Mechanisms allow a user to test a BPEL process by simulating partner Web services. To simulate the partner Web services, a development environment is extended to include an intermediate component that intercepts messages intended for a partner Web service and displays those messages in a testing user interface. In the testing user interface, a user can create and send messages in direct response to the messages invoked while executing the BPEL process. The development environment may also include an alternate deployment descriptor, as part of a BPEL engine. The alternate deployment descriptor identifies where a message should be routed. For example, the alternate deployment descriptor lists the address of the intermediate component, rather than the actual partner Web service address, so that messages can be redirected to the testing user interface.