api.yaml

openapi: 3.0.0
info:
paths:
components:
  schemas:
    SomeObj:
      type: object
      required:
        - state
      properties:
        state:
          $ref: '#/components/schemas/State'
    State:
      oneOf:
        - $ref: '#/components/schemas/StateA'
        - $ref: '#/components/schemas/StateB'
      discriminator:
        propertyName: status
        mapping:
          a: '#/components/schemas/StateA'
          b: '#/components/schemas/StateB'
    Status:
      type: string
      enum:
        - a
        - b
    StateA:
      type: object
      required:
        - status
      properties:
        status:
          $ref: '#/components/schemas/Status'
    StateB:
      type: object
      required:
        - status
        - mode
      properties:
        status:
          $ref: '#/components/schemas/Status'
        mode:
          type: string
          enum:
            - mode1
            - mode2