fix(core): fail closed on empty tool arguments (#4545)

src/agents/util/_approvals.py

@@ -17,9 +17,11 @@ def _reject_nonstandard_json_constant(value: str) -> NoReturn:
 
 def parse_function_tool_arguments(arguments: str | None) -> dict[str, Any] | None:
     """Return parsed object arguments, or None when an approval policy cannot inspect them."""
+    if arguments is None or not arguments.strip():
+        return None
     try:
         parsed = json.loads(
-            arguments or "{}",
+            arguments,
             parse_constant=_reject_nonstandard_json_constant,
         )
     except ValueError:

tests/realtime/test_session.py

@@ -3125,6 +3125,7 @@ class TestToolCallExecution:
     @pytest.mark.parametrize(
         "arguments",
         [
+            "",
             '{"subject": "refund"',
             "null",
             "[]",

tests/test_hitl_error_scenarios.py

@@ -919,6 +919,7 @@ async def test_function_needs_approval_invalid_type_raises() -> None:
 @pytest.mark.parametrize(
     "arguments",
     [
+        "",
         '{"subject": "refund"',
         "null",
         "[]",