Extract server capabilities into dedicated Capabilities class

Refactor capabilities handling into MCP::Server::Capabilities to improve
code organization and separation of concerns. This provides a cleaner API
for configuring server capabilities including prompts, resources, tools,
completions, logging, and experimental features.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

lib/mcp/server.rb

@@ -3,6 +3,7 @@
 require "json_rpc_handler"
 require_relative "instrumentation"
 require_relative "methods"
+require_relative "server/capabilities"
 
 module MCP
   class Server

lib/mcp/server/capabilities.rb

@@ -0,0 +1,96 @@
+# frozen_string_literal: true
+
+module MCP
+  class Server
+    class Capabilities
+      def initialize(capabilities_hash = nil)
+        @completions = nil
+        @experimental = nil
+        @logging = nil
+        @prompts = nil
+        @resources = nil
+        @tools = nil
+
+        if capabilities_hash
+          support_completions if capabilities_hash.key?(:completions)
+          support_experimental(capabilities_hash[:experimental]) if capabilities_hash.key?(:experimental)
+          support_logging if capabilities_hash.key?(:logging)
+
+          if capabilities_hash.key?(:prompts)
+            support_prompts
+            prompts_config = capabilities_hash[:prompts] || {}
+            support_prompts_list_changed if prompts_config[:listChanged]
+          end
+
+          if capabilities_hash.key?(:resources)
+            support_resources
+            resources_config = capabilities_hash[:resources] || {}
+            support_resources_list_changed if resources_config[:listChanged]
+            support_resources_subscribe if resources_config[:subscribe]
+          end
+
+          if capabilities_hash.key?(:tools)
+            support_tools
+            tools_config = capabilities_hash[:tools] || {}
+            support_tools_list_changed if tools_config[:listChanged]
+          end
+        end
+      end
+
+      def support_completions
+        @completions ||= {}
+      end
+
+      def support_experimental(config = {})
+        @experimental = config || {}
+      end
+
+      def support_logging
+        @logging ||= {}
+      end
+
+      def support_prompts
+        @prompts ||= {}
+      end
+
+      def support_prompts_list_changed
+        support_prompts
+        @prompts[:listChanged] = true
+      end
+
+      def support_resources
+        @resources ||= {}
+      end
+
+      def support_resources_list_changed
+        support_resources
+        @resources[:listChanged] = true
+      end
+
+      def support_resources_subscribe
+        support_resources
+        @resources[:subscribe] = true
+      end
+
+      def support_tools
+        @tools ||= {}
+      end
+
+      def support_tools_list_changed
+        support_tools
+        @tools[:listChanged] = true
+      end
+
+      def to_h
+        {
+          completions: @completions,
+          experimental: @experimental,
+          logging: @logging,
+          prompts: @prompts,
+          resources: @resources,
+          tools: @tools,
+        }.compact
+      end
+    end
+  end
+end

test/mcp/server/capabilities_test.rb

@@ -0,0 +1,197 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+module MCP
+  class Server
+    class CapabilitiesTest < ActiveSupport::TestCase
+      test "can be initialized with a capabilities hash" do
+        capabilities = MCP::Server::Capabilities.new(prompts: {})
+
+        assert_equal({ prompts: {} }, capabilities.to_h)
+      end
+
+      test "ignores unknown values in capabilities hash" do
+        capabilities = MCP::Server::Capabilities.new(prompts: {}, invalid: :value)
+
+        assert_equal({ prompts: {} }, capabilities.to_h)
+      end
+
+      test "support prompts" do
+        capabilities = MCP::Server::Capabilities.new
+
+        capabilities.support_prompts
+
+        assert_operator capabilities.to_h, :key?, :prompts
+      end
+
+      test "support resources" do
+        capabilities = MCP::Server::Capabilities.new
+
+        capabilities.support_resources
+
+        assert_operator capabilities.to_h, :key?, :resources
+      end
+
+      test "support tools" do
+        capabilities = MCP::Server::Capabilities.new
+
+        capabilities.support_tools
+
+        assert_operator capabilities.to_h, :key?, :tools
+      end
+
+      test "support completions" do
+        capabilities = MCP::Server::Capabilities.new
+
+        capabilities.support_completions
+
+        assert_operator capabilities.to_h, :key?, :completions
+        assert_empty(capabilities.to_h[:completions])
+      end
+
+      test "support logging" do
+        capabilities = MCP::Server::Capabilities.new
+
+        capabilities.support_logging
+
+        assert_operator capabilities.to_h, :key?, :logging
+        assert_empty(capabilities.to_h[:logging])
+      end
+
+      test "support experimental with custom config" do
+        capabilities = MCP::Server::Capabilities.new
+
+        capabilities.support_experimental({ myFeature: { enabled: true } })
+
+        assert_operator capabilities.to_h, :key?, :experimental
+        assert_equal({ myFeature: { enabled: true } }, capabilities.to_h[:experimental])
+      end
+
+      test "support prompts list changed" do
+        capabilities = MCP::Server::Capabilities.new
+
+        capabilities.support_prompts_list_changed
+
+        assert_equal({ prompts: { listChanged: true } }, capabilities.to_h)
+      end
+
+      test "support resources list changed" do
+        capabilities = MCP::Server::Capabilities.new
+
+        capabilities.support_resources_list_changed
+
+        assert_equal({ resources: { listChanged: true } }, capabilities.to_h)
+      end
+
+      test "support resources subscribe" do
+        capabilities = MCP::Server::Capabilities.new
+
+        capabilities.support_resources_subscribe
+
+        assert_equal({ resources: { subscribe: true } }, capabilities.to_h)
+      end
+
+      test "support resources with both list changed and subscribe" do
+        capabilities = MCP::Server::Capabilities.new
+
+        capabilities.support_resources_list_changed
+        capabilities.support_resources_subscribe
+
+        assert_equal({ resources: { listChanged: true, subscribe: true } }, capabilities.to_h)
+      end
+
+      test "support tools list changed" do
+        capabilities = MCP::Server::Capabilities.new
+
+        capabilities.support_tools_list_changed
+
+        assert_equal({ tools: { listChanged: true } }, capabilities.to_h)
+      end
+
+      test "initializes with prompts list changed from hash" do
+        capabilities = MCP::Server::Capabilities.new(prompts: { listChanged: true })
+
+        assert_equal({ prompts: { listChanged: true } }, capabilities.to_h)
+      end
+
+      test "initializes with resources list changed and subscribe from hash" do
+        capabilities = MCP::Server::Capabilities.new(resources: { listChanged: true, subscribe: true })
+
+        assert_equal({ resources: { listChanged: true, subscribe: true } }, capabilities.to_h)
+      end
+
+      test "initializes with tools list changed from hash" do
+        capabilities = MCP::Server::Capabilities.new(tools: { listChanged: true })
+
+        assert_equal({ tools: { listChanged: true } }, capabilities.to_h)
+      end
+
+      test "initializes with completions from hash" do
+        capabilities = MCP::Server::Capabilities.new(completions: {})
+
+        assert_equal({ completions: {} }, capabilities.to_h)
+      end
+
+      test "initializes with logging from hash" do
+        capabilities = MCP::Server::Capabilities.new(logging: {})
+
+        assert_equal({ logging: {} }, capabilities.to_h)
+      end
+
+      test "initializes with experimental config from hash" do
+        capabilities = MCP::Server::Capabilities.new(experimental: { feature1: { enabled: true }, feature2: "config" })
+
+        assert_equal({ experimental: { feature1: { enabled: true }, feature2: "config" } }, capabilities.to_h)
+      end
+
+      test "initializes with all capabilities from hash" do
+        capabilities = MCP::Server::Capabilities.new(
+          completions: {},
+          experimental: { customFeature: true },
+          logging: {},
+          prompts: { listChanged: true },
+          resources: { listChanged: true, subscribe: true },
+          tools: { listChanged: true },
+        )
+
+        expected = {
+          completions: {},
+          experimental: { customFeature: true },
+          logging: {},
+          prompts: { listChanged: true },
+          resources: { listChanged: true, subscribe: true },
+          tools: { listChanged: true },
+        }
+
+        assert_equal(expected, capabilities.to_h)
+      end
+
+      test "handles nil values in capabilities hash gracefully" do
+        capabilities = MCP::Server::Capabilities.new(
+          prompts: nil,
+          resources: nil,
+          tools: nil,
+        )
+
+        assert_equal({ prompts: {}, resources: {}, tools: {} }, capabilities.to_h)
+      end
+
+      test "to_h returns empty hash when no capabilities are set" do
+        capabilities = MCP::Server::Capabilities.new
+
+        assert_empty(capabilities.to_h)
+      end
+
+      test "calling support methods multiple times does not duplicate entries" do
+        capabilities = MCP::Server::Capabilities.new
+
+        capabilities.support_prompts
+        capabilities.support_prompts
+        capabilities.support_prompts_list_changed
+
+        assert_equal({ prompts: { listChanged: true } }, capabilities.to_h)
+      end
+    end
+  end
+end