API Code-first is a method of building APIs by first creating the API and then using annotations to generate API documentation in formats like OpenAPI, with Springdoc being a popular tool for this in Spring applications. The process involves setting up prerequisites such as Java 21, Maven or Gradle, and a Spring Boot application, followed by installing the Springdoc dependency and configuring it to generate OpenAPI descriptions. This initial setup allows for basic OpenAPI description generation, which can be improved by adding Java annotations to provide more context and detailed information about API operations, parameters, and responses. Annotations such as @Operation, @Tag, and @Parameter are used to enrich the documentation with summaries, descriptions, and parameter details, making it more useful for API consumers. The guide also covers deploying the OpenAPI description to Bump.sh, a platform for hosting and viewing API documentation, and suggests automating the deployment process through CI/CD pipelines to ensure the documentation remains up-to-date with any changes in the API code.