fix(items): empty list/tuple tool output dropped via all([]) == True

src/agents/items.py

@@ -801,7 +801,10 @@ def _convert_tool_output(cls, output: Any) -> str | ResponseFunctionCallOutputIt
             maybe_converted_output_list = [
                 cls._maybe_get_output_as_structured_function_output(item) for item in output
             ]
-            if all(maybe_converted_output_list):
+            # An empty list/tuple has no structured items; ``all([])`` is ``True``,
+            # so guard against it to avoid emitting an empty structured-output list
+            # (which would drop the tool result) and stringify instead.
+            if maybe_converted_output_list and all(maybe_converted_output_list):
                 return [
                     cls._convert_single_tool_output_pydantic_model(item)
                     for item in maybe_converted_output_list

tests/test_tool_output_conversion.py

@@ -370,3 +370,26 @@ def test_tool_call_output_item_mixed_list_partial_invalid_not_converted() -> Non
     # All-or-nothing: if any item is invalid, convert entire list to string
     assert isinstance(payload["output"], str)
     assert payload["output"] == "[{'type': 'text', 'text': 'hello'}, {'msg': 'foobar'}]"
+
+
+def test_tool_call_output_item_empty_list_not_converted() -> None:
+    """An empty list has no structured items, so it should stringify rather than
+    produce an empty structured-output list (which would drop the tool result)."""
+    call = _make_tool_call()
+    payload = ItemHelpers.tool_call_output_item(call, [])
+
+    assert payload["type"] == "function_call_output"
+    assert payload["call_id"] == call.call_id
+    assert isinstance(payload["output"], str)
+    assert payload["output"] == "[]"
+
+
+def test_tool_call_output_item_empty_tuple_not_converted() -> None:
+    """An empty tuple should stringify, mirroring the empty-list behavior."""
+    call = _make_tool_call()
+    payload = ItemHelpers.tool_call_output_item(call, ())
+
+    assert payload["type"] == "function_call_output"
+    assert payload["call_id"] == call.call_id
+    assert isinstance(payload["output"], str)
+    assert payload["output"] == "()"