From 0927cc492afe96c657219551be9bc0c0d4f30fe4 Mon Sep 17 00:00:00 2001
From: Gyanu Mayank <gyanumayank0@gmail.com>
Date: Fri, 18 Sep 2026 08:47:47 +0530
Subject: [PATCH] fix: reject *args and **kwargs on tool signatures

A tool with a variadic parameter is advertised with a schema that can
never be satisfied, so the call always fails validation. Raise
InvalidSignature at registration, the same way a leading underscore
already is. Resource templates still allow **kwargs for URI variables
that are only known at match time.
---
 .../server/mcpserver/resources/templates.py   |  5 +++-
 .../mcpserver/utilities/func_metadata.py      | 19 +++++++++++++++
 tests/server/mcpserver/test_func_metadata.py  | 24 +++++++++++++++++++
 3 files changed, 47 insertions(+), 1 deletion(-)

diff --git a/src/mcp/server/mcpserver/resources/templates.py b/src/mcp/server/mcpserver/resources/templates.py
index 621b2e9448..4964974cb7 100644
--- a/src/mcp/server/mcpserver/resources/templates.py
+++ b/src/mcp/server/mcpserver/resources/templates.py
@@ -151,7 +151,10 @@ def from_function(
 
         # Only the argument model is needed; a resource has no output schema to derive
         func_arg_metadata = func_metadata(
-            fn, skip_names=[context_kwarg] if context_kwarg is not None else [], structured_output=False
+            fn,
+            skip_names=[context_kwarg] if context_kwarg is not None else [],
+            structured_output=False,
+            allow_variadic=True,
         )
         parameters = func_arg_metadata.arg_model.model_json_schema()
 
diff --git a/src/mcp/server/mcpserver/utilities/func_metadata.py b/src/mcp/server/mcpserver/utilities/func_metadata.py
index 0ffac07c4e..f25165224a 100644
--- a/src/mcp/server/mcpserver/utilities/func_metadata.py
+++ b/src/mcp/server/mcpserver/utilities/func_metadata.py
@@ -276,6 +276,8 @@ def func_metadata(
     func: Callable[..., Any],
     skip_names: Sequence[str] = (),
     structured_output: bool | None = None,
+    *,
+    allow_variadic: bool = False,
 ) -> FuncMetadata:
     """Given a function, return metadata including a Pydantic model representing its signature.
 
@@ -293,6 +295,11 @@ 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_variadic: If False (the default, used for tools), ``*args`` and
+            ``**kwargs`` parameters raise :class:`InvalidSignature` because JSON
+            Schema cannot express them and the resulting tool would be uncallable.
+            Resource templates pass True so a catch-all ``**kwargs`` can receive
+            URI variables that are only known at match time.
         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)
@@ -331,6 +338,18 @@ def func_metadata(
             raise InvalidSignature(f"Parameter {param.name} of {func.__name__} cannot start with '_'")
         if param.name in skip_names:
             continue
+        if param.kind in (inspect.Parameter.VAR_POSITIONAL, inspect.Parameter.VAR_KEYWORD):
+            if allow_variadic:
+                continue
+            kind = (
+                "variadic positional (*args)"
+                if param.kind is inspect.Parameter.VAR_POSITIONAL
+                else "variadic keyword (**kwargs)"
+            )
+            raise InvalidSignature(
+                f"Parameter {param.name} of {func.__name__} is a {kind} parameter; "
+                "JSON Schema cannot express *args or **kwargs"
+            )
 
         annotation = param.annotation if param.annotation is not inspect.Parameter.empty else Any
         field_name = param.name
diff --git a/tests/server/mcpserver/test_func_metadata.py b/tests/server/mcpserver/test_func_metadata.py
index dba0637ded..556a8927d9 100644
--- a/tests/server/mcpserver/test_func_metadata.py
+++ b/tests/server/mcpserver/test_func_metadata.py
@@ -1520,3 +1520,27 @@ def fn() -> StepA | StepB: ...  # pragma: no branch
 
     meta = func_metadata(fn)
     assert meta.output_schema is None
+
+
+def test_variadic_kwargs_rejected_for_tools():
+    def with_kwargs(x: int, **kwargs: str) -> str:  # pragma: no cover
+        return f"{x} {kwargs}"
+
+    with pytest.raises(InvalidSignature, match=r"\*\*kwargs"):
+        func_metadata(with_kwargs)
+
+    schema = func_metadata(with_kwargs, allow_variadic=True).arg_model.model_json_schema()
+    assert "kwargs" not in schema.get("properties", {})
+    assert "x" in schema.get("properties", {})
+
+
+def test_variadic_args_rejected_for_tools():
+    def with_args(x: int, *args: str) -> str:  # pragma: no cover
+        return f"{x} {args}"
+
+    with pytest.raises(InvalidSignature, match=r"\*args"):
+        func_metadata(with_args)
+
+    schema = func_metadata(with_args, allow_variadic=True).arg_model.model_json_schema()
+    assert "args" not in schema.get("properties", {})
+    assert "x" in schema.get("properties", {})