Cannot handle `Unexpected content type` when initializing MCP session

[bystepii]

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 trying to connect to an MCP server using streamablehttp_client and ClientSession, the client prints Unexpected content type error if the server responds with anything other than the expected MCP JSON stream (e.g. error page, misconfigured endpoint, etc.).

This makes it difficult to handle cases where the server is not reachable or not serving MCP correctly, since no exception is raised.

Example Code

import asyncio
from mcp.client.streamable_http import streamablehttp_client
from mcp import ClientSession

async def main():
    async with streamablehttp_client(
        url="http://localhost:3001/mcp",
    ) as (read, write, _):
        async with ClientSession(read, write) as session:
            try:
                await session.initialize()
            except Exception as e:
                print(f"Failed to initialize MCP session: {e}")
                return
            print("MCP session initialized")

if __name__ == "__main__":
    asyncio.run(main())

Output

Unexpected content type: text/html; charset=utf-8

Python & MCP Python SDK

python 3.13.5
mcp 1.14.1