fix(core): raise a clear UserError when a tool parameter is named model_config

src/agents/function_schema.py

@@ -511,6 +511,13 @@ def function_schema(
                 )
 
     # 3. Dynamically build a Pydantic model
+    if "model_config" in fields:
+        # Pydantic reads a ``model_config`` keyword as the model's configuration, not as a
+        # field, so create_model() fails deep inside Pydantic with an unhelpful TypeError.
+        raise UserError(
+            f"Parameter `model_config` in function {func_name} is reserved by Pydantic and cannot"
+            " be a tool argument. Rename the parameter."
+        )
     dynamic_model = create_model(f"{func_name}_args", __base__=BaseModel, **fields)
 
     # 4. Build JSON schema from that model

tests/test_function_schema.py

@@ -1350,3 +1350,14 @@ def test_to_call_args_allows_kwargs_key_matching_var_positional_param() -> None:
     args, kwargs_dict = fs.to_call_args(parsed)
 
     assert _kwargs_var_positional_name(*args, **kwargs_dict) == ((1,), {"rest": 5})
+
+
+def test_model_config_parameter_raises_a_clear_user_error():
+    """Pydantic treats a ``model_config`` kwarg to create_model() as the configuration, which
+    surfaced as ``TypeError: 'FieldInfo' object is not iterable``. Name the real problem."""
+
+    def configure(model_config: int) -> int:
+        return model_config
+
+    with pytest.raises(UserError, match="`model_config` in function configure is reserved"):
+        function_schema(configure)