The authors present a formal model for incremental protocol test sequence generation based on executable rule-based specification and implementation. In order to have direct control, instead of using heterogeneous representation mechanisms, a homogeneous representation mechanism, the OPS5 production system, is used in both the design phase and the test phase. Since the tested paths can be recorded in a globally shared dataspace (working memory) of the OPS5 production system, the test process can continue by searching for the untested transitions incrementally. This method uses a status message to verify the entrance state instead of using distinguishing sequences. Therefore, each transition is tested exactly once.