The new TestState API allows you to thoroughly test every state in a state machine, achieving nearly 100% coverage. It solves the problem of testing hard-to-reach execution paths and provides a way to iterate on your state definitions without redeploying the project. The API is not a local simulation tool and does not improve the speed of feedback loops, but it helps you test individual states directly and can be used for input or output processing logic. You should still perform component-level testing on Lambda functions involved in the state machine using "remocal tests" to maintain a fast feedback loop. Step Functions Local is no longer necessary, as the TestState API can invoke remote resources referenced by Task states. End-to-end tests are still valuable for business-critical execution paths to ensure that individual states function together as a unit and catch integration problems early.