unsupportedgranttype

Unsupportedgranttype is a term sometimes used to describe an OAuth 2.0 error condition in which the authorization server does not support the grant type requested by a client. In the official OAuth 2.0 error model, this condition is communicated by the error parameter unsupported_grant_type in the token endpoint response, usually accompanied by a 400 Bad Request status.

In OAuth 2.0, a client makes a token request with a grant_type parameter that indicates the preferred flow, such as authorization_code, client_credentials, password, or refresh_token. The authorization server maintains a list of supported grant types. If a client requests a grant type that is not supported or not enabled on the server, the server should respond with error=unsupported_grant_type and may include an error_description to clarify the issue.

Common causes include using a grant type that is not enabled on the server, requesting a deprecated or custom grant type without server support, or misconfiguring the token endpoint. If encountered, the recommended action is to verify the server’s supported grant types and ensure the client uses a compatible grant_type. Configuration changes may be required to enable the desired flow, or the client may need to switch to a supported grant type.

This error is distinct from other OAuth errors such as invalid_request, invalid_client, or access_denied. Proper handling involves checking the grant_type value, aligning client capabilities with server configurations, and consulting server documentation for supported flows.