diff --git a/src/app/utils/scheduledMessages.ts b/src/app/utils/scheduledMessages.ts
index b1aefd05d..f7e8b6e5c 100644
--- a/src/app/utils/scheduledMessages.ts
+++ b/src/app/utils/scheduledMessages.ts
@@ -2,11 +2,9 @@ import { IContent, MatrixClient, Method } from 'matrix-js-sdk';
 
 /**
  * Schedule a message via MSC4140 (delayed messages).
- * @param mx - Matrix client instance
- * @param roomId - The room to send the message in
- * @param content - The message event content
- * @param sendAtMs - Unix timestamp (ms) when the message should be sent
- * @returns The delay_id returned by the server (use to cancel/restart)
+ * Synapse implementation: pass org.matrix.msc4140.delay on the standard
+ * /v3/rooms send endpoint. The server stores the event and returns delay_id
+ * instead of event_id.
  */
 export async function scheduleMessage(
   mx: MatrixClient,
@@ -16,18 +14,21 @@ export async function scheduleMessage(
 ): Promise<string> {
   const delayMs = Math.max(1000, Math.round(sendAtMs - Date.now()));
   const txnId = `sched_${Date.now()}_${Math.random().toString(36).slice(2)}`;
-  // Use the path relative to the MSC4140 prefix — authedRequest prepends prefix to path.
   const path = `/rooms/${encodeURIComponent(roomId)}/send/m.room.message/${txnId}`;
-  const res = (await mx.http.authedRequest(Method.Put, path, { delay: delayMs }, content, {
-    prefix: '/_matrix/client/unstable/org.matrix.msc4140',
-  })) as { delay_id: string };
+  // Use standard v3 prefix with the MSC4140 delay query param.
+  const res = (await mx.http.authedRequest(
+    Method.Put,
+    path,
+    {
+      'org.matrix.msc4140.delay': delayMs,
+    },
+    content,
+  )) as { delay_id: string };
   return res.delay_id;
 }
 
 /**
  * Cancel a scheduled message via MSC4140.
- * @param mx - Matrix client instance
- * @param delayId - The delay_id from scheduleMessage
  */
 export async function cancelScheduledMessage(mx: MatrixClient, delayId: string): Promise<void> {
   const path = `/delayed_events/${encodeURIComponent(delayId)}`;
@@ -36,9 +37,7 @@ export async function cancelScheduledMessage(mx: MatrixClient, delayId: string):
     path,
     undefined,
     { action: 'cancel' },
-    {
-      prefix: '/_matrix/client/unstable/org.matrix.msc4140',
-    },
+    { prefix: '/_matrix/client/unstable/org.matrix.msc4140' },
   );
 }
 
@@ -49,8 +48,6 @@ export async function restartScheduledMessage(mx: MatrixClient, delayId: string)
     path,
     undefined,
     { action: 'restart' },
-    {
-      prefix: '/_matrix/client/unstable/org.matrix.msc4140',
-    },
+    { prefix: '/_matrix/client/unstable/org.matrix.msc4140' },
   );
 }