Official MCP Rust SDK implements 2026-07-28 stable spec

The repository provides the core rmcp crate and the rmcp-macros proc-macro crate, built on the tokio async runtime. The SDK implements the 2026-07-28 stable MCP specification while staying compatible with 2025-11-25 and earlier versions. The new spec brings server discovery and negotiation, transport-agnostic subscriptions, long-running tasks, response caching, multi-round-trip requests, and standard HTTP routing headers. The client lifecycle adds Discover and Auto modes, with Auto falling back when an older server does not support server/discover. On the tool side, returns can include images, audio, embedded resources, and mixed content, and structured content can be any JSON value rather than objects only. The audience that needs to adapt is clear: teams building MCP servers in Rust can hand protocol negotiation, pagination, and OAuth boilerplate to the SDK and focus on business tools, while Rust client developers launching MCP services via npx can use serve_with_lifecycle to control version-probing strategy. Separating tool-level errors from protocol errors also directly affects whether callers can see the failure reason.