The interfaces are part of the list of deliverables, surely? I can’t see why this is’t just a restatement of the obvious. It’s where all the effort used to go in the old days, e.g. if you were putting in Oracle or SAP.
Sounds like an old, old lesson needs to be relearned again. And, of course, if you are using the orthogonal UML models: Use Case/Interaction/Class you should have seen this coming if you were paying attention.
Of course, all of the old PM’s that knew this stuff have been fired because they were “too expensive”. Now we know why they were … they don’t make beginner’s mistakes like this.