MCP仕様2026-07-28版の最新アップデート、ステートレス化の要点

This document lists changes made to the Model Context Protocol (MCP) specification since the previous revision, 2025-11-25. ／ Remove protocol-level sessions and the Mcp-Session-Id header from the Streamable HTTP transport. List endpoints (tools/list, resources/list, prompts/list) no longer vary per-connection. Servers that need cross-call state use explicit, server-minted handles passed as ordinary tool arguments (SEP-2567). ／ Servers SHOULD return tools from tools/list in a deterministic order to enable client-side caching and improve LLM prompt cache hit rates. ／ Require ttlMs and cacheScope fields on results returned by tools/list, prompts/list, resources/list, resources/read, and resources/templates/list via a new CacheableResult interface. ttlMs is a freshness hint (in milliseconds) allowing clients to cache responses and reduce polling; cacheScope ("public" or "private") controls whether shared intermediaries may cache the response. — 対象となる版、セッション廃止、一覧の決定的順序とキャッシュ関連フィールドに関する記述より

Make MCP stateless: remove the initialize/notifications/initialized handshake. Every request now carries its protocol version and client capabilities in _meta (io.modelcontextprotocol/protocolVersion, io.modelcontextprotocol/clientCapabilities). Clients SHOULD identify themselves on each request (io.modelcontextprotocol/clientInfo), and servers SHOULD identify themselves in each result's _meta (io.modelcontextprotocol/serverInfo). Version mismatches return UnsupportedProtocolVersionError (SEP-2575). ／ Add server/discover: servers MUST implement this RPC to advertise their supported protocol versions, capabilities, and identity. Clients MAY call it before any other request for up-front version selection, or use it as a backward-compatibility probe on STDIO (SEP-2575). — ステートレス化の内容と、server/discover の追加および呼び出し方に関する記述より

Remove SSE stream resumability and message redelivery (the Last-Event-ID header and SSE event IDs) from the Streamable HTTP transport. A broken response stream loses the in-flight request; clients MUST re-issue it as a new request with a new request ID (SEP-2575). ／ Replace the HTTP GET endpoint and resources/subscribe/resources/unsubscribe with subscriptions/listen: a single long-lived POST-response stream for opted-in server-to-client change notifications. Clients opt in to specific types (toolsListChanged, promptsListChanged, resourcesListChanged, resourceSubscriptions); the server acknowledges and tags notifications with io.modelcontextprotocol/subscriptionId. Request-scoped notifications such as notifications/progress and notifications/message continue to flow on the response stream of the request they relate to, not the subscriptions/listen stream (SEP-2575). — SSE再開の削除と、通知の受け取り方の変更およびリクエスト単位の通知との区別に関する記述より

Multi Round-Trip Requests (MRTR) pattern introduced which replaces the previous approach of sending server-initiated requests, such as roots/list, sampling/createMessage, or elicitation/create. Servers return an InputRequiredResult (resultType: "input_required") whose inputRequests field carries the requests for the additional information needed to process the request. Clients respond with inputResponses on a retry of the original request providing the requested information. (SEP-2322). ／ All results now carry a required resultType field: "complete" for ordinary results and "input_required" for multi round-trip request interim results. Clients MUST treat results from earlier-protocol servers that omit the field as "complete" (SEP-2322). — MRTR パターンの導入と、resultType フィールドの必須化に関する記述より

Move experimental tasks out of the core protocol and into an official extension (io.modelcontextprotocol/tasks). The redesigned extension replaces the blocking tasks/result method with polling via tasks/get and a new tasks/update for client-to-server input, removes tasks/list, and allows servers to return task handles unsolicited without per-request opt-in (SEP-2663). — tasks の拡張への移動と再設計に関する記述より

Remove ping, logging/setLevel, and notifications/roots/list_changed. Log level is now set per-request via io.modelcontextprotocol/logLevel in _meta; servers MUST NOT emit notifications/message for requests that did not include this field (SEP-2575). ／ Change resource not found error code from -32002 to -32602 (Invalid Params) to align with JSON-RPC specification. ／ Define an error code allocation policy partitioning the JSON-RPC server-error range: -32000 to -32019 remains implementation-defined (existing SDK usage is grandfathered), -32020 to -32099 is reserved for the MCP specification. ／ Loosen inputSchema and outputSchema to allow any JSON Schema 2020-12 keywords, and structuredContent to allow any JSON value. — 削除された呼び出しとログ水準、エラーコードの変更と割り当て方針、スキーマ制約の緩和に関する記述より