diff --git a/litellm/responses/streaming_iterator.py b/litellm/responses/streaming_iterator.py
index c4e72cb7dc5..a7eecf53dfe 100644
--- a/litellm/responses/streaming_iterator.py
+++ b/litellm/responses/streaming_iterator.py
@@ -1212,6 +1212,18 @@ def _build_synthetic_response_events(
             response=transformed,
         )
     )
+
+    # Assign monotonic sequence_number to every event. The helpers above
+    # build events without consistently passing sequence_number, and several
+    # event types now declare `sequence_number: int = 0` (default), which
+    # would otherwise serialize as 0 for most events and break the strict
+    # monotonic ordering guarantee expected by some Responses API clients.
+    for idx, event in enumerate(events):
+        try:
+            event.sequence_number = idx
+        except (AttributeError, ValueError):
+            pass
+
     return events