diff --git a/api/client-server/sync.yaml b/api/client-server/sync.yaml
index bd659369..2c504295 100644
--- a/api/client-server/sync.yaml
+++ b/api/client-server/sync.yaml
@@ -129,6 +129,50 @@ paths:
                       title: Joined Room
                       type: object
                       properties:
+                        summary:
+                          title: RoomSummary
+                          type: object
+                          description: |-
+                            Information about the room which clients may need to
+                            correctly render it to users.
+                          properties:
+                            "m.heroes":
+                              type: array
+                              description: |-
+                                The users which can be used to generate a room name
+                                if the room does not have one. Required if the room
+                                does not have a ``m.room.name`` or ``m.room.canonical_alias``
+                                state event with non-empty content.
+
+                                This should be the first 5 members of the room, ordered
+                                by stream ordering, which are joined or invited. The
+                                list must never include the client's own user ID. When
+                                no joined or invited members are available, this should
+                                consist of the banned and left users. More than 5 members
+                                may be provided, however less than 5 should only be provided
+                                when there are less than 5 members to represent.
+
+                                When lazy-loading room members is enabled, the membership
+                                events for the heroes MUST be included in the ``state``,
+                                unless they are redundant. When the list of users changes,
+                                the server notifies the client by sending a fresh list of
+                                heroes. If there are no changes since the last sync, this
+                                field may be omitted.
+                              items:
+                                type: string
+                            "m.joined_member_count":
+                              type: integer
+                              description: |-
+                                 The number of users with ``membership`` of ``join``,
+                                 including the client's own user ID. If this field has
+                                 not changed since the last sync, it may be omitted.
+                                 Required otherwise.
+                            "m.invited_member_count":
+                              type: integer
+                              description: |-
+                                 The number of users with ``membership`` of ``invite``.
+                                 If this field has not changed since the last sync, it
+                                 may be omitted. Required otherwise.
                         state:
                           title: State
                           type: object
@@ -334,6 +378,14 @@ paths:
                 "rooms": {
                   "join": {
                     "!726s6s6q:example.com": {
+                      "summary": {
+                        "m.heroes": [
+                          "@alice:example.com",
+                          "@bob:example.com"
+                        ],
+                        "m.joined_member_count": 2,
+                        "m.invited_member_count": 0
+                      },
                       "state": {
                         "events": [
                           {