model context protocol - error - " is not valid JSON {"context":"connection","stack":"SyntaxError: Unexpected token 'm'

This is quite a generic error when i tried to start my mcp server with claude. The only hint I got was - it has something to do with JSON. 

And after some debugging and looking at the logs from Claude - i was able to figure that my class that i was going to pass to my target API endpoint needs pydantic BaseModel support. 

So I had to update my class to which resolves the issue for me:-

from pydantic import BaseModel

class PizzaOrder(BaseModel):

    def __init__(self, pizza_type: str, crust_size: str):

        self.pizza_type = pizza_type

        self.crust_size = crust_size

    def __str__(self):

        return f"{self.crust_size} crust {self.pizza_type} pizza"