Hmm, that definitely get me closer, but not quite. Now I see the correct schema definition, however there's no error for the "invalid" item. The invalid item is neither removed from the array, nor does it trigger a validation error.

Note: I'm using @Optional as I want to maintain compatibility for clients who aren't aware of this property. Removing the @Property and @Optional decorators has no effect on the validation outcome.

Using the following decorators

    @Property()
    @Optional()
    @Any(ObjectWithId, UserWithEmail)
    notify?: (ObjectWithId | UserWithEmail)[];

Generates the following schema. To my untrained eyes, the schema looks correct so I'm not sure why the "invalid" item still passes validation.

{
  "type": "array",
  "items": {
    "anyOf": [
      {
        "$ref": "#/definitions/ObjectWithId"
      },
      {
        "$ref": "#/definitions/UserWithEmail"
      }
    ]
  }
}

Property and optionable isn't necessary when you use at least a schema decorator.

You can try OneOf instead Any (AnyOf).

Also you can test json schema behavior here:

https://www.jsonschemavalidator.net

This tool is helpful to understand json schema behavior ;)

See you