The Official Ruby SDK for MCP Reaches Tier 1

What it claims

The mcp gem, the official Ruby SDK for the Model Context Protocol, was confirmed at Tier 1 in September 2026. The latest release at the time of writing is v1.6.1. Tier 1 requires full feature support, a 100% conformance pass rate and a maintenance commitment with defined response times. The SDK moved fast: Tier 3 in March 2026, Tier 2 with the 1.0 release in July, a Tier 1 application in August, and confirmation in September.

How it works

Version 1.2.0 serves both protocol lifecycle eras from one server. Pre-2026 clients keep the classic session handshake. Clients on the 2026-07-28 revision use the stateless lifecycle (SEP-2575), including server/discover and the subscriptions/listen notification stream.

flowchart LR
  C[Client connects] --> V{Protocol version}
  V -->|pre-2026| S[Classic session handshake]
  V -->|2026-07-28| L[Stateless path]
  S --> H[Same handlers]
  L --> H

Handlers written in the new input_required multi round-trip style (SEP-2322) also serve pre-2026 clients unchanged, through a built-in shim. Cache hints (SEP-2549) and the standard and custom HTTP request headers (SEP-2243) are emitted and validated only on the modern path. Roots, Sampling and Logging are deprecated per SEP-2577. Clients that connect with 2026-07-28 and declare Roots or Sampling get a warning.

Evidence

The SDK passes 67/67 server and 50/50 client conformance scenario runs for both the 2025-11-25 and 2026-07-28 requirement sets. These runs use the suite's frozen per-revision scoring at each revision's own wire version. Issue triage was 100% within the Tier 1 window, with no P0 issues filed. Reproduction steps are in the linked Tier 1 assessment.

How to use it

$ gem install mcp

Gemfiles still pinned to 0.x need a new constraint:

gem 'mcp', '>= 1.6'

or, to stay on 1.x:

gem 'mcp', '~> 1.6'

An existing 1.x server needs only an upgrade to serve 2026-07-28 clients alongside current ones. From 1.3.0, the documentation lives on a dedicated site with one page per topic, covering Rails integration and the full OAuth client flow.

Limits

Several 1.x minor releases, starting with 1.2.0, tighten validation under the spec-conformance and security exceptions of the versioning policy. Existing 1.x applications should review the changelog before upgrading. The stable release supporting 2026-07-28 shipped about two weeks after the spec, short of the Tier 1 expectation of support on release day. The team has committed to support the next revision, scheduled for 2026-12-15, by its release date.