Reject variadic tool parameters

src/mcp/server/mcpserver/tools/base.py

@@ -102,6 +102,7 @@ def from_function(
             fn,
             skip_names=skip_names,
             structured_output=structured_output,
+            allow_var_params=False,
         )
         parameters = func_arg_metadata.arg_model.model_json_schema(by_alias=True)
 

src/mcp/server/mcpserver/utilities/func_metadata.py

@@ -276,6 +276,7 @@ def func_metadata(
     func: Callable[..., Any],
     skip_names: Sequence[str] = (),
     structured_output: bool | None = None,
+    allow_var_params: bool = True,
 ) -> FuncMetadata:
     """Given a function, return metadata including a Pydantic model representing its signature.
 
@@ -293,6 +294,11 @@ def func_metadata(
         func: The function to convert to a Pydantic model
         skip_names: A list of parameter names to skip. These will not be included in
             the model.
+        allow_var_params: When False, a `*args` or `**kwargs` parameter raises
+            `InvalidSignature`; a variadic parameter has no scalar schema and would
+            otherwise be modelled as an unusable normal field. Tools pass False;
+            resource templates leave it True since they use `**kwargs` for
+            runtime-determined URI variables.
         structured_output: Controls whether the tool's output is structured or unstructured
             - If None, auto-detects based on the function's return type annotation
             - If True, creates a structured tool (return type annotation permitting)
@@ -329,6 +335,12 @@ def func_metadata(
     for param in params.values():
         if param.name.startswith("_"):  # pragma: no cover
             raise InvalidSignature(f"Parameter {param.name} of {func.__name__} cannot start with '_'")
+        if not allow_var_params and param.kind in (
+            inspect.Parameter.VAR_POSITIONAL,
+            inspect.Parameter.VAR_KEYWORD,
+        ):
+            variadic = "*args" if param.kind is inspect.Parameter.VAR_POSITIONAL else "**kwargs"
+            raise InvalidSignature(f"Function {func.__name__} cannot have a {variadic} parameter")
         if param.name in skip_names:
             continue
 

tests/server/mcpserver/test_func_metadata.py

@@ -1520,3 +1520,36 @@ def fn() -> StepA | StepB: ...  # pragma: no branch
 
     meta = func_metadata(fn)
     assert meta.output_schema is None
+
+
+def test_var_positional_param_is_rejected_for_tools():
+    """A tool signature with *args has no scalar schema and is rejected at registration."""
+
+    def func_with_var_args(a: int, *args: int) -> int:  # pragma: no cover
+        return a
+
+    with pytest.raises(InvalidSignature) as exc_info:
+        func_metadata(func_with_var_args, allow_var_params=False)
+    assert "*args" in str(exc_info.value)
+
+
+def test_var_keyword_param_is_rejected_for_tools():
+    """A tool signature with **kwargs has no scalar schema and is rejected at registration."""
+
+    def func_with_var_kwargs(a: int, **kwargs: int) -> int:  # pragma: no cover
+        return a
+
+    with pytest.raises(InvalidSignature) as exc_info:
+        func_metadata(func_with_var_kwargs, allow_var_params=False)
+    assert "**kwargs" in str(exc_info.value)
+
+
+def test_var_keyword_param_is_allowed_by_default():
+    """Resource templates keep using **kwargs for runtime-determined URI variables, so the
+    default (allow_var_params=True) must not reject a variadic parameter."""
+
+    def func_with_var_kwargs(**kwargs: str) -> str:  # pragma: no cover
+        return ""
+
+    meta = func_metadata(func_with_var_kwargs)
+    assert isinstance(meta, FuncMetadata)