diff --git a/changelogs/client_server/newsfragments/2403.feature b/changelogs/client_server/newsfragments/2403.feature
new file mode 100644
index 00000000..b4fd7c2a
--- /dev/null
+++ b/changelogs/client_server/newsfragments/2403.feature
@@ -0,0 +1 @@
+Specify `unsigned.replaces_state` in client-formatted events. Contributed by @nexy7574.
diff --git a/data/api/client-server/definitions/client_event_without_room_id.yaml b/data/api/client-server/definitions/client_event_without_room_id.yaml
index b8011ab1..02623c02 100644
--- a/data/api/client-server/definitions/client_event_without_room_id.yaml
+++ b/data/api/client-server/definitions/client_event_without_room_id.yaml
@@ -124,6 +124,8 @@ properties:
           by the local homeserver, and is only returned if the event is a state event.
           Unlike `prev_content`, this field is included regardless of history visibility.
         type: string
+        format: mx-event-id
+        pattern: "^\\$"
         x-addedInMatrixVersion: "1.19"
       membership:
         description: |