Fix Streamable HTTP Accept header quality-factor handling

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

Streamable HTTP currently ignores q=0 values in the Accept header, so requests that explicitly mark a media type as unacceptable can still be accepted. This violates RFC 7231 semantics for content negotiation and can cause clients and servers to disagree about which response format is actually allowed.

Example Code

from starlette.requests import Request

def check_accept_headers(request: Request) -> tuple[bool, bool]:
    accept_header = request.headers.get("accept", "")
    accept_types = [media_type.strip().split(";")[0].strip().lower() for media_type in accept_header.split(",")]

    has_wildcard = "*/*" in accept_types
    has_json = has_wildcard or any(t in ("application/json", "application/*") for t in accept_types)
    has_sse = has_wildcard or any(t in ("text/event-stream", "text/*") for t in accept_types)

    return has_json, has_sse

# Example: client explicitly rejects JSON
request = Request({"type": "http", "method": "GET", "headers": [(b"accept", b"application/json;q=0, text/event-stream;q=1.0")]})

print(check_accept_headers(request))
# -> (True, True)

Python & MCP Python SDK

Python 3.12.10
MCP Python SDK: 1.28.1

Commencez par suivre le traitement des requêtes Streamable HTTP et l’exemple check_accept_headers dans l’issue, en vous concentrant sur la manière dont les paramètres Accept sont analysés. Vérifiez le comportement avec la requête fournie application/json;q=0 et text/event-stream;q=1.0, et envisagez des cas supplémentaires de facteurs de qualité ; le travail est terminé lorsque les types de média explicitement inacceptables ne sont pas traités comme acceptés selon la sémantique de RFC 7231.