Initial Checks
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 @ 6affe5c
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.
Initial Checks
Release line
2.x (current stable)
Description
Registering a tool whose function signature has a
*argsor**kwargsparameter succeeds, and the tool shows up in
list_tools, but it can neveractually be called: the generated JSON schema lists
args/kwargsas arequired 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/kwargsis "missing"; there's no way to actually supply it, since aJSON object has no way to express "and then also a variable number of
positional values" for a single scalar field.
Example Code
Listing tools shows:
Calling it with
{"x": 1}(the only arguments a caller could reasonablyguess) fails:
Same shape with
*argsinstead of**kwargs.Python & MCP Python SDK
Python 3.14.7, mcp-python-sdk main @ 6affe5c
I have a patch that raises
InvalidSignaturefor a*args/**kwargstoolparameter at registration time, scoped so it doesn't affect resource
templates (which legitimately use
**kwargsfor runtime-determined URIvariables); happy to open a PR against this issue if that's the direction
you'd want.