fix: handle cancelled single function tools as tool failures

src/agents/agent.py

@@ -850,6 +850,24 @@ async def dispatch_stream_events() -> None:
             if custom_output_extractor:
                 return await custom_output_extractor(run_result)
 
+            if run_result.final_output is not None and (
+                not isinstance(run_result.final_output, str) or run_result.final_output != ""
+            ):
+                return run_result.final_output
+
+            from .items import ItemHelpers
+
+            text_output = ItemHelpers.text_message_outputs(run_result.new_items)
+            if text_output:
+                return text_output
+
+            for item in reversed(run_result.to_input_list()):
+                if item.get("type") != "function_call_output":
+                    continue
+                output = item.get("output")
+                if isinstance(output, str) and output:
+                    return output
+
             return run_result.final_output
 
         run_agent_tool = _build_wrapped_function_tool(

src/agents/run_internal/tool_execution.py

@@ -1400,9 +1400,10 @@ async def _drain_cancelled_tasks(
         self,
         tasks: set[asyncio.Task[Any]],
     ) -> tuple[_FunctionToolFailure | None, set[asyncio.Task[Any]]]:
-        late_failure_sources: dict[asyncio.Task[Any], _FunctionToolFailureSource] = {
-            task: "cancelled_teardown" for task in tasks
-        }
+        late_failure_sources: dict[asyncio.Task[Any], _FunctionToolFailureSource] = dict.fromkeys(
+            tasks,
+            "cancelled_teardown",
+        )
         return await _drain_cancelled_function_tool_tasks(
             pending_tasks=tasks,
             task_states=self.task_states,
@@ -1415,9 +1416,9 @@ async def _wait_post_invoke_tasks(
         self,
         tasks: set[asyncio.Task[Any]],
     ) -> tuple[_FunctionToolFailure | None, set[asyncio.Task[Any]]]:
-        post_invoke_failure_sources: dict[asyncio.Task[Any], _FunctionToolFailureSource] = {
-            task: "post_invoke" for task in tasks
-        }
+        post_invoke_failure_sources: dict[asyncio.Task[Any], _FunctionToolFailureSource] = (
+            dict.fromkeys(tasks, "post_invoke")
+        )
         return await _wait_pending_function_tool_tasks_for_timeout(
             pending_tasks=tasks,
             task_states=self.task_states,
@@ -1638,7 +1639,7 @@ async def _invoke_tool_and_run_post_invoke(
                 arguments=tool_call.arguments,
             )
         except asyncio.CancelledError as e:
-            if not self.isolate_parallel_failures or outer_task in self.teardown_cancelled_tasks:
+            if outer_task in self.teardown_cancelled_tasks:
                 raise
 
             result = await maybe_invoke_function_tool_failure_error_function(

tests/test_agent_runner.py

@@ -641,6 +641,45 @@ async def _cancel_tool() -> str:
     ]
 
 
+@pytest.mark.asyncio
+async def test_single_tool_call_with_cancelled_tool_reaches_final_output() -> None:
+    async def _cancel_tool() -> str:
+        raise asyncio.CancelledError("tool-cancelled")
+
+    model = FakeModel()
+    agent = Agent(
+        name="test",
+        model=model,
+        tools=[function_tool(_cancel_tool, name_override="cancel_tool")],
+    )
+
+    model.add_multiple_turn_outputs(
+        [
+            [get_function_tool_call("cancel_tool", "{}", call_id="call_cancel")],
+            [get_text_message("final answer")],
+        ]
+    )
+
+    result = await Runner.run(agent, input="user_message")
+
+    assert result.final_output == "final answer"
+    assert len(result.raw_responses) == 2
+
+    second_turn_input = cast(list[dict[str, Any]], model.last_turn_args["input"])
+    tool_outputs = [
+        item for item in second_turn_input if item.get("type") == "function_call_output"
+    ]
+    assert tool_outputs == [
+        {
+            "call_id": "call_cancel",
+            "output": (
+                "An error occurred while running the tool. Please try again. Error: tool-cancelled"
+            ),
+            "type": "function_call_output",
+        },
+    ]
+
+
 @pytest.mark.asyncio
 async def test_reasoning_item_id_policy_omits_follow_up_reasoning_ids() -> None:
     model = FakeModel()

tests/test_agent_runner_streamed.py

@@ -496,6 +496,46 @@ async def _cancel_tool() -> str:
     ]
 
 
+@pytest.mark.asyncio
+async def test_streamed_single_tool_call_with_cancelled_tool_reaches_final_output() -> None:
+    async def _cancel_tool() -> str:
+        raise asyncio.CancelledError("tool-cancelled")
+
+    model = FakeModel()
+    agent = Agent(
+        name="test",
+        model=model,
+        tools=[function_tool(_cancel_tool, name_override="cancel_tool")],
+    )
+
+    model.add_multiple_turn_outputs(
+        [
+            [get_function_tool_call("cancel_tool", "{}", call_id="call_cancel")],
+            [get_text_message("final answer")],
+        ]
+    )
+
+    result = Runner.run_streamed(agent, input="user_message")
+    await consume_stream(result)
+
+    assert result.final_output == "final answer"
+    assert len(result.raw_responses) == 2
+
+    second_turn_input = cast(list[dict[str, Any]], model.last_turn_args["input"])
+    tool_outputs = [
+        item for item in second_turn_input if item.get("type") == "function_call_output"
+    ]
+    assert tool_outputs == [
+        {
+            "call_id": "call_cancel",
+            "output": (
+                "An error occurred while running the tool. Please try again. Error: tool-cancelled"
+            ),
+            "type": "function_call_output",
+        },
+    ]
+
+
 @pytest.mark.asyncio
 async def test_streamed_reasoning_item_id_policy_omits_follow_up_reasoning_ids() -> None:
     model = FakeModel()

tests/test_run_step_execution.py

@@ -740,6 +740,29 @@ async def _manual_on_invoke_tool(_ctx: ToolContext[Any], _args: str) -> str:
     )
 
 
+@pytest.mark.asyncio
+async def test_single_tool_call_uses_default_failure_error_function_for_cancelled_tool():
+    async def _cancel_tool() -> str:
+        raise asyncio.CancelledError("tool-cancelled")
+
+    cancel_tool = function_tool(_cancel_tool, name_override="cancel_tool")
+    agent = Agent(name="test", tools=[cancel_tool])
+    response = ModelResponse(
+        output=[get_function_tool_call("cancel_tool", "{}", call_id="1")],
+        usage=Usage(),
+        response_id=None,
+    )
+
+    result = await get_execute_result(agent, response)
+
+    assert len(result.generated_items) == 2
+    assert isinstance(result.next_step, NextStepRunAgain)
+    assert_item_is_function_tool_call_output(
+        result.generated_items[1],
+        "An error occurred while running the tool. Please try again. Error: tool-cancelled",
+    )
+
+
 @pytest.mark.asyncio
 async def test_multiple_tool_calls_surface_hook_failure_over_sibling_cancellation():
     hook_started = asyncio.Event()