Positional-only handler parameters break tools, prompts and resource templates #3760

Handlers registered with a positional-only parameter (def echo(x: str, /)) are accepted and advertised, but every call fails, for tools, prompts and resource templates alike.

Reproduction

from mcp.server.mcpserver import MCPServer
from mcp import Client

mcp = MCPServer("posonly")

@mcp.tool()
def echo(x: str, /) -> str:
    return f"got:{x}"

async with Client(mcp) as client:
    result = await client.call_tool("echo", {"x": "hi"})

Actual: tools/list advertises {"properties": {"x": {"type": "string"}}, "required": ["x"]}, but the call returns is_error=True with Error executing tool echo; the server logs TypeError: echo() got some positional-only arguments passed as keyword arguments: 'x'.

The same happens for prompts (def greet(name: str, /) → client.get_prompt raises MCPError: Internal server error, log ValueError: Error rendering prompt greet) and resource templates (@mcp.resource("posonly://{name}") def read(name: str, /) → MCPError: Error creating resource from template posonly://thing).

Expected: got:hi, the rendered prompt, and the resource content respectively.

Cause

The handlers are invoked with keyword arguments only:

• src/mcp/server/mcpserver/utilities/func_metadata.py (call_fn: fn(**kwargs) / functools.partial(fn, **kwargs))
• src/mcp/server/mcpserver/prompts/base.py (Prompt.render: same)
• src/mcp/server/mcpserver/resources/templates.py (ResourceTemplate.create_resource: same)

There is no POSITIONAL_ONLY handling anywhere in src. The tool/prompt/resource input schemas are derived from the type hints, so the parameter is advertised as required and then cannot be bound.

A positional-only parameter is valid Python and the docs describe the handler signature as "just Python" (docs/servers/tools.md), so either the parameters should be bound positionally (a small helper that splits the leading positional-only run out of the kwargs dict, using declared defaults when omitted) or registration should reject positional-only parameters instead of advertising an uncallable handler.

Happy to prepare the binding change if maintainers want it — not opening a PR because the contributing guide asks for an assigned issue first.