RH-SSO server returns the invalid_request error response when a client sent requests with grant_type=<unsupported value>, for example, "urn:ietf:params:oauth:grant-type:device_code".
According to RFC6749, it seems to be better to return the unsupported_grant_type error response.
5.2. Error Response
The authorization server responds with an HTTP 400 (Bad Request)
status code (unless specified otherwise) and includes the following
parameters with the response:
REQUIRED. A single ASCII [USASCII] error code from the
The request is missing a required parameter, includes an
unsupported parameter value (other than grant type),
repeats a parameter, includes multiple credentials,
utilizes more than one mechanism for authenticating the
client, or is otherwise malformed.
The authorization grant type is not supported by the