matrix-spec/api/openapi_extensions.md

OpenAPI Extensions

For some functionality that is not directly provided by the OpenAPI v2 specification, some extensions have been added that are to be consistent across the specification. The defined extensions are listed below. Extensions should not break parsers, however if extra functionality is required, aware parsers should be able to take advantage of the added syntax.

Extensible Query Parameters

If a unknown amount of query parameters can be added to a request, the name must be fields..., with the trailing ellipses representing the possibility of more fields.

Example:

  - in: query
    name: fields...
    type: string

Using oneOf to provide type alternatives

oneOf (available in JSON Schema and Swagger/OpenAPI v3 but not in v2) is used in cases when a simpler type specification as a list of types doesn't work, as in the following example:

  properties:
    old: # compliant with old Swagger
      type:
        - string
        - object # Cannot specify a schema here
    new: # uses oneOf extension
      oneOf:
        - type: string
        - type: object
          title: CustomSchemaForTheWin
          properties:
            ...