Have you ever designed an API? Did you use OpenAPI, Swagger?

Yes, I have had to design APIs. To describe and document RESTful APIs, I used the OpenAPI Specification (formerly known as Swagger). This allows formalizing the API structure, describing endpoints, methods, parameters, and data models, as well as automatically generating documentation and client SDKs.

An example of a simple endpoint description in OpenAPI (YAML):

paths:
  /users:
    get:
      summary: Get a list of users
      responses:
        '200':
          description: Successful response
          content:
            application/json:
              schema:
                type: array
                items:
                  $ref: '#/components/schemas/User'
components:
  schemas:
    User:
      type: object
      properties:
        id:
          type: integer
        name:
          type: string