Since the customer tests are automated, the customer has to be very specific about what the test will do. It is impossible to do this if the customer does not understand the story in intimate detail from a user perspective. Writing the customer tests will force the customer to go into all the details of the story. Stories must be testable.

Once the story is well understood and selected for the iteration, the automated customer tests are written. If the customer team includes testers, the customer could communicate the test criteria to the tester. This can be verbal or written. The criterion describes the tests for the normal and exceptional behavior of the system.