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The 2026-07-28 MCP Specification: Stateless Core, MRTR, Header-Based Routing

Official blog post from the MCP maintainers announcing the 2026-07-28 spec revision: the protocol moves from stateful/bidirectional to a stateless request/response core, replaces server-initiated calls (elicitation, sampling, roots) with Multi Round-Trip Requests (MRTR), and requires Mcp-Method/Mcp-Name headers so gateways can route and authorize without parsing JSON bodies. It also details significant authorization hardening — RFC 9207 issuer (iss) validation to close authorization-server mix-up attacks, binding client credentials to the issuer that minted them, and formal deprecation of Dynamic Client Registration in favor of Client ID Metadata Documents (CIMD) — alongside cacheable list responses and a new extensions framework (Tasks, MCP Apps, EMA).