[Nijikokun]

https://www.rfc-editor.org/rfc/rfc6749#section-4.1.2.1

[tadfisher]

It's in Section 5, actually. That one is for the implicit-grant flow, so the fields are URL-encoded and appended to the redirect URI's query fragment.

[marcellus23]

Yep, from section 5:

> error

> REQUIRED. A single ASCII [USASCII] error code from the following...