Rust SDK rmcp 3.1.0 adds SEP-2260 request association and conformance work meeting Tier 1 requirements

The MCP Rust SDK released rmcp 3.1.0 on 2026-07-31, three days after the 3.0.0 line added support for the 2026-07-28 revision. The release tightens the SDK's handling of the stateless protocol and adds work aimed at Tier 1 status.

Three additions stand out. The SDK now classifies authorization-required errors distinctly, applies strict validation to stateless protocol metadata, and enforces client receive-side request association through the stream layer per SEP-2260. The release also finalizes Tier 1 feature documentation and a roadmap, and includes conformance improvements the notes describe as meeting Tier 1 requirements. Rust is currently listed as a Tier 2 SDK.

Fixes in the same release cover metadata decoding for input-required results affecting multi round-trip requests, HTTP request metadata requirements, and protocol version negotiation during initialization.

A follow-up, rmcp 3.1.1, shipped on 2026-08-05 with handler-level fixes: emitting cache hints from handler macros, exposing multi round-trip request state to tool handlers, and disambiguating input-required results. It also makes async-trait an optional dependency.