jlowin
Optional string parameters for tools get [not] parsed as JSON in MCP Inspector
Description
Hey folks,
Not sure whether this is a MCP Inspector issue or FastMCP so figured I'd start here.
I've got a simple tool function (below) that takes an optional role_name which seems to not be getting populated in the request coming in BEFORE even my middleware can take a look at it:
It's clearly different in the MCP Inspector UI as it seems to expect JSON for this field now instead of just a plain old string:
It seems like even the calls from MCP inspector is being sent as null, even when the role_name field is populated:
The spec on the MCP Inspector side seems correct:
Example Code
@mcp.tool
async def netsuite_create_new_user(
first_name: str,
last_name: str,
email: str,
job_title: str,
role_name: str | None = None,
) -> str:
return await create_new_user(first_name, last_name, email, job_title, role_name)
Version Information
fastmcp==2.10.1
Python 3.12.8
Additional Context
No response
I'm encountering the same issue where an optional string parameter in a tool call is being set to None—likely because it's expecting JSON format—even though a string value is provided. It only works correctly when the string is explicitly wrapped in double quotes, like "string_content".
Same issue. Doesn't matter if I add explicit arguments and types to their function using the Optional or Union objects in the typing library, or if I use the python 3.10+ syntax with the pipe operator.
@mcp.tool()
def get_incidents(
limit: int = 10, # Optional - has default value
offset: int = 0, # Optional - has default value
state: Optional[str] = None, # Optional - can be None
assigned_to: str | None = None, # Optional - can be None
category: Union[str, None] = None, # Optional - can be None
priority: Optional[str] = None, # Optional - can be None
number: Optional[str] = None, # Optional - can be None
date_filters: Optional[dict] = None, # Optional - can be None
) -> dict:
...
produces this output in the MCP Inspector
Version Information
fastmcp==2.12.5
Python 3.12.10
