Initialize call hangs forever if MCP server does not return a `Content-Type: text/plain`

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

We had some timeouts on our server while using MCP client from the SDK.
When users try to integrate our client with their MCP server, we do an initialize call to the server and we are waiting infinitely to the following:

sync with self:
    ...

After investigation, it seems the error happens when the MCP server is misconfigured, and returns something different than Content-Type: text/event-stream as a response from the initialize call.

Although it is on the server side, the client should be able to fail immediately with a understandable error to warn the user something went wrong with the server's initialize output.

Example Code

# server.py

from http.server import BaseHTTPRequestHandler, HTTPServer

class PlainTextHandler(BaseHTTPRequestHandler):
    def do_POST(self):
        length = int(self.headers.get("Content-Length", 0))
        self.rfile.read(length)

        body = b"this is not json"
        self.send_response(200)
        self.send_header("Content-Type", "text/plain")
        self.send_header("Content-Length", str(len(body)))
        self.end_headers()
        self.wfile.write(body)

    def log_message(self, format, *args):
        print(f"[server] {format % args}")

server = HTTPServer(("localhost", 9000), PlainTextHandler)
print("[server] listening on http://localhost:9000")
server.serve_forever()

# client.py
import asyncio
import mcp
from mcp.client.streamable_http import streamable_http_client

async def main():
    print("connecting…")
    async with streamable_http_client("http://localhost:9000/mcp") as (
        read_stream, write_stream, _,
    ):
        async with mcp.ClientSession(read_stream, write_stream) as session:
            await session.initialize()  # ← hangs here forever

    print("done (you should never see this)")

Python & MCP Python SDK

- Python 3.12.12
- MCP - 1.27.0

client.py に示されている streamable_http_client エントリーポイントから始め、server.py の PlainTextHandler の例を使って動作を再現します。initialize のレスポンス処理を追跡し、non-text/event-stream レスポンスのカバレッジを追加します。完了の条件は、クライアントがハングせず、理解しやすいエラーですぐに失敗することです。