
The lesson here for defense counsel is that from now on, in any case in which a client has proffered to the government, counsel must make sure that either the above language (or anything like it) is not included in the plea agreement or that a copy of the proffer letter (or any other document which should in fairness bind the government) is attached to the plea agreement and specifically incorporated into the plea agreement.