This is a waivable issue, as SCOTUSBlog reports.
The Supreme Court ruled Wednesday that, if federal prosecutors violated
a plea bargain with an accused who pleads guilty, that must be brought
out at the trial and thus cannot be challenged for the first time on
appeal. On a 7-2 vote, the Court cleared up a conflict among lower
courts by narrowing a federal rule that allows an exception to the
usual rule that a claim not made at trial is forfeited and cannot be
raised on appeal. The ruling came in the case of Puckett v. U.S. (07-9712). Justice Antonin Scalia wrote for the Court; Justice David H. Souter dissented, joined by Justice John Paul Stevens.
Here is the SCOTUSWiki site on the relevant documents.