This project illustrates the bug reported in : https://github.com/jlowin/fastmcp/issues/1280

Main files

• api.json : dummy openapi 3.0 spec. This is used to generate the MCP server.
• api.py : dummy api. This is used to expose the API.
• server.py : MCP server based on api.json
• client.py : MCP client that connects via STDIO to the MCP server to call tools

Note : I first have tried running the API using FastAPI then use the spec from that API to generate the MCP Server. The problem is this doesn't work since FastAPI generates API that conform to the OAS 3.1 spec. Here, we want to test for OAS 3.0.

Test

Create a python venv, source it and install the dependencies :

python3.13 -m venv venv
source venv/bin/activate
pip install -r requirements.txt

Run the fastapi API :

fastapi dev api.py --port 8000

Run the fastmcp client :

python client.py

The client will connect to the MCP server, and try and execute the "get_item" tool. This will return an item with a price: null which, even though the "price" property has a key "nullable" set to true in api.json, will cause the following error :

fastmcp.exceptions.ToolError: Output validation error: None is not of type 'number'