A tool with a *args or **kwargs parameter is registered with a schema it can never satisfy

Initial Checks

• I confirm that I'm using the newest release of my line (2.x, main at 6affe5c)
• I confirm that I searched for my issue in the issue tracker before opening this issue

Release line

2.x (current stable)

Description

Registering a tool whose function signature has a *args or **kwargs parameter succeeds, and the tool shows up in list_tools, but it can never actually be called: the generated JSON schema lists args/kwargs as a required parameter of a plain scalar type, which doesn't match how the function actually receives those values.

Expected: either the decorator rejects a signature it can't turn into a schema (it already does this for a leading underscore in a parameter name), or the schema reflects what the function accepts.

Actual: the tool is silently registered and permanently uncallable. Calling it with only the named parameters filled in fails schema validation because args/kwargs is "missing"; there's no way to actually supply it, since a JSON object has no way to express "and then also a variable number of positional values" for a single scalar field.

Example Code

from mcp.server.mcpserver import MCPServer

server = MCPServer("demo")


@server.tool()
def with_kwargs(x: int, **kwargs: str) -> str:
    return f"{x} {kwargs}"

Listing tools shows:

{'type': 'object', 'properties': {'x': {'title': 'X', 'type': 'integer'}, 'kwargs': {'title': 'Kwargs', 'type': 'string'}}, 'required': ['x', 'kwargs'], 'title': 'with_kwargsArguments'}

Calling it with {"x": 1} (the only arguments a caller could reasonably guess) fails:

Error executing tool with_kwargs: 1 validation error for with_kwargsArguments
kwargs
  Field required [type=missing, input_value={'x': 1}, input_type=dict]

Same shape with *args instead of **kwargs.

Python & MCP Python SDK

Python 3.14.7, mcp-python-sdk main @ 6affe5c0d3588fd1705713b3703dc68015cfe3eb

I have a patch that raises InvalidSignature for a *args/**kwargs tool parameter at registration time, scoped so it doesn't affect resource templates (which legitimately use **kwargs for runtime-determined URI variables); happy to open a PR against this issue if that's the direction you'd want.