This talk explores a contract-first approach to API development using the OpenAPI generator, a powerful tool for automating API generation from a standardized specification. We will cover (1) what would you need to run to have a standard implementation of the FastAPI endpoints and data models; (2) how to customize the mustache templates that are used to generate the API stubs; (3) share some ideas how to customize the CLI and (4) how to maintain the contract and how to handle breaking changes to the contract. We will close the session with a discussion of the challenges of implementing the OpenAPI generator.