Is this a new feature, an improvement, or a change to existing functionality?

Improvement

How would you describe the priority of this feature request

Critical (currently preventing usage)

Please provide a clear description of problem this feature solves

I would like to be able to use a MCP server that has a tool with a union type.
An example tool that has a union type:

field_name: new_line field_properties: {'type': ['number', 'null'], 'description': 'Line number after change'}

Will cause a fatal error:

[ERROR] Failed to fetch tools via MCPBuilder: unhashable type: 'list'

The MCP server being used https://github.com/zereight/gitlab-mcp

Describe your ideal solution

Add a new elif clause in mcp_client.py

        elif isinstance(json_type, list):
            nullable = "null" in json_type
            main_type = next((t for t in json_type if t != "null"), "string")
            field_type = _type_map.get(main_type, Any)
            if nullable:
                field_type = field_type | None
            return field_type, Field(
                default=field_properties.get("default", None if nullable else ...),
                description=field_properties.get("description", "")
            )

Additional context

No response

Code of Conduct

• I agree to follow this project's Code of Conduct
• I have searched the open feature requests and have found no duplicates for this feature request