convert_openapi_to_mcp_tools adds "type" field, breaking nullable schemas

[NTSER]

Here is sample code:

from fastapi_mcp.openapi.convert import convert_openapi_to_mcp_tools
from jsonschema import validate
openapi_schema = {
    "openapi": "3.0.3",
    "info": {
        "title": "Union Test",
        "version": "1.0.0"
    },
    "paths": {
        "/test": {
            "post": {
                "operationId": "test_union",
                "summary": "Test endpoint with a union type",
                "requestBody": {
                    "required": True,
                    "content": {
                        "application/json": {
                            "schema": {
                                "type": "object",
                                "properties": {
                                    "value": {
                                        "anyOf": [
                                            {"type": "string", "maxLength": 2000},
                                            {"type": "null"}
                                        ],
                                        "title": "value"
                                    }
                                },
                                "required": ["value"]
                            }
                        }
                    }
                },
                "responses": {
                    "200": {
                        "description": "OK"
                    }
                }
            }
        }
    }
}

mcp_tools = convert_openapi_to_mcp_tools(openapi_schema=openapi_schema)
first_tool = mcp_tools[0][0]
mcp_input_schema = first_tool.inputSchema


# Validate using original OpenAPI schema (works)
validate(instance={"value": None}, schema=openapi_schema)

# Validate using MCP-generated schema (fails)
validate(instance={"value": None}, schema=mcp_input_schema)
# jsonschema.exceptions.ValidationError: None is not of type 'string'

I am new in the field and maybe that is expected behaviour but still for me it was very confusing.
The unexpected behaviour happens inside the convert_openapi_to_mcp_tools function. It modifies the schema

from:

{
    "value": {
        "anyOf": [
            {"type": "string", "maxLength": 2000},
            {"type": "null"}
        ],
        "title": "value"
    }
}

to:

{
    "value": {
        "anyOf": [
            {"type": "string", "maxLength": 2000},
            {"type": "null"}
        ],
        "title": "value",
        "type": "string"
    }
}

In my use case, I had a FastAPI app and added MCP. When sending requests via LangChain, I ran into issues because the LLM sometimes sent None, which then triggered a validation error. This caused a lot of debugging time.

If it's not a bug and is expected behavior, It might be helpful if the function issued a warning in such cases to alert users about potential type conflicts, which could significantly reduce debugging effort.

For now, I believe the solution is to make the original field non-nullable. If there is a more appropriate approach, I would appreciate suggestions.