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ruby-sdk/docs/protocol-versions.md
Koichi ITO c9d36b04bf [Doc] Align the Docs Page Names With the MCP Specification 
Three pages carried names the 2026-07-28 specification does not use: "Multi-Round-Trip Results" for
the pattern the specification titles Multi Round-Trip Requests (MRTR) and serves at basic/patterns/mrtr -
the results are the `InputRequiredResult` wire type, not the feature - plus "Completions" for the utility
the specification titles Completion, and "Notification Subscriptions" for the pattern it titles Subscriptions.
Retitle the three pages to the specification's names, move them to /server/mrtr/ and /client/mrtr/,
/server/completion/, and /server/subscriptions/ following the specification's slugs, update every internal reference,
and keep the URLs published with v1.3.0 working via jekyll-redirect-from.
The Discovery page keeps its title but moves from /server/discovery/ to /server/discover/, matching
the specification's slug and the `server/discover` RPC, with its published URL redirected the same way.
2026-08-22 18:07:38 +09:00

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layout title nav_order permalink
default Protocol Versions 4 /protocol-versions/

Protocol Versions

The SDK supports the following MCP protocol versions:

Version Era Notes
2026-07-28 Modern Stateless lifecycle: no handshake, version carried on every request (SEP-2575)
2025-11-25 Handshake The default handshake version; adds URL mode elicitation, enum schemas, and sampling tools
2025-06-18 Handshake Adds elicitation, structured tool output, and OAuth resource servers; removes JSON-RPC batching
2025-03-26 Handshake Adds Streamable HTTP, OAuth 2.1 authorization, tool annotations, and audio content
2024-11-05 Handshake Initial protocol revision

The Era Model

Per the SEP-2575 era model, an era is a property of the protocol version itself:

  • The modern version (2026-07-28) has no handshake at all: clients discover the server through server/discover, and every request carries its version in the _meta envelope, validated per request.
  • Handshake versions (2025-11-25 and earlier) establish a session through the initialize handshake. The server offers 2025-11-25 by default, and the version can be pinned with MCP::Configuration.new(protocol_version:); see Server Protocol Version.

Pinning the server's handshake version:

configuration = MCP::Configuration.new(protocol_version: "2025-06-18")
MCP::Server.new(name: "my_server", configuration: configuration)

The handshake never negotiates a modern version: a client asking initialize for 2026-07-28 is counter-offered the latest handshake version, matching the TypeScript and Python SDKs. The bundled transports serve both eras side by side with no configuration needed.

Client Negotiation

MCP::Client#connect negotiates the lifecycle automatically by default: it probes server/discover and adopts the modern lifecycle when the server serves it, falling back to the initialize handshake otherwise. connect(mode: :modern), connect(mode: :legacy), and an explicit protocol_version: pin select a lifecycle directly; see Lifecycle.

client.connect                                 # negotiate automatically (default)
client.connect(mode: :modern)                  # require the modern lifecycle
client.connect(mode: :legacy)                  # force the classic initialize handshake
client.connect(protocol_version: "2025-11-25") # pin the handshake version, no probe

Deprecations

The 2026-07-28 revision deprecates Roots, Sampling, and Logging per SEP-2577; all three remain fully supported on the handshake versions.

Check the MCP specification to understand what each protocol version includes.