Creating the contract with your consumers is the first thing you do. You can automatically create your API from this OpenAPI definitions (e.g. skip the whole configuration of your API Gateway on AWS by using OpenAPI). You’ll have a beautiful visual overview of your API. Generate your class definitions based on this OpenAPI definitions. Validate incoming and outgoing requests with the OpenAPI definition. Did you notice that those are already five advantages of using OpenAPI? It’s only the beginning. And that’s why you should practice API First development with OpenAPI.