fix(core): read tool args without Pydantic property shadowing

src/agents/function_schema.py

@@ -51,14 +51,18 @@ def to_call_args(self, data: BaseModel) -> tuple[list[Any], dict[str, Any]]:
         positional_args: list[Any] = []
         keyword_args: dict[str, Any] = {}
         seen_var_positional = False
+        # Read instance storage first so Pydantic properties such as ``model_extra``
+        # and ``model_fields_set`` do not shadow tool parameters of the same name.
+        # ``model_dump()`` is unsuitable here because it converts nested models to dicts.
+        instance_values = object.__getattribute__(data, "__dict__")
 
         # Use enumerate() so we can skip the first parameter if it's context.
         for idx, (name, param) in enumerate(self.signature.parameters.items()):
             # If the function takes a RunContextWrapper and this is the first parameter, skip it.
             if self.takes_context and idx == 0:
                 continue
 
-            value = getattr(data, name, None)
+            value = instance_values[name] if name in instance_values else getattr(data, name, None)
             if param.kind == param.VAR_POSITIONAL:
                 # e.g. *args: extend positional args and mark that *args is now seen
                 positional_args.extend(value or [])

tests/test_function_schema.py

@@ -93,6 +93,44 @@ def test_simple_function():
         func_schema.params_pydantic_model(**{"a": "not an integer"})
 
 
+def function_with_model_extra_param(query: str, model_extra: str) -> str:
+    return f"{query}:{model_extra}"
+
+
+def function_with_model_fields_set_param(query: str, model_fields_set: int) -> str:
+    return f"{query}:{model_fields_set}"
+
+
+def test_to_call_args_does_not_shadow_pydantic_model_extra():
+    """A parameter named ``model_extra`` must not be replaced by BaseModel.model_extra."""
+
+    with pytest.warns(UserWarning, match="model_extra"):
+        func_schema = function_schema(function_with_model_extra_param, use_docstring_info=False)
+    parsed = func_schema.params_pydantic_model.model_validate(
+        {"query": "hello", "model_extra": "gpt-4.1"}
+    )
+
+    args, kwargs_dict = func_schema.to_call_args(parsed)
+    result = function_with_model_extra_param(*args, **kwargs_dict)
+    assert result == "hello:gpt-4.1"
+
+
+def test_to_call_args_does_not_shadow_pydantic_model_fields_set():
+    """A parameter named ``model_fields_set`` must not be replaced by BaseModel.model_fields_set."""
+
+    with pytest.warns(UserWarning, match="model_fields_set"):
+        func_schema = function_schema(
+            function_with_model_fields_set_param, use_docstring_info=False
+        )
+    parsed = func_schema.params_pydantic_model.model_validate(
+        {"query": "hello", "model_fields_set": 42}
+    )
+
+    args, kwargs_dict = func_schema.to_call_args(parsed)
+    result = function_with_model_fields_set_param(*args, **kwargs_dict)
+    assert result == "hello:42"
+
+
 def varargs_function(x: int, *numbers: float, flag: bool = False, **kwargs: Any):
     return x, numbers, flag, kwargs