Better logic/reasoning

synapse/handlers/federation.py

@@ -1155,12 +1155,16 @@ class FederationHandler:
         # > server MAY respond to invites with a `400 M_MISSING_PARAM` standard Matrix
         # > error (new to the endpoint). For invites to room version 12+ rooms, servers
         # > SHOULD rather than MAY respond to such requests with `400 M_MISSING_PARAM`.
+        #
+        # FIXME: Always validate for all room versions after 2027-01-01. Given Synapse
+        # claimed to support room version 12 but didn't adhere to this behavior until
+        # 2026-05-04, we will skip for now.
         invite_room_state = event.unsigned.get("invite_room_state")
-        if invite_room_state is not None and room_version.msc4311_stripped_state:
+        if room_version.msc4311_stripped_state:
             try:
                 # Scrutinize JSON values
                 assert isinstance(invite_room_state, list), (
-                    "`invite_room_state` must be a list of PDU's"
+                    "`invite_room_state` must be a list of PDU's that includes the `m.room.create` event"
                 )
                 includes_create_event = False
                 for raw_stripped_event in invite_room_state:
@@ -1205,7 +1209,7 @@ class FederationHandler:
 
         # With MSC4311: `invite_room_state` over federation can use full PDUs so we need
         # to convert them into "stripped state events" so they don't end up being sent
-        # down to the client.
+        # down to the client as full PDU's.
         #
         # We do this separate from the validation above as sending full PDU's can happen
         # in any room version.