While it is relatively difficult to successfully hold a supplier liable in an agile project, this is by no means impossible. The most promising base will generally be the duty of care of the IT service provider. In particular, a supplier must clearly warn the customer when the customer (in its role of product owner) changes the course of the project in ways that affect the progress of the project or the suitability of the underlying architecture. As is evident from other IT cases, under certain circumstances, this could mean that a supplier must warn insistently, suggest alternatives or even refuse to proceed on a certain course.

Source