fix(realtime): include tool arguments in RealtimeToolStart events

Fixes #1663

Author: ruskaruma

src/agents/realtime/events.py

@@ -69,6 +69,10 @@ class RealtimeToolStart:
     """The agent that updated."""
 
     tool: Tool
+    """The tool being called."""
+
+    arguments: str
+    """The arguments passed to the tool as a JSON string."""
 
     info: RealtimeEventInfo
     """Common info for all events, such as the context."""

src/agents/realtime/session.py

@@ -411,6 +411,7 @@ async def _handle_tool_call(
                     info=self._event_info,
                     tool=function_map[event.name],
                     agent=agent,
+                    arguments=event.arguments,
                 )
             )
 

tests/realtime/test_session.py

@@ -986,6 +986,7 @@ async def test_function_tool_execution_success(
         assert isinstance(tool_start_event, RealtimeToolStart)
         assert tool_start_event.tool == mock_function_tool
         assert tool_start_event.agent == mock_agent
+        assert tool_start_event.arguments == '{"param": "value"}'
 
         # Check tool end event
         tool_end_event = await session._event_queue.get()
@@ -1111,6 +1112,7 @@ async def test_function_tool_exception_handling(
         assert session._event_queue.qsize() == 1
         tool_start_event = await session._event_queue.get()
         assert isinstance(tool_start_event, RealtimeToolStart)
+        assert tool_start_event.arguments == "{}"
 
         # But no tool output should have been sent and no end event queued
         assert len(mock_model.sent_tool_outputs) == 0
@@ -1133,10 +1135,15 @@ async def test_tool_call_with_complex_arguments(
 
         await session._handle_tool_call(tool_call_event)
 
-        # Verify arguments were passed correctly
+        # Verify arguments were passed correctly to tool
         call_args = mock_function_tool.on_invoke_tool.call_args
         assert call_args[0][1] == complex_args
 
+        # Verify tool_start event includes arguments
+        tool_start_event = await session._event_queue.get()
+        assert isinstance(tool_start_event, RealtimeToolStart)
+        assert tool_start_event.arguments == complex_args
+
     @pytest.mark.asyncio
     async def test_tool_call_with_custom_call_id(self, mock_model, mock_agent, mock_function_tool):
         """Test that tool context receives correct call_id"""