fastmcp-issue-demo

This repository is a minimal example to reproduce an input_schema property key related error occurring in FastMCP (an OpenAPI-based MCP server).

Purpose

• Reproduces a property key rule error that occurs when using a specific OpenAPI spec structure in FastMCP.
• Intended for issue reporting and community inquiries.

Features

• Minimal FastMCP server example
• Uses a Notion Postman converted to OpenAPI
• Reproduces property key rule error

Installation & Setup

1. Create Notion OAuth Integration

1. Notion Integrations page visit
2. Click "Create new integration".
3. Fill in the integration name and other required info.
4. Set the Redirect URI to http://localhost:8000/callback (this must match your .env setting).
5. After creation, copy the Client ID and Client Secret.

2. Clone the repository

git clone https://github.com/yourusername/fastmcp-issue-demo.git
cd fastmcp-issue-demo

3. Create and activate virtual environment

python -m venv .venv
# Linux/macOS
source .venv/bin/activate
# Windows
.venv\Scripts\activate

4. Install dependencies

pip install -r requirements.txt

5. Create .env file

• Create a .env file by referring to .env.example.

6. Get Notion Token

cd oauth
uvicorn oauth_server:app --host localhost --reload

• Proceed with OAuth authentication in your browser, get the token, and copy it.

6. Apply Token

• In the mcp/mcp_server.py file, find the following section:

client = httpx.AsyncClient(
    base_url="https://api.notion.com",
    headers={
        "Authorization": "Bearer <your_token>",
        "Notion-Version": "2022-06-28"
    }
)

• Replace <your_token> with the copied token.

7. Run Server

cd mcp
python mcp_client.py mcp_server.py

• Enter any chat message.

Expected Outcome and Problem Situation

• The server should start normally, but the following error occurs:

Error: Error code: 400 - {'type': 'error', 'error': {'type': 'invalid_request_error', 'message': "tools.8.custom.input_schema.properties: Property keys should match pattern '^[a-zA-Z0-9_.-]{1,64}$'"}}

• tools.8 == retrieveAPage
• Thus, it was identified that a specific OpenAPI spec structure is problematic, but I wasn't able to pinpoint what exactly was wrong despite careful examination.
• When using notion_openapi_fixed.json (with the retrieveAPage part removed), the server works normally.
• You can confirm normal operation by changing the filename in mcp/mcp_server.py as follows and re-running:

with open("./json/notion_openapi_fixed.json", "r", encoding="utf-8") as f:
    openapi_spec = json.load(f)

Environment

• Python 3.10 or higher recommended
• fastmcp version: Latest version

Security Precautions

• Sensitive information such as API tokens and Client Secrets must be separated into a .env file and strictly added to .gitignore.
• This repository is for issue reproduction/reporting purposes only, not for production use.