fix: avoid auto response for unknown realtime tools (ref: #3287)

src/agents/realtime/session.py

@@ -776,7 +776,7 @@ async def _handle_tool_call(
                 RealtimeModelSendToolOutput(
                     tool_call=event,
                     output=error_message,
-                    start_response=True,
+                    start_response=False,
                 )
             )
             await self._put_event(

tests/realtime/test_session.py

@@ -1288,7 +1288,7 @@ async def test_handoff_tool_handling(self, mock_model):
 
     @pytest.mark.asyncio
     async def test_unknown_tool_handling(self, mock_model, mock_agent, mock_function_tool):
-        """Test that unknown tools complete the model call and emit a RealtimeError event"""
+        """Test that unknown tools complete the model call without starting a response."""
         # Set up agent to return different tool than what's called
         mock_function_tool.name = "known_tool"
         mock_agent.get_all_tools.return_value = [mock_function_tool]
@@ -1307,7 +1307,7 @@ async def test_unknown_tool_handling(self, mock_model, mock_agent, mock_function
         sent_call, sent_output, start_response = mock_model.sent_tool_outputs[0]
         assert sent_call == tool_call_event
         assert "Tool unknown_tool not found" in sent_output
-        assert start_response is True
+        assert start_response is False
 
         # Should have emitted a RealtimeError event
         assert session._event_queue.qsize() >= 1