ClosedResourceError when using FastMCP(..., stateless_http=True, json_response=True) with MCP Inspector

[yabramow]

Initial Checks

• I confirm that I'm using the latest version of MCP Python SDK
• I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue

Description

Summary

Running the example MCP server fastmcp_quickstart.py with:
FastMCP("Demo", stateless_http=True, json_response=True)
and connecting to it using MCP Inspector triggers an exception inside the server’s message router:
ClosedResourceError

This occurs immediately after the Inspector connects.

Steps to Reproduce

1. Use the example server from the Python MCP SDK:
   uv run --with mcp examples/snippets/servers/fastmcp_quickstart.py
2. Ensure the server instance is initialized as:
   FastMCP("Demo", stateless_http=True, json_response=True)
3. Open MCP Inspector and connect to this server.
4. Observe the server error.

Environment

• MCP Python SDK version: latest
• Python version: 3.13
• OS: macOS (Apple Silicon)
• MCP Inspector version: latest

Example Code

"""
FastMCP quickstart example.

Run from the repository root:
    uv run examples/snippets/servers/fastmcp_quickstart.py
"""

from mcp.server.fastmcp import FastMCP

# Create an MCP server
mcp = FastMCP("Demo", stateless_http=True, json_response=True)


# Add an addition tool
@mcp.tool()
def add(a: int, b: int) -> int:
    """Add two numbers"""
    return a + b


# Add a dynamic greeting resource
@mcp.resource("greeting://{name}")
def get_greeting(name: str) -> str:
    """Get a personalized greeting"""
    return f"Hello, {name}!"


# Add a prompt
@mcp.prompt()
def greet_user(name: str, style: str = "friendly") -> str:
    """Generate a greeting prompt"""
    styles = {
        "friendly": "Please write a warm, friendly greeting",
        "formal": "Please write a formal, professional greeting",
        "casual": "Please write a casual, relaxed greeting",
    }

    return f"{styles.get(style, styles['friendly'])} for someone named {name}."


# Run with streamable HTTP transport
if __name__ == "__main__":
    mcp.run(transport="streamable-http")

Python & MCP Python SDK

Python: 3.13
MCP Python SDK: 1.22.1.dev4+b19fa6f