fix: send tool output to model when function tool raises exception

src/agents/realtime/session.py

@@ -694,11 +694,21 @@ async def _handle_tool_call(
                 tool_arguments=event.arguments,
                 agent=agent,
             )
-            result = await invoke_function_tool(
-                function_tool=func_tool,
-                context=tool_context,
-                arguments=event.arguments,
-            )
+            try:
+                result = await invoke_function_tool(
+                    function_tool=func_tool,
+                    context=tool_context,
+                    arguments=event.arguments,
+                )
+            except Exception:
+                await self._model.send_event(
+                    RealtimeModelSendToolOutput(
+                        tool_call=event,
+                        output=f"Tool '{event.name}' failed",
+                        start_response=True,
+                    )
+                )
+                raise
 
             await self._model.send_event(
                 RealtimeModelSendToolOutput(
@@ -729,7 +739,17 @@ async def _handle_tool_call(
             )
 
             # Execute the handoff to get the new agent
-            result = await handoff.on_invoke_handoff(self._context_wrapper, event.arguments)
+            try:
+                result = await handoff.on_invoke_handoff(self._context_wrapper, event.arguments)
+            except Exception:
+                await self._model.send_event(
+                    RealtimeModelSendToolOutput(
+                        tool_call=event,
+                        output=f"Handoff '{event.name}' failed",
+                        start_response=True,
+                    )
+                )
+                raise
             if not isinstance(result, RealtimeAgent):
                 raise UserError(
                     f"Handoff {handoff.tool_name} returned invalid result: {type(result)}"

tests/realtime/test_session.py

@@ -1109,7 +1109,7 @@ async def invoke_slow_tool(_ctx: ToolContext[Any], _arguments: str) -> str:
         with pytest.raises(ToolTimeoutError, match="timed out"):
             await session._handle_tool_call(tool_call_event)
 
-        assert len(mock_model.sent_tool_outputs) == 0
+        assert len(mock_model.sent_tool_outputs) == 1
         assert session._event_queue.qsize() == 1
 
         tool_start_event = await session._event_queue.get()
@@ -1196,7 +1196,7 @@ async def invoke_slow_tool(_ctx: ToolContext[Any], _arguments: str) -> str:
 
         assert isinstance(session._stored_exception, ToolTimeoutError)
         assert session._stored_exception.tool_name == "slow_tool"
-        assert len(mock_model.sent_tool_outputs) == 0
+        assert len(mock_model.sent_tool_outputs) == 1
 
         events = []
         while True:
@@ -1626,8 +1626,8 @@ async def test_function_tool_exception_handling(
         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
+        # An error tool output should be sent to the model before re-raising
+        assert len(mock_model.sent_tool_outputs) == 1
 
     @pytest.mark.asyncio
     async def test_tool_call_with_complex_arguments(