diff --git a/spec3.json b/spec3.json
index 2cf09bd..1bd9ec8 100644
--- a/spec3.json
+++ b/spec3.json
@@ -7780,7 +7780,8 @@
                   },
                   "clientId": {
                     "type": "string",
-                    "description": "Public identifier for the OAuth client.",
+                    "maxLength": 1024,
+                    "description": "Public identifier for the OAuth client. This may also be the URL of a client ID metadata document, in which case the client's details are resolved from that document.",
                     "example": "2bquf8avrpdv31par42a"
                   }
                 }
diff --git a/spec3.yml b/spec3.yml
index 9c54e74..288cfcf 100644
--- a/spec3.yml
+++ b/spec3.yml
@@ -5503,7 +5503,11 @@ paths:
                   format: uuid
                 clientId:
                   type: string
-                  description: Public identifier for the OAuth client.
+                  maxLength: 1024
+                  description: >-
+                    Public identifier for the OAuth client. This may also be the
+                    URL of a client ID metadata document, in which case the
+                    client's details are resolved from that document.
                   example: 2bquf8avrpdv31par42a
       responses:
         "200":