WS-BPEL eases programming in the large by composing web services, but poses new challenges to classical white-box testing techniques. These have to be updated to take context into account and cope with its specific instructions for web service management. Takuan is an open-source system that dynamically generates invariants reflecting the internal logic of a WS-BPEL composition. After several improvements and the development of a graphical interface, we consider Takuan to be a mature tool that can help find both bugs in the WS-BPEL composition and missing test cases in the test suite.