discriminator property must be required ?

[champtar]

Describe the bug

I'm not certain, but I think the discriminator property must be required

https://spec.openapis.org/oas/v3.1.0.html#fixed-fields-20

The expectation now is that a property with name petType MUST be present in the response payload, and the value will correspond to the name of a schema defined in the OAS document.

To Reproduce

Here an example

openapi: 3.1.0
info:
  title: test
  version: v1
servers:
  - url: /
paths:
  /aaa:
    get:
      operationId: aaa
      summary: aaa
      responses:
        "200":
          description: successful operation
          content:
            application/json:
              schema:
                $ref: "#/components/schemas/objX"
components:
  schemas:
    objstr:
      type: object
      #required: [objectType]
      properties:
        objectType:
          type: string
          enum: ["str"]
    objint:
      type: object
      #required: [objectType]
      properties:
        objectType:
          type: string
          enum: ["int"]
    objX:
      anyOf:
        - $ref: "#/components/schemas/objstr"
        - $ref: "#/components/schemas/objint"
      discriminator:
        propertyName: objectType
        mapping:
          "int": "#/components/schemas/objint"
          "str": "#/components/schemas/objstr"

Expected behavior

Validation fails because we are missing required: [objectType]

Additional context

oapi-codegen fails to generate correct code with this example, but I'm not 100% clear if it is valid or not https://github.com/oapi-codegen/oapi-codegen/pull/1789#issuecomment-2759047235

If it's not valid it would be great if redocly-cli was rejecting such example

[champtar]

OpenAPI 3.1.1 is clearer: https://github.com/OAI/OpenAPI-Specification/blob/main/versions/3.1.1.md#fixed-fields-22

This property SHOULD be required in the payload schema, as the behavior when the property is absent is undefined.

[jeremyfiel]

Forcing it to be required is against the spec language, so while it's not totally incorrect to assume that, an optional definition does not violate the spec

---

this behavior will slightly change with the 3.2 minor release coming soon, hopefully by Summer 25.

https://github.com/OAI/OpenAPI-Specification/pull/4339

[champtar]

Should we output a warning then ? As of 3.1, it's clearly called out as undefined behavior when missing.

[jeremyfiel]

You can write a custom configurable rule that checks the anyOf / oneOf schemas against the discriminating value

[tatomyr]

@champtar, as @jeremyfiel said, you'll need to write a custom rule using plugin if you want to use that now. You can check out our cookbook as well for more practical examples.

[baywet]

I think #2320 gives us a way forward here. I'd be happy to address this issue in a follow up PR assuming this one gets merged in first and we agree that this is something that needs to be fixed. CC @jeremyfiel