errorunsupportedgranttype

ErrorUnsupportedGrantType is an OAuth 2.0 error response defined in RFC 6749 that indicates a request uses an unsupported or unrecognized grant type. The grant type defines how a client obtains an access token, such as authorization_code, implicit, password, client_credentials, or refresh_token. The error is returned in the JSON body of an HTTP error response, typically with status code 400 (Bad Request). The response format includes the key "error" with the value "unsupported_grant_type" and may include an optional "error_description" providing additional details.

Triggers for this error include:

- The client requests a grant type that the authorization server does not implement or permit for the given client identifier.

- The client submits an invalid or misspelled grant_type parameter.

- The grant type is not allowed due to policy constraints, such as a public client attempting to use the password grant, which is reserved for confidential clients.

- The server operates in a mode where only specific grant types are enabled and others are disabled.

Typical handling steps for developers:

1. Verify the grant_type value is correctly spelled and corresponds to a supported type documented by the authorization server.

2. Check client configuration to ensure the requested grant type is authorized for that client.

3. Review server-side policies or configuration to confirm the grant type is enabled.

4. If the server uses a custom or extension grant type, confirm the grant type is properly registered and advertised.

ErrorUnsupportedGrantType is distinct from other OAuth 2.0 errors such as "invalid_client" or "invalid_grant". It is intended to inform the client that the request format is correct but the requested grant type is not supported by the server at that time. Correctly handling this error allows clients to recover by selecting an appropriate grant type or adjusting client registration.