Some type hints lead to MCPO not running

[dleemueller]

Hi, a tool in my local MCP Server is returning a custom DataTypeCustom that inherits the BaseModel.

class DataTypeCustom(BaseModel):
# Some code

This does not work:

@mcp.tool(name="Something")
def tool_name(args) -> DataTypeCustom:
# some code that returns a value of type DataTypeCustom

Error (the same as in issue #189):

...
assert ref in schema_defs, "Custom field not found" 
TypeError: argument of type 'NoneType' is not iterable

These work:

@mcp.tool(name="Something")
def tool_name(args):
# some code that returns a value of type DataTypeCustom

@mcp.tool(name="Something")
def tool_name(args) -> List[DataTypeCustom]:
# some code that returns the value of type DataTypeCustom in a list

Do you have an ideas what the issue could be here? Is the schema not good enough for mcpo?