Support Ruby 2.7 - Ruby 3.1

## Summary

Unlike applications, libraries provide value by remaining usable for a broad range of users,
even when that means supporting older Ruby versions beyond Ruby's own EOL timeline.

I maintain RuboCop, a Ruby linter. Because linters need to work across the ecosystem,
they often need to keep compatibility with older Ruby versions to some extent.

The trade-off is that the MCP Ruby SDK must remain compatible with Ruby 2.7.
However, given the ecosystem impact, supporting Ruby 2.7 is still worthwhile.

For example, bundled gems such as `csv` (Ruby >= 2.5), `bigdecimal` (Ruby >= 2.5), `json` (Ruby >= 2.7),
and `language_server-protocol` (Ruby >= 2.5) tend to keep compatibility with older Ruby versions.

Unlike dropping support, which imposes restrictions on users,
this change broadens compatibility and does not introduce breaking changes.

## Development Note

`rubocop-shopify` (2.18) supports Ruby 3.1+, and this PR does not change that.
Since RuboCop can configure the runtime Ruby version and the target Ruby version for analysis independently,
CI continues to run RuboCop on Ruby 4.0, while analyzing code with `TargetRubyVersion: 2.7`.

When setting `TargetRubyVersion: 2.7`, several RuboCop offenses were resolved.

Additionally, Sorbet-related libraries support Ruby 3.0+. Since these are not dependencies of the MCP Ruby SDK itself,
they are versioned and tested in the development Gemfile.

.github/workflows/ci.yml

@@ -7,6 +7,9 @@ jobs:
     strategy:
       matrix:
         entry:
+          - { ruby: '2.7', allowed-failure: false }
+          - { ruby: '3.0', allowed-failure: false }
+          - { ruby: '3.1', allowed-failure: false }
           - { ruby: '3.2', allowed-failure: false }
           - { ruby: '3.3', allowed-failure: false }
           - { ruby: '3.4', allowed-failure: false }
@@ -29,7 +32,7 @@ jobs:
       - uses: actions/checkout@v6
       - uses: ruby/setup-ruby@v1
         with:
-          ruby-version: 3.2 # Specify the oldest supported Ruby version.
+          ruby-version: 4.0 # Specify the latest supported Ruby version.
           bundler-cache: true
       - run: bundle exec rake rubocop
 
@@ -40,6 +43,6 @@ jobs:
       - uses: actions/checkout@v6
       - uses: ruby/setup-ruby@v1
         with:
-          ruby-version: 3.2 # Specify the oldest supported Ruby version.
+          ruby-version: 4.0 # Specify the latest supported Ruby version.
           bundler-cache: true
       - run: bundle exec yard --no-output

.rubocop.yml

@@ -5,6 +5,9 @@ plugins:
   - rubocop-minitest
   - rubocop-rake
 
+AllCops:
+  TargetRubyVersion: 2.7
+
 Gemspec/DevelopmentDependencies:
   Enabled: true
 

AGENTS.md

@@ -6,7 +6,7 @@ This is the official Ruby SDK for the Model Context Protocol (MCP), implementing
 
 ## Dev environment setup
 
-- Ruby 3.2.0+ required
+- Ruby 3.2.0+ required to run the full test suite, including all Sorbet-related features
 - Run `bundle install` to install dependencies
 - Dependencies: `json-schema` >= 4.1 - Schema validation
 

Gemfile

@@ -8,17 +8,18 @@ gemspec
 # Specify development dependencies below
 gem "rubocop-minitest", require: false
 gem "rubocop-rake", require: false
-gem "rubocop-shopify", require: false
+gem "rubocop-shopify", ">= 2.18", require: false if RUBY_VERSION >= "3.1"
 
 gem "puma", ">= 5.0.0"
 gem "rackup", ">= 2.1.0"
 
 gem "activesupport"
-gem "debug"
+# Fix io-console install error when Ruby 3.0.
+gem "debug" if RUBY_VERSION >= "3.1"
 gem "rake", "~> 13.0"
-gem "sorbet-static-and-runtime"
+gem "sorbet-static-and-runtime" if RUBY_VERSION >= "3.0"
 gem "yard", "~> 0.9"
-gem "yard-sorbet", "~> 0.9"
+gem "yard-sorbet", "~> 0.9" if RUBY_VERSION >= "3.1"
 
 group :test do
   gem "faraday", ">= 2.0"

lib/json_rpc_handler.rb

@@ -22,14 +22,14 @@ module JsonRpcHandler
 
   def handle(request, id_validation_pattern: DEFAULT_ALLOWED_ID_CHARACTERS, &method_finder)
     if request.is_a?(Array)
-      return error_response(id: :unknown_id, id_validation_pattern:, error: {
+      return error_response(id: :unknown_id, id_validation_pattern: id_validation_pattern, error: {
         code: ErrorCode::INVALID_REQUEST,
         message: "Invalid Request",
         data: "Request is an empty array",
       }) if request.empty?
 
       # Handle batch requests
-      responses = request.map { |req| process_request(req, id_validation_pattern:, &method_finder) }.compact
+      responses = request.map { |req| process_request(req, id_validation_pattern: id_validation_pattern, &method_finder) }.compact
 
       # A single item is hoisted out of the array
       return responses.first if responses.one?
@@ -38,9 +38,9 @@ module JsonRpcHandler
       responses if responses.any?
     elsif request.is_a?(Hash)
       # Handle single request
-      process_request(request, id_validation_pattern:, &method_finder)
+      process_request(request, id_validation_pattern: id_validation_pattern, &method_finder)
     else
-      error_response(id: :unknown_id, id_validation_pattern:, error: {
+      error_response(id: :unknown_id, id_validation_pattern: id_validation_pattern, error: {
         code: ErrorCode::INVALID_REQUEST,
         message: "Invalid Request",
         data: "Request must be an array or a hash",
@@ -51,9 +51,9 @@ module JsonRpcHandler
   def handle_json(request_json, id_validation_pattern: DEFAULT_ALLOWED_ID_CHARACTERS, &method_finder)
     begin
       request = JSON.parse(request_json, symbolize_names: true)
-      response = handle(request, id_validation_pattern:, &method_finder)
+      response = handle(request, id_validation_pattern: id_validation_pattern, &method_finder)
     rescue JSON::ParserError
-      response = error_response(id: :unknown_id, id_validation_pattern:, error: {
+      response = error_response(id: :unknown_id, id_validation_pattern: id_validation_pattern, error: {
         code: ErrorCode::PARSE_ERROR,
         message: "Parse error",
         data: "Invalid JSON",
@@ -74,7 +74,7 @@ module JsonRpcHandler
       'Method name must be a string and not start with "rpc."'
     end
 
-    return error_response(id: :unknown_id, id_validation_pattern:, error: {
+    return error_response(id: :unknown_id, id_validation_pattern: id_validation_pattern, error: {
       code: ErrorCode::INVALID_REQUEST,
       message: "Invalid Request",
       data: error,
@@ -84,7 +84,7 @@ module JsonRpcHandler
     params = request[:params]
 
     unless valid_params?(params)
-      return error_response(id:, id_validation_pattern:, error: {
+      return error_response(id: id, id_validation_pattern: id_validation_pattern, error: {
         code: ErrorCode::INVALID_PARAMS,
         message: "Invalid params",
         data: "Method parameters must be an array or an object or null",
@@ -95,7 +95,7 @@ module JsonRpcHandler
       method = method_finder.call(method_name)
 
       if method.nil?
-        return error_response(id:, id_validation_pattern:, error: {
+        return error_response(id: id, id_validation_pattern: id_validation_pattern, error: {
           code: ErrorCode::METHOD_NOT_FOUND,
           message: "Method not found",
           data: method_name,
@@ -104,9 +104,9 @@ module JsonRpcHandler
 
       result = method.call(params)
 
-      success_response(id:, result:)
+      success_response(id: id, result: result)
     rescue StandardError => e
-      error_response(id:, id_validation_pattern:, error: {
+      error_response(id: id, id_validation_pattern: id_validation_pattern, error: {
         code: ErrorCode::INTERNAL_ERROR,
         message: "Internal error",
         data: e.message,
@@ -136,8 +136,8 @@ module JsonRpcHandler
   def success_response(id:, result:)
     {
       jsonrpc: Version::V2_0,
-      id:,
+      id: id,
-      result:,
+      result: result,
     } unless id.nil?
   end
 

lib/mcp/client/http.rb

@@ -18,42 +18,42 @@ module MCP
       rescue Faraday::BadRequestError => e
         raise RequestHandlerError.new(
           "The #{method} request is invalid",
-          { method:, params: },
+          { method: method, params: params },
           error_type: :bad_request,
           original_error: e,
         )
       rescue Faraday::UnauthorizedError => e
         raise RequestHandlerError.new(
           "You are unauthorized to make #{method} requests",
-          { method:, params: },
+          { method: method, params: params },
           error_type: :unauthorized,
           original_error: e,
         )
       rescue Faraday::ForbiddenError => e
         raise RequestHandlerError.new(
           "You are forbidden to make #{method} requests",
-          { method:, params: },
+          { method: method, params: params },
           error_type: :forbidden,
           original_error: e,
         )
       rescue Faraday::ResourceNotFound => e
         raise RequestHandlerError.new(
           "The #{method} request is not found",
-          { method:, params: },
+          { method: method, params: params },
           error_type: :not_found,
           original_error: e,
         )
       rescue Faraday::UnprocessableEntityError => e
         raise RequestHandlerError.new(
           "The #{method} request is unprocessable",
-          { method:, params: },
+          { method: method, params: params },
           error_type: :unprocessable_entity,
           original_error: e,
         )
       rescue Faraday::Error => e # Catch-all
         raise RequestHandlerError.new(
           "Internal error handling #{method} request",
-          { method:, params: },
+          { method: method, params: params },
           error_type: :internal_error,
           original_error: e,
         )

lib/mcp/configuration.rb

@@ -85,10 +85,10 @@ module MCP
       validate_tool_call_arguments = other.validate_tool_call_arguments
 
       Configuration.new(
-        exception_reporter:,
+        exception_reporter: exception_reporter,
-        instrumentation_callback:,
+        instrumentation_callback: instrumentation_callback,
-        protocol_version:,
+        protocol_version: protocol_version,
-        validate_tool_call_arguments:,
+        validate_tool_call_arguments: validate_tool_call_arguments,
       )
     end
 

lib/mcp/content.rb

@@ -11,7 +11,7 @@ module MCP
       end
 
       def to_h
-        { text:, annotations:, type: "text" }.compact
+        { text: text, annotations: annotations, type: "text" }.compact
       end
     end
 
@@ -25,7 +25,7 @@ module MCP
       end
 
       def to_h
-        { data:, mime_type:, annotations:, type: "image" }.compact
+        { data: data, mime_type: mime_type, annotations: annotations, type: "image" }.compact
       end
     end
   end

lib/mcp/instrumentation.rb

@@ -6,7 +6,7 @@ module MCP
       start_time = Process.clock_gettime(Process::CLOCK_MONOTONIC)
       begin
         @instrumentation_data = {}
-        add_instrumentation_data(method:)
+        add_instrumentation_data(method: method)
 
         result = yield block
 

lib/mcp/prompt.rb

@@ -96,7 +96,7 @@ module MCP
           icons icons
           arguments arguments
           define_singleton_method(:template) do |args, server_context: nil|
-            instance_exec(args, server_context:, &block)
+            instance_exec(args, server_context: server_context, &block)
           end
           meta meta
         end

lib/mcp/prompt/message.rb

@@ -11,7 +11,7 @@ module MCP
       end
 
       def to_h
-        { role:, content: content.to_h }.compact
+        { role: role, content: content.to_h }.compact
       end
     end
   end

lib/mcp/prompt/result.rb

@@ -11,7 +11,7 @@ module MCP
       end
 
       def to_h
-        { description:, messages: messages.map(&:to_h) }.compact
+        { description: description, messages: messages.map(&:to_h) }.compact
       end
     end
   end

lib/mcp/resource/contents.rb

@@ -11,7 +11,7 @@ module MCP
       end
 
       def to_h
-        { uri:, mime_type: }.compact
+        { uri: uri, mime_type: mime_type }.compact
       end
     end
 

lib/mcp/resource/embedded.rb

@@ -10,7 +10,7 @@ module MCP
       end
 
       def to_h
-        { resource: resource.to_h, annotations: }.compact
+        { resource: resource.to_h, annotations: annotations }.compact
       end
     end
   end

lib/mcp/server.rb

@@ -114,7 +114,7 @@ module MCP
     end
 
     def define_tool(name: nil, title: nil, description: nil, input_schema: nil, annotations: nil, meta: nil, &block)
-      tool = Tool.define(name:, title:, description:, input_schema:, annotations:, meta:, &block)
+      tool = Tool.define(name: name, title: title, description: description, input_schema: input_schema, annotations: annotations, meta: meta, &block)
       tool_name = tool.name_value
 
       @tool_names << tool_name
@@ -124,7 +124,7 @@ module MCP
     end
 
     def define_prompt(name: nil, title: nil, description: nil, arguments: [], &block)
-      prompt = Prompt.define(name:, title:, description:, arguments:, &block)
+      prompt = Prompt.define(name: name, title: title, description: description, arguments: arguments, &block)
       @prompts[prompt.name_value] = prompt
 
       validate!
@@ -289,11 +289,11 @@ module MCP
 
     def server_info
       @server_info ||= {
-        description:,
+        description: description,
-        icons:,
+        icons: icons,
-        name:,
+        name: name,
-        title:,
+        title: title,
-        version:,
+        version: version,
         websiteUrl: website_url,
       }.compact
     end
@@ -316,13 +316,13 @@ module MCP
 
       tool = tools[tool_name]
       unless tool
-        add_instrumentation_data(tool_name:, error: :tool_not_found)
+        add_instrumentation_data(tool_name: tool_name, error: :tool_not_found)
 
         return error_tool_response("Tool not found: #{tool_name}")
       end
 
       arguments = request[:arguments] || {}
-      add_instrumentation_data(tool_name:)
+      add_instrumentation_data(tool_name: tool_name)
 
       if tool.input_schema&.missing_required_arguments?(arguments)
         add_instrumentation_data(error: :missing_required_arguments)
@@ -360,7 +360,7 @@ module MCP
         raise RequestHandlerError.new("Prompt not found #{prompt_name}", request, error_type: :prompt_not_found)
       end
 
-      add_instrumentation_data(prompt_name:)
+      add_instrumentation_data(prompt_name: prompt_name)
 
       prompt_args = request[:arguments]
       prompt.validate_arguments!(prompt_args)

lib/mcp/server/transports/streamable_http_transport.rb

@@ -42,7 +42,7 @@ module MCP
 
           notification = {
             jsonrpc: "2.0",
-            method:,
+            method: method,
           }
           notification[:params] = params if params
 

lib/mcp/tool/annotations.rb

@@ -19,7 +19,7 @@ module MCP
           idempotentHint: idempotent_hint,
           openWorldHint: open_world_hint,
           readOnlyHint: read_only_hint,
-          title:,
+          title: title,
         }.compact
       end
     end

lib/mcp/tool/response.rb

@@ -23,7 +23,7 @@ module MCP
       end
 
       def to_h
-        { content:, isError: error?, structuredContent: @structured_content }.compact
+        { content: content, isError: error?, structuredContent: @structured_content }.compact
       end
     end
   end

mcp.gemspec

@@ -13,7 +13,8 @@ Gem::Specification.new do |spec|
   spec.homepage      = "https://github.com/modelcontextprotocol/ruby-sdk"
   spec.license       = "MIT"
 
-  spec.required_ruby_version = ">= 3.2.0"
+  # Since this library is used by a broad range of users, it does not align its support policy with Ruby's EOL.
+  spec.required_ruby_version = ">= 2.7.0"
 
   spec.metadata["allowed_push_host"] = "https://rubygems.org"
   spec.metadata["changelog_uri"] = "https://github.com/modelcontextprotocol/ruby-sdk/releases/tag/v#{spec.version}"

test/json_rpc_handler_test.rb

@@ -94,7 +94,7 @@ describe JsonRpcHandler do
       register("add") { |params| params[:a] + params[:b] }
       id = "request-123_abc"
 
-      handle jsonrpc: "2.0", id:, method: "add", params: { a: 1, b: 2 }
+      handle jsonrpc: "2.0", id: id, method: "add", params: { a: 1, b: 2 }
 
       assert_rpc_success expected_result: 3
       assert_equal id, @response[:id]
@@ -104,7 +104,7 @@ describe JsonRpcHandler do
       register("add") { |params| params[:a] + params[:b] }
       id = 42
 
-      handle jsonrpc: "2.0", id:, method: "add", params: { a: 1, b: 2 }
+      handle jsonrpc: "2.0", id: id, method: "add", params: { a: 1, b: 2 }
 
       assert_rpc_success expected_result: 3
       assert_equal id, @response[:id]
@@ -124,7 +124,7 @@ describe JsonRpcHandler do
       register("add") { |params| params[:a] + params[:b] }
       id = "request-123_ABC"
 
-      handle jsonrpc: "2.0", id:, method: "add", params: { a: 1, b: 2 }
+      handle jsonrpc: "2.0", id: id, method: "add", params: { a: 1, b: 2 }
 
       assert_rpc_success expected_result: 3
       assert_equal id, @response[:id]
@@ -134,7 +134,7 @@ describe JsonRpcHandler do
       register("add") { |params| params[:a] + params[:b] }
       id = "550e8400-e29b-41d4-a716-446655440000"
 
-      handle jsonrpc: "2.0", id:, method: "add", params: { a: 1, b: 2 }
+      handle jsonrpc: "2.0", id: id, method: "add", params: { a: 1, b: 2 }
 
       assert_rpc_success expected_result: 3
       assert_equal id, @response[:id]