Testers on an XP team help customers choose and write automated system-level tests in advance of implementation and coach programmers on testing techniques. On XP teams much of the responsibility for catching trivial mistakes is accepted by the programmers. Test-first programming results in a suite of tests that help keep the project stable. The role of testers shifts to early in development, helping define and specify what will constitute acceptable functioning of the system before the functionality has been implemented.
In Weekly Cycle, the first thing that happens to the chosen stories is that they are turned into automated system-level tests. This is one leveraged place for strong testing skills. Customers may have a good idea of the general behavior they want to see, but testers are good at looking at "happy paths" and asking what should happen if something goes wrong. "Okay, but what if login fails three times? What should happen then?" In this role testers amplify communication. They ensure that the system-level tests succeed only when the stories are fully implemented and ready for deployment.
Once the tests for the week are written and failing, testers continue to write new tests as implementation uncovers new details that need to be specified. Testers can also work to further automate and tune tests. Finally, when a programmer gets stuck on a knotty testing problem, a tester can pair with the programmer to help solve the problem.