Merge pull request #516 from koic/counter_offer_legacy_version_on_initialize
Stop negotiating modern protocol versions through the `initialize` handshake
This commit is contained in:
Koichi ITO
GitHub
commit
a62e56ef48
2 changed files with 472 additions and 207 deletions
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@ -327,8 +327,9 @@ module MCP
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#
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# @param client_info [Hash, nil] `{ name:, version: }` identifying the client.
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# Defaults to `{ name: "mcp-ruby-client", version: MCP::VERSION }`.
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# @param protocol_version [String, nil] Protocol version to offer. Defaults
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# to `MCP::Configuration::LATEST_STABLE_PROTOCOL_VERSION`.
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# @param protocol_version [String, nil] Protocol version to offer on the legacy handshake.
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# Defaults to `MCP::Configuration::LATEST_HANDSHAKE_PROTOCOL_VERSION`; a modern version
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# raises `ArgumentError` here (modern versions are selected via `mode: :modern`/`:auto`).
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# @param capabilities [Hash] Capabilities advertised by the client. Defaults to `{}`.
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# @return [Hash] The server's `InitializeResult`.
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# @raise [RequestHandlerError] If the server responds with a JSON-RPC error
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@ -341,6 +342,9 @@ module MCP
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def connect(client_info: nil, protocol_version: nil, capabilities: {}, mode: :legacy)
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return @server_info if connected?
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# Per the SEP-2575 era model, a modern version cannot ride the legacy `initialize` handshake.
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MCP::Configuration.reject_modern_handshake_version!(protocol_version) if mode == :legacy
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client_info ||= { name: "mcp-ruby-client", version: MCP::VERSION }
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case mode
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@ -559,7 +563,7 @@ module MCP
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attr_reader :headers
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def connect_legacy(client_info:, protocol_version:, capabilities:)
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protocol_version ||= MCP::Configuration::LATEST_STABLE_PROTOCOL_VERSION
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protocol_version ||= MCP::Configuration::LATEST_HANDSHAKE_PROTOCOL_VERSION
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response = send_request(request: {
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jsonrpc: JsonRpcHandler::Version::V2_0,
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@ -593,7 +597,10 @@ module MCP
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@server_info = response["result"]
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negotiated_protocol_version = @server_info["protocolVersion"]
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unless MCP::Configuration::SUPPORTED_STABLE_PROTOCOL_VERSIONS.include?(negotiated_protocol_version)
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# A modern version in an `InitializeResult` is rejected along with unknown ones: the handshake
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# settles on a legacy version by definition, and the TypeScript and Python clients refuse
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# a modern counter-offer the same way.
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unless MCP::Configuration::SUPPORTED_HANDSHAKE_PROTOCOL_VERSIONS.include?(negotiated_protocol_version)
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clear_session
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raise RequestHandlerError.new(
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"Server initialization failed: unsupported protocol version #{negotiated_protocol_version.inspect}",
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@ -602,8 +609,6 @@ module MCP
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)
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end
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MCP::ProtocolDeprecations.warn_for_client_capabilities(capabilities, protocol_version: negotiated_protocol_version, uplevel: 1)
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begin
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send_request(request: {
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jsonrpc: JsonRpcHandler::Version::V2_0,
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@ -647,6 +652,10 @@ module MCP
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)
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end
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# SEP-2577 deprecates roots and sampling at 2026-07-28, the revision every modern connection speaks,
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# so the warning lives here now that the handshake cannot land on one.
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MCP::ProtocolDeprecations.warn_for_client_capabilities(capabilities, protocol_version: version, uplevel: 1)
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@server_info = result
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@connected = true
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@server_info
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@ -658,8 +667,13 @@ module MCP
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# version as well: during the 2026-07-28 rollout a server may answer discovery while
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# only serving legacy versions.
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def connect_auto(client_info:, protocol_version:, capabilities:)
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connect_modern(client_info: client_info, protocol_version: nil, capabilities: capabilities)
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modern_pin = protocol_version if protocol_version && MCP::Configuration.modern_protocol_version?(protocol_version)
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connect_modern(client_info: client_info, protocol_version: modern_pin, capabilities: capabilities)
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rescue RequestHandlerError
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# An explicitly requested modern version is never downgraded by the fallback: the legacy handshake cannot negotiate it,
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# so the probe's failure is the real answer and propagates.
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raise if modern_pin
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connect_legacy(client_info: client_info, protocol_version: protocol_version, capabilities: capabilities)
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end
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@ -77,8 +77,9 @@ module MCP
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#
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# @param client_info [Hash, nil] `{ name:, version: }` identifying the client.
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# Defaults to `{ name: "mcp-ruby-client", version: MCP::VERSION }`.
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# @param protocol_version [String, nil] Protocol version to offer. Defaults
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# to `MCP::Configuration::LATEST_STABLE_PROTOCOL_VERSION`.
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# @param protocol_version [String, nil] Protocol version to offer on the legacy handshake.
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# Defaults to `MCP::Configuration::LATEST_HANDSHAKE_PROTOCOL_VERSION`; a modern version
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# raises `ArgumentError` here (modern versions are selected via `mode: :modern`/`:auto`).
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# @param capabilities [Hash] Capabilities advertised by the client. Defaults to `{}`.
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# @return [Hash] The server's `InitializeResult`.
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# @raise [RequestHandlerError] If the server responds with a JSON-RPC error,
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@ -91,6 +92,9 @@ module MCP
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def connect(client_info: nil, protocol_version: nil, capabilities: {}, mode: :legacy)
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return @server_info if connected?
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# Validated before `start` so a pure argument error never spawns the server process.
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MCP::Configuration.reject_modern_handshake_version!(protocol_version) if mode == :legacy
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start unless @started
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client_info ||= { name: "mcp-ruby-client", version: MCP::VERSION }
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@ -221,7 +225,7 @@ module MCP
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private
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def connect_legacy(client_info:, protocol_version:, capabilities:)
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protocol_version ||= MCP::Configuration::LATEST_STABLE_PROTOCOL_VERSION
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protocol_version ||= MCP::Configuration::LATEST_HANDSHAKE_PROTOCOL_VERSION
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init_request = {
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jsonrpc: JsonRpcHandler::Version::V2_0,
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@ -257,9 +261,11 @@ module MCP
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@server_info = response["result"]
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negotiated_protocol_version = @server_info["protocolVersion"]
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unless MCP::Configuration::SUPPORTED_STABLE_PROTOCOL_VERSIONS.include?(negotiated_protocol_version)
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unless MCP::Configuration::SUPPORTED_HANDSHAKE_PROTOCOL_VERSIONS.include?(negotiated_protocol_version)
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# Per spec, if the client does not support the server's returned protocol version,
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# the client SHOULD disconnect. Roll back the cached `InitializeResult` before raising
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# the client SHOULD disconnect. A modern version is rejected along with unknown ones:
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# the handshake settles on a legacy version by definition, and the TypeScript and Python clients refuse
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# a modern counter-offer the same way. Roll back the cached `InitializeResult` before raising
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# so a retry starts without a stale `server_info`.
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# https://modelcontextprotocol.io/specification/2025-11-25/basic/lifecycle#version-negotiation
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@server_info = nil
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@ -270,8 +276,6 @@ module MCP
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)
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end
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MCP::ProtocolDeprecations.warn_for_client_capabilities(capabilities, protocol_version: negotiated_protocol_version, uplevel: 1)
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begin
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notification = {
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jsonrpc: JsonRpcHandler::Version::V2_0,
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@ -317,6 +321,10 @@ module MCP
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)
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end
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# SEP-2577 deprecates roots and sampling at 2026-07-28, the revision every modern connection speaks,
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# so the warning lives here now that the handshake cannot land on one.
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MCP::ProtocolDeprecations.warn_for_client_capabilities(capabilities, protocol_version: version, uplevel: 1)
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@server_info = result
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@server_info
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end
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@ -327,8 +335,13 @@ module MCP
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# version as well: during the 2026-07-28 rollout a server may answer discovery while
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# only serving legacy versions.
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def connect_auto(client_info:, protocol_version:, capabilities:)
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connect_modern(client_info: client_info, protocol_version: nil, capabilities: capabilities)
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modern_pin = protocol_version if protocol_version && MCP::Configuration.modern_protocol_version?(protocol_version)
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connect_modern(client_info: client_info, protocol_version: modern_pin, capabilities: capabilities)
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rescue RequestHandlerError
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# An explicitly requested modern version is never downgraded by the fallback: the legacy handshake cannot negotiate it,
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# so the probe's failure is the real answer and propagates.
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raise if modern_pin
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connect_legacy(client_info: client_info, protocol_version: protocol_version, capabilities: capabilities)
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end
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