AWS AgentCore Gateway supports MCP 2026-07-28 on day zero via a single UpdateGateway call

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AWS shipped same-day support for the new MCP spec, letting one gateway advertise multiple protocol versions simultaneously so clients negotiate per-request instead of at handshake. The gateway drops session pinning entirely, routes and throttles on the Mcp-Method/Mcp-Name headers at the HTTP layer, honors ttlMs/cacheScope freshness metadata, and moves elicitation and sampling state into opaque tokens rather than long-lived streams. Transport failures now return real HTTP status codes instead of burying errors in a 200 with a JSON-RPC body, and tool schemas accept full JSON Schema 2020-12 including composition keywords and conditionals.