This invention provides a method, computer program and system for updating a model of a system under test for use in model-based testing. In one embodiment, the system includes a rules engine, a comparator and a message dictionary. The rules engine runs an action against the system under test and runs the action against a model under test. The comparator compares an output from the system under test with a corresponding output from the model under test, wherein the output from the model under test is selected from one of a plurality of output message records. The dictionary engine updates the output message record by changing the message record if the output of the model under test is different from the output of the system under test. If the output message record is empty, the dictionary engine adds an output message record based on the system under test.