diff --git a/api/client-server/room_send.yaml b/api/client-server/room_send.yaml
index 6963f76c..fc8f3339 100644
--- a/api/client-server/room_send.yaml
+++ b/api/client-server/room_send.yaml
@@ -85,5 +85,7 @@ paths:
                 type: string
                 description: |-
                   A unique identifier for the event.
+            required:
+              - event_id
       tags:
         - Room participation
diff --git a/api/client-server/room_state.yaml b/api/client-server/room_state.yaml
index 62168f26..640e2009 100644
--- a/api/client-server/room_state.yaml
+++ b/api/client-server/room_state.yaml
@@ -92,6 +92,8 @@ paths:
                 type: string
                 description: |-
                   A unique identifier for the event.
+            required:
+              - event_id
         403:
           description: |-
             The sender doesn't have permission to send the event into the room.
diff --git a/changelogs/client_server/newsfragments/2525.clarification b/changelogs/client_server/newsfragments/2525.clarification
new file mode 100644
index 00000000..df1fc538
--- /dev/null
+++ b/changelogs/client_server/newsfragments/2525.clarification
@@ -0,0 +1 @@
+Clarify that an ``event_id`` is returned when sending events.