diff --git a/src/alerting.py b/src/alerting.py
index f5a95f7..726a542 100644
--- a/src/alerting.py
+++ b/src/alerting.py
@@ -52,7 +52,9 @@ class MatrixNotifier:
     def send_message(self, message: str, msgtype: str = "m.text") -> bool:
         """Send message to Matrix room"""
         try:
-            url = f"{self.homeserver}/_matrix/client/v3/rooms/{self.room_id}/send/m.room.message"
+            # Add transaction ID to prevent duplicates
+            txn_id = datetime.datetime.now().strftime("%Y%m%d_%H%M%S_%f")
+            url = f"{self.homeserver}/_matrix/client/v3/rooms/{self.room_id}/send/m.room.message/{txn_id}"
 
             headers = {
                 "Authorization": f"Bearer {self.access_token}",
@@ -64,11 +66,8 @@ class MatrixNotifier:
                 "body": message
             }
 
-            # Add transaction ID to prevent duplicates
-            txn_id = datetime.datetime.now().strftime("%Y%m%d_%H%M%S_%f")
-            url += f"/{txn_id}"
-
-            response = self.session.post(url, headers=headers, json=data, timeout=10)
+            # Matrix API requires PUT when transaction ID is in the URL path
+            response = self.session.put(url, headers=headers, json=data, timeout=10)
             response.raise_for_status()
 
             logger.info(f"Matrix message sent successfully: {response.json().get('event_id')}")