fix(gemini realtime): tolerate sibling-only frames (e.g. standalone usageMetadata)

A Gemini Live frame that contains only metadata keys outside
_KNOWN_GEMINI_TOP_LEVEL_KEYS (e.g. a bare {"usageMetadata": {...}}
emitted between turns) leaves returned_message empty after the
transform loop and was tripping the 'Unknown message type' guard,
which raised ValueError and terminated the WebSocket session.

Treat such frames as no-ops and return the unchanged state instead.

litellm/llms/gemini/realtime/transformation.py

@@ -1528,6 +1528,23 @@ class GeminiRealtimeConfig(BaseRealtimeConfig):
             else:
                 raise ValueError(f"Unknown openai event: {openai_event}")
         if len(returned_message) == 0:
+            # A frame whose only top-level keys are sibling metadata (e.g.
+            # a standalone ``{"usageMetadata": {...}}`` emitted by Gemini
+            # Live between turns) is not an error — there is just nothing
+            # to forward to the OpenAI-shaped client. Returning the
+            # unchanged state keeps the WebSocket alive; raising would
+            # terminate the session for a benign no-op frame.
+            if not any(key in _KNOWN_GEMINI_TOP_LEVEL_KEYS for key in json_message):
+                return {
+                    "response": returned_message,
+                    "current_output_item_id": current_output_item_id,
+                    "current_response_id": current_response_id,
+                    "current_delta_chunks": current_delta_chunks,
+                    "current_conversation_id": current_conversation_id,
+                    "current_item_chunks": current_item_chunks,
+                    "current_delta_type": current_delta_type,
+                    "session_configuration_request": session_configuration_request,
+                }
             if isinstance(message, bytes):
                 message_str = message.decode("utf-8", errors="replace")
             else:

tests/test_litellm/llms/gemini/realtime/test_gemini_realtime_transformation.py

@@ -1145,3 +1145,43 @@ def test_gemini_tool_call_id_to_name_evicts_oldest_when_capped():
         "call_6",
         "call_7",
     ]
+
+
+def test_gemini_standalone_usage_metadata_does_not_crash_websocket():
+    """A Gemini frame containing only sibling metadata (e.g. a standalone
+    ``usageMetadata`` block emitted between turns) must not trip the
+    ``Unknown message type`` guard — that would terminate the WebSocket
+    session on a benign no-op frame."""
+    config = GeminiRealtimeConfig()
+    logging_obj = MagicMock()
+    logging_obj.litellm_trace_id = "trace_usage_only"
+
+    result = config.transform_realtime_response(
+        json.dumps(
+            {
+                "usageMetadata": {
+                    "promptTokenCount": 12,
+                    "responseTokenCount": 34,
+                    "totalTokenCount": 46,
+                }
+            }
+        ),
+        "gemini-2.5-flash",
+        logging_obj,
+        realtime_response_transform_input={
+            "session_configuration_request": None,
+            "current_output_item_id": "item_existing",
+            "current_response_id": "resp_existing",
+            "current_conversation_id": "conv_existing",
+            "current_delta_chunks": [],
+            "current_item_chunks": [],
+            "current_delta_type": None,
+        },
+    )
+
+    assert result["response"] == []
+    # State must be returned unchanged so subsequent frames continue the
+    # in-flight response correctly.
+    assert result["current_output_item_id"] == "item_existing"
+    assert result["current_response_id"] == "resp_existing"
+    assert result["current_conversation_id"] == "conv_existing"