According to the OpenAPI Spec the responses object should be able to have extensions added to it https://spec.openapis.org/oas/v3.0.3#responses-object This object MAY be extended with [Specification Extensions](https://spec.openapis.org/oas/v3.0.3#specificationExtensions).

But the latest version of kin-openapi doesn't expose extensions for the openapi3.Responses type which at the moment looks to be just a type alias to a map