Lifespan enters and exits for each request when stateless_http=True

[savvaki]

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

When stateless_http=True, the lifespan of the MCP server enters and exits with each request. Surely this is unintended behavior?

Example Code

from mcp.server.fastmcp import Context, FastMCP
from contextlib import asynccontextmanager

@asynccontextmanager
async def tmp_lifespan(server: FastMCP):
    print(f"Starting lifespan...")
    yield
    print(f"Ending lifespan...")

mcp = FastMCP("localmcp", host="127.0.0.1", lifespan=tmp_lifespan, port=8000, stateless_http=True)


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

if __name__ == "__main__":
    print(f"Starting MCP server...")
    mcp.run(transport="streamable-http")

Python & MCP Python SDK

1.13.0

[felixweinberger]

This is actually by design as stateless mode is currently spinning up a new transport for each request

python-sdk/src/mcp/server/streamable_http_manager.py

Lines 161 to 162 in c260e29

	# No session ID needed in stateless mode
	http_transport = StreamableHTTPServerTransport(

Likely could be improved though, marking this as something for potential enhancements in v2

[Kludex]

This is a bug.