Handle built-in tool errors better in tool registration

pydantic_ai_slim/pydantic_ai/_function_schema.py

@@ -96,16 +96,20 @@ def function_schema(  # noqa: C901
     config = ConfigDict(title=function.__name__, use_attribute_docstrings=True)
     config_wrapper = ConfigWrapper(config)
     gen_schema = _generate_schema.GenerateSchema(config_wrapper)
+    errors: list[str] = []
 
-    sig = signature(function)
+    try:
+        sig = signature(function)
+    except ValueError as e:
+        errors.append(str(e))
+        sig = signature(lambda: None)
 
     type_hints = _typing_extra.get_function_type_hints(function)
 
     var_kwargs_schema: core_schema.CoreSchema | None = None
     fields: dict[str, core_schema.TypedDictField] = {}
     positional_fields: list[str] = []
     var_positional_field: str | None = None
-    errors: list[str] = []
     decorators = _decorators.DecoratorInfos()
 
     description, field_descriptions = doc_descriptions(function, sig, docstring_format=docstring_format)
@@ -235,14 +239,19 @@ def _takes_ctx(function: TargetFunc[P, R]) -> TypeIs[WithCtx[P, R]]:
     Returns:
         `True` if the function takes a `RunContext` as first argument, `False` otherwise.
     """
-    sig = signature(function)
+    try:
+        sig = signature(function)
+    except ValueError:  # pragma: no cover
+        return False  # pragma: no cover
     try:
         first_param_name = next(iter(sig.parameters.keys()))
     except StopIteration:
         return False
     else:
         type_hints = _typing_extra.get_function_type_hints(function)
-        annotation = type_hints[first_param_name]
+        annotation = type_hints.get(first_param_name)
+        if annotation is None:
+            return False  # pragma: no cover
         return True is not sig.empty and _is_call_ctx(annotation)
 
 

tests/test_tools.py

@@ -55,6 +55,26 @@ async def invalid_tool(ctx: RunContext[None]) -> str:  # pragma: no cover
     )
 
 
+def test_builtin_tool_registration():
+    """
+    Test that built-in functions can't be registered as tools.
+    """
+
+    with pytest.raises(
+        UserError,
+        match='Error generating schema for min:\n  no signature found for builtin <built-in function min>',
+    ):
+        agent = Agent(TestModel())
+        agent.tool_plain(min)
+
+    with pytest.raises(
+        UserError,
+        match='Error generating schema for max:\n  no signature found for builtin <built-in function max>',
+    ):
+        agent = Agent(TestModel())
+        agent.tool_plain(max)
+
+
 def test_tool_ctx_second():
     agent = Agent(TestModel())