OpenAPI/Swagger is a specification for building APIs. It provides a way to describe the structure of an API and the operations that can be performed on it. Using OpenAPI/Swagger with API design first principle means defining the API specification before writing any code. This approach helps to ensure that the API is well-designed, consistent, and easy to use.

By defining the API specification first, developers can get a clear understanding of the API's functionality and how it will be used. This helps to identify any potential issues or limitations early in the development process, which can save time and resources later on. Additionally, having a well-designed API specification can make it easier for other developers to understand and use the API, which can lead to increased adoption and usage.

Here is an example of an OpenAPI/Swagger specification for a simple API endpoint that returns a list of users:

openapi: 3.0.0
info:
  title: User API
  version: 1.0.0
servers:
  - url: http://localhost:3000
paths:
  /users:
    get:
      summary: Get a list of users
      responses:
        '200':
          description: OK
          content:
            application/json:
              schema:
                type: array
                items:
                  type: object
                  properties:
                    id:
                      type: integer
                      description: The user ID
                    name:
                      type: string
                      description: The user's name
                    email:
                      type: string
                      description: The user's email address

In this example, the API specification defines a single endpoint at /users that returns a list of users in JSON format. The specification includes information about the endpoint's summary, response codes, and the structure of the response data. By defining the API specification first, developers can ensure that the API is well-designed and easy to use, which can lead to increased adoption and usage.