Streamable HTTP server never returns the spec-mandated 405 for GET requests it won't serve as SSE (406/400 instead) — breaks client SSE-probe fallback

Summary

The streamable-HTTP server transport never answers a GET it won't serve with 405 Method Not Allowed, even though the spec makes 405 the required signal:

The server MUST either return Content-Type: text/event-stream in response to this HTTP GET, or else return HTTP 405 Method Not Allowed, indicating that the server does not offer an SSE stream at this endpoint.
— Streamable HTTP § Listening for Messages from the Server

Instead, a GET that can't be served yields:

So in stateful mode there is no request shape for which a pre-session GET returns 405. (405 is used elsewhere: DELETE without a session and unsupported methods.)

Why it matters — real interop break

Several client transports probe with exactly this GET before initialize to ask "do you offer a standalone SSE stream?", and treat only 405 as the graceful "no SSE — fall through to POST" signal (e.g. the TypeScript SDK's _startOrAuthSse explicitly special-cases 405). Any other status is a transport error, so against a stock python-SDK server the connection aborts before initialize is ever sent.

We hit this in production between two independently-built agents: the client authenticated successfully, then its GET probe got 406 (its relay sent Accept: */*), the transport threw, and the handshake never reached initialize — surfacing as "server doesn't respond to MCP protocol" while the server looked perfectly healthy to every python-SDK client (which only GETs after initialize, with a session id). We've deployed a workaround (a small ASGI shim returning 405 + Allow: POST for session-less GETs on the MCP path), but every stock python-SDK deployment presumably reproduces this.

Repro (mcp 1.28.1, stateful StreamableHTTP server)

curl -i -X GET http://localhost:8000/mcp -H 'Accept: */*'                 # → 406, expected 405
curl -i -X GET http://localhost:8000/mcp -H 'Accept: text/event-stream'  # → 400, expected 405 (no session yet)

Suggested behavior

For a GET the server cannot serve as an SSE stream, respond 405 with an Allow header per the spec quote above — at minimum for the pre-session case (no mcp-session-id), where returning 400 makes the spec'd client probe impossible to satisfy. Separately (or as part of this), _check_accept_headers honoring */* / text/* per RFC 9110 §12.5.1 would remove the 406 arm for wildcard clients.

Related

• #2349 covers the POST-side strict dual-Accept requirement; this issue is about the GET/405 contract, which is distinct.
• #1641 raised wildcard-Accept non-compliance and is closed, but on 1.28.1 _check_accept_headers still does literal prefix matching, and the GET path 406s wildcard clients.

Happy to provide full header traces or a PR if the maintainers agree on the intended shape.

まず、issue に記載されている 2 つの curl リクエストを再現し、続いて streamable HTTP サーバーのエントリポイント _handle_get_request、_check_accept_headers、_validate_session を追跡します。セッション開始前の GET の動作と Accept ヘッダーのケースを確認し、その後、サポートされていない GET が Allow ヘッダー付きで 405 を返しつつ、有効な SSE 処理が維持されるように、テストカバレッジを追加または更新します。