diff --git a/litellm/litellm_core_utils/realtime_streaming.py b/litellm/litellm_core_utils/realtime_streaming.py
index 3550363601b..1ad2dd80e10 100644
--- a/litellm/litellm_core_utils/realtime_streaming.py
+++ b/litellm/litellm_core_utils/realtime_streaming.py
@@ -636,6 +636,27 @@ class RealTimeStreaming:
 
                 ## LOGGING
                 self.store_input(message=message)
+                
+                ## GUARDRAIL: Inject turn_detection into first session.update if needed
+                try:
+                    msg_obj = json.loads(message)
+                    if (
+                        msg_obj.get("type") == "session.update"
+                        and self.session_configuration_request is None
+                        and not self._guardrail_turn_detection_update_sent
+                        and self._has_audio_transcription_guardrails()
+                    ):
+                        # Inject turn_detection into the first session.update
+                        session = msg_obj.setdefault("session", {})
+                        session.setdefault("turn_detection", {})["create_response"] = False
+                        message = json.dumps(msg_obj)
+                        self._guardrail_turn_detection_update_sent = True
+                        verbose_logger.debug(
+                            "Injected turn_detection into first session.update for audio transcription guardrails"
+                        )
+                except (json.JSONDecodeError, AttributeError):
+                    pass
+                
                 ## FORWARD TO BACKEND
                 if self.provider_config:
                     message = self.provider_config.transform_realtime_request(
diff --git a/tests/test_litellm/litellm_core_utils/test_realtime_streaming.py b/tests/test_litellm/litellm_core_utils/test_realtime_streaming.py
index e4b85cd35d4..f7fc3184f10 100644
--- a/tests/test_litellm/litellm_core_utils/test_realtime_streaming.py
+++ b/tests/test_litellm/litellm_core_utils/test_realtime_streaming.py
@@ -1117,3 +1117,56 @@ async def test_guardrail_update_respects_idempotency_flag():
     # Second call should be a no-op (idempotent)
     await streaming._maybe_send_guardrail_turn_detection_update()
     assert backend_ws.send.await_count == 1  # Still 1, not 2
+
+
+@pytest.mark.asyncio
+async def test_guardrail_turn_detection_injected_into_first_session_update_deferred_mode():
+    """Verify turn_detection is injected into first session.update in deferred mode."""
+    client_ws = AsyncMock()
+    client_ws.receive_text = AsyncMock(
+        side_effect=[
+            json.dumps({
+                "type": "session.update",
+                "session": {
+                    "modalities": ["text", "audio"],
+                    "tools": [{"type": "function", "name": "get_weather"}],
+                }
+            }),
+            ConnectionClosed(None, None),
+        ]
+    )
+    backend_ws = MagicMock()
+    backend_ws.send = AsyncMock()
+
+    logging_obj = MagicMock()
+    logging_obj.litellm_trace_id = "trace_1"
+    logging_obj.async_success_handler = AsyncMock()
+    logging_obj.success_handler = MagicMock()
+
+    provider_config = MagicMock()
+    transformed_messages = []
+    def mock_transform(msg, model, session_config):
+        transformed_messages.append((msg, session_config))
+        return [msg]  # Pass through for simplicity
+    provider_config.transform_realtime_request = MagicMock(side_effect=mock_transform)
+
+    streaming = RealTimeStreaming(
+        websocket=client_ws,
+        backend_ws=backend_ws,
+        logging_obj=logging_obj,
+        provider_config=provider_config,
+        model="gemini-2.5-flash",
+    )
+    streaming._has_audio_transcription_guardrails = MagicMock(return_value=True)  # type: ignore[method-assign]
+    
+    # Simulate first session.update in deferred mode
+    await streaming.client_ack_messages()
+    
+    # Verify turn_detection was injected into the session.update
+    assert len(transformed_messages) == 1
+    transformed_msg, session_config = transformed_messages[0]
+    msg_obj = json.loads(transformed_msg)
+    assert msg_obj["type"] == "session.update"
+    assert "turn_detection" in msg_obj["session"]
+    assert msg_obj["session"]["turn_detection"]["create_response"] is False
+    assert streaming._guardrail_turn_detection_update_sent is True