convert_openapi_to_mcp_tools adds "type" field, breaking nullable schemas
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.
Source: tadata-org/fastapi_mcp