fix(tool): route non-object JSON inputs through the JSON-error path

src/agents/tool.py

@@ -1391,6 +1391,13 @@ def _extract_tool_argument_json_error(error: Exception) -> json.JSONDecodeError
     return _extract_json_decode_error(error)
 
 
+def _is_tool_argument_input_error(error: Exception) -> bool:
+    """Return True when an error represents an invalid-JSON-input failure for a function tool."""
+    return isinstance(error, ModelBehaviorError) and str(error).startswith(
+        "Invalid JSON input for tool"
+    )
+
+
 def _build_handled_function_tool_error_handler(
     *,
     span_message: str,
@@ -1411,6 +1418,11 @@ def _on_handled_error(
         if json_decode_error is not None and span_message_for_json_decode_error is not None:
             resolved_span_message = span_message_for_json_decode_error
             span_error_detail = str(json_decode_error)
+        elif span_message_for_json_decode_error is not None and _is_tool_argument_input_error(
+            error
+        ):
+            resolved_span_message = span_message_for_json_decode_error
+            span_error_detail = str(error)
         else:
             resolved_span_message = span_message
             span_error_detail = str(error)
@@ -1481,6 +1493,12 @@ def default_tool_error_function(ctx: RunContextWrapper[Any], error: Exception) -
             "Please try again with valid JSON. "
             f"Error: {json_decode_error}"
         )
+    if _is_tool_argument_input_error(error):
+        return (
+            "An error occurred while parsing tool arguments. "
+            "Please try again with valid JSON. "
+            f"Error: {error}"
+        )
     return f"An error occurred while running the tool. Please try again. Error: {str(error)}"
 
 

tests/test_function_tool.py

@@ -759,6 +759,31 @@ def echo(value: str) -> str:
         )
 
 
+@pytest.mark.asyncio
+@pytest.mark.parametrize("input_json", ["[]", '"value"', "123", "null", "true"])
+async def test_default_error_function_treats_non_object_input_as_json_error(
+    input_json: str,
+) -> None:
+    def echo(value: str) -> str:
+        return value
+
+    tool = function_tool(echo, name_override="echo_tool")
+
+    result = await tool.on_invoke_tool(
+        ToolContext(
+            None,
+            tool_name="echo_tool",
+            tool_call_id="1",
+            tool_arguments=input_json,
+        ),
+        input_json,
+    )
+
+    assert isinstance(result, str)
+    assert result.startswith("An error occurred while parsing tool arguments.")
+    assert "expected a JSON object" in result
+
+
 @pytest.mark.asyncio
 async def test_default_failure_error_function_survives_deepcopy() -> None:
     def boom() -> None: