Aggregate model-based testing, particularly action-state testing, emerges as a versatile method for both stateful and stateless systems by allowing for implementation-independent modeling that is adaptable to various requirements. The technique, introduced in the book "Paradigm Shift in Software Testing," is implemented in Harmony and facilitates the generation of abstract test cases from textual models containing user actions, system responses, and inner test states. This approach enhances testing efficiency by detecting missing actions and states, and it helps prevent errors through early validation against specifications. Action-state testing offers a significant advantage by removing the need for coding, thus simplifying the testing process and supporting a shift-left strategy in software development. The comprehensive modeling ensures a thorough coverage of requirements, while the statechart representation aids in identifying incomplete transitions and improving the overall test model.