Ruby SDK 1.2.0 completes the 2026-07-28 stateless lifecycle, and a Tier 1 promotion is proposed

The official MCP Ruby SDK released v1.2.0 on 2026-08-15. Its release notes state that the release completes the SEP-2575 stateless lifecycle of the 2026-07-28 revision, together with the SEP-2322, SEP-2549 and SEP-2243 features that revision builds on.

On the server side the release adds the SEP-2575 request envelope, an era lock so stdio can serve both lifecycle eras on one connection, the sessionless modern path over Streamable HTTP, and the subscriptions/listen notification stream. server/discover is finalized, results carry the resultType and cache hints the revision requires, and tool parameters marked x-mcp-header are mirrored into Mcp-Param-* headers per SEP-2243. Handlers can return multi round-trip input_required results, on both the modern and the legacy wire.

Several entries are breaking. The Mcp-Method header is now required on the modern path, server-to-client requests are refused in the modern lifecycle and bounded by a timeout elsewhere, methods the revision removed are rejected before the connection's era locks, and the SDK no longer negotiates modern protocol versions through the initialize handshake. Clients that connect on the modern lifecycle declaring the Roots or Sampling capabilities now get a deprecation warning, per SEP-2577. The notes say those changes are incompatible with 1.1.0 and ship in a minor release under the spec-conformance and security exceptions written into the SDK's VERSIONING.md. Three of the fixes touch security: exception messages no longer leak to clients through JSON-RPC error data, OAuth discovery is restricted to same-origin metadata URLs, and private-network destinations are refused.

A pull request opened the same week proposes moving Ruby up the tier ladder. PR #3248, filed 2026-08-15 and still open when checked, would promote the SDK from Tier 2 to Tier 1 in the official SDK listing for the 2026-07-28 revision and the draft, citing the Tier 1 application in #3247: server conformance 67 of 67 and client conformance 50 of 50 against the frozen 2025-11-25 and 2026-07-28 requirement sets, full issue triage, no P0 issues ever filed, a stable v1.2.0, and a four-day spec-tracking gap. The published listing still shows Ruby at Tier 2, alongside Java and Rust.