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.