Support icons parameter

This PR adds support for the optional `icons` parameter in prompt, resource,
resource template, and server info.

The specifications for `icons` are as follows.
https://modelcontextprotocol.io/specification/2025-11-25/schema#icon

Resolves https://github.com/modelcontextprotocol/ruby-sdk/issues/127.

lib/mcp.rb

@@ -3,6 +3,7 @@
 require_relative "json_rpc_handler"
 require_relative "mcp/configuration"
 require_relative "mcp/content"
+require_relative "mcp/icon"
 require_relative "mcp/instrumentation"
 require_relative "mcp/methods"
 require_relative "mcp/prompt"

lib/mcp/icon.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+module MCP
+  class Icon
+    SUPPORTED_THEMES = ["light", "dark"]
+
+    attr_reader :mime_type, :sizes, :src, :theme
+
+    def initialize(mime_type: nil, sizes: nil, src: nil, theme: nil)
+      raise ArgumentError, 'The value of theme must specify "light" or "dark".' if theme && !SUPPORTED_THEMES.include?(theme)
+
+      @mime_type = mime_type
+      @sizes = sizes
+      @src = src
+      @theme = theme
+    end
+
+    def to_h
+      { mimeType: mime_type, sizes: sizes, src: src, theme: theme }.compact
+    end
+  end
+end

lib/mcp/prompt.rb

@@ -7,6 +7,7 @@ module MCP
 
       attr_reader :title_value
       attr_reader :description_value
+      attr_reader :icons_value
       attr_reader :arguments_value
       attr_reader :meta_value
 
@@ -19,6 +20,7 @@ module MCP
           name: name_value,
           title: title_value,
           description: description_value,
+          icons: icons&.map(&:to_h),
           arguments: arguments_value&.map(&:to_h),
           _meta: meta_value,
         }.compact
@@ -29,6 +31,7 @@ module MCP
         subclass.instance_variable_set(:@name_value, nil)
         subclass.instance_variable_set(:@title_value, nil)
         subclass.instance_variable_set(:@description_value, nil)
+        subclass.instance_variable_set(:@icons_value, nil)
         subclass.instance_variable_set(:@arguments_value, nil)
         subclass.instance_variable_set(:@meta_value, nil)
       end
@@ -61,6 +64,14 @@ module MCP
         end
       end
 
+      def icons(value = NOT_SET)
+        if value == NOT_SET
+          @icons_value
+        else
+          @icons_value = value
+        end
+      end
+
       def arguments(value = NOT_SET)
         if value == NOT_SET
           @arguments_value
@@ -77,11 +88,12 @@ module MCP
         end
       end
 
-      def define(name: nil, title: nil, description: nil, arguments: [], meta: nil, &block)
+      def define(name: nil, title: nil, description: nil, icons: [], arguments: [], meta: nil, &block)
         Class.new(self) do
           prompt_name name
           title title
           description description
+          icons icons
           arguments arguments
           define_singleton_method(:template) do |args, server_context: nil|
             instance_exec(args, server_context:, &block)

lib/mcp/resource.rb

@@ -2,13 +2,14 @@
 
 module MCP
   class Resource
-    attr_reader :uri, :name, :title, :description, :mime_type
+    attr_reader :uri, :name, :title, :description, :icons, :mime_type
 
-    def initialize(uri:, name:, title: nil, description: nil, mime_type: nil)
+    def initialize(uri:, name:, title: nil, description: nil, icons: [], mime_type: nil)
       @uri = uri
       @name = name
       @title = title
       @description = description
+      @icons = icons
       @mime_type = mime_type
     end
 
@@ -18,6 +19,7 @@ module MCP
         name: name,
         title: title,
         description: description,
+        icons: icons.map(&:to_h),
         mimeType: mime_type,
       }.compact
     end

lib/mcp/resource_template.rb

@@ -2,13 +2,14 @@
 
 module MCP
   class ResourceTemplate
-    attr_reader :uri_template, :name, :title, :description, :mime_type
+    attr_reader :uri_template, :name, :title, :description, :icons, :mime_type
 
-    def initialize(uri_template:, name:, title: nil, description: nil, mime_type: nil)
+    def initialize(uri_template:, name:, title: nil, description: nil, icons: [], mime_type: nil)
       @uri_template = uri_template
       @name = name
       @title = title
       @description = description
+      @icons = icons
       @mime_type = mime_type
     end
 
@@ -18,6 +19,7 @@ module MCP
         name: name,
         title: title,
         description: description,
+        icons: icons.map(&:to_h),
         mimeType: mime_type,
       }.compact
     end

lib/mcp/server.rb

@@ -40,10 +40,11 @@ module MCP
 
     include Instrumentation
 
-    attr_accessor :description, :name, :title, :version, :website_url, :instructions, :tools, :prompts, :resources, :server_context, :configuration, :capabilities, :transport
+    attr_accessor :description, :icons, :name, :title, :version, :website_url, :instructions, :tools, :prompts, :resources, :server_context, :configuration, :capabilities, :transport
 
     def initialize(
       description: nil,
+      icons: [],
       name: "model_context_protocol",
       title: nil,
       version: DEFAULT_VERSION,
@@ -59,6 +60,7 @@ module MCP
       transport: nil
     )
       @description = description
+      @icons = icons
       @name = name
       @title = title
       @version = version
@@ -288,6 +290,7 @@ module MCP
     def server_info
       @server_info ||= {
         description:,
+        icons:,
         name:,
         title:,
         version:,

lib/mcp/tool.rb

@@ -7,6 +7,7 @@ module MCP
 
       attr_reader :title_value
       attr_reader :description_value
+      attr_reader :icons_value
       attr_reader :annotations_value
       attr_reader :meta_value
 
@@ -19,6 +20,7 @@ module MCP
           name: name_value,
           title: title_value,
           description: description_value,
+          icons: icons&.map(&:to_h),
           inputSchema: input_schema_value.to_h,
           outputSchema: @output_schema_value&.to_h,
           annotations: annotations_value&.to_h,
@@ -31,6 +33,7 @@ module MCP
         subclass.instance_variable_set(:@name_value, nil)
         subclass.instance_variable_set(:@title_value, nil)
         subclass.instance_variable_set(:@description_value, nil)
+        subclass.instance_variable_set(:@icons_value, nil)
         subclass.instance_variable_set(:@input_schema_value, nil)
         subclass.instance_variable_set(:@output_schema_value, nil)
         subclass.instance_variable_set(:@annotations_value, nil)
@@ -71,6 +74,14 @@ module MCP
         end
       end
 
+      def icons(value = NOT_SET)
+        if value == NOT_SET
+          @icons_value
+        else
+          @icons_value = value
+        end
+      end
+
       def input_schema(value = NOT_SET)
         if value == NOT_SET
           input_schema_value
@@ -107,11 +118,12 @@ module MCP
         end
       end
 
-      def define(name: nil, title: nil, description: nil, input_schema: nil, output_schema: nil, meta: nil, annotations: nil, &block)
+      def define(name: nil, title: nil, description: nil, icons: [], input_schema: nil, output_schema: nil, meta: nil, annotations: nil, &block)
         Class.new(self) do
           tool_name name
           title title
           description description
+          icons icons
           input_schema input_schema
           meta meta
           output_schema output_schema

test/mcp/icon_test.rb

@@ -0,0 +1,48 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+module MCP
+  class IconTest < ActiveSupport::TestCase
+    def test_initialization
+      icon = Icon.new(mime_type: "image/png", sizes: ["48x48", "96x96"], src: "https://example.com", theme: "light")
+
+      assert_equal("image/png", icon.mime_type)
+      assert_equal(["48x48", "96x96"], icon.sizes)
+      assert_equal("https://example.com", icon.src)
+      assert_equal("light", icon.theme)
+
+      assert_equal({ mimeType: "image/png", sizes: ["48x48", "96x96"], src: "https://example.com", theme: "light" }, icon.to_h)
+    end
+
+    def test_initialization_by_default
+      icon = Icon.new
+
+      assert_nil(icon.mime_type)
+      assert_nil(icon.sizes)
+      assert_nil(icon.src)
+      assert_nil(icon.theme)
+
+      assert_empty(icon.to_h)
+    end
+
+    def test_valid_theme_for_light
+      assert_nothing_raised do
+        Icon.new(theme: "light")
+      end
+    end
+
+    def test_valid_theme_for_dark
+      assert_nothing_raised do
+        Icon.new(theme: "dark")
+      end
+    end
+
+    def test_invalid_theme
+      exception = assert_raises(ArgumentError) do
+        Icon.new(theme: "unexpected")
+      end
+      assert_equal('The value of theme must specify "light" or "dark".', exception.message)
+    end
+  end
+end

test/mcp/prompt_test.rb

@@ -6,6 +6,7 @@ module MCP
   class PromptTest < ActiveSupport::TestCase
     class TestPrompt < Prompt
       description "Test prompt"
+      icons [{ mimeType: "image/png", sizes: ["48x48", "96x96"], src: "https://example.com", theme: "light" }]
       arguments [
         Prompt::Argument.new(name: "test_argument", description: "Test argument", required: true),
       ]
@@ -43,6 +44,7 @@ module MCP
       class MockPrompt < Prompt
         prompt_name "my_mock_prompt"
         description "a mock prompt for testing"
+        icons [{ mimeType: "image/png", sizes: ["48x48", "96x96"], src: "https://example.com", theme: "light" }]
         arguments [
           Prompt::Argument.new(name: "test_argument", description: "Test argument", required: true),
         ]
@@ -64,6 +66,7 @@ module MCP
 
       assert_equal "my_mock_prompt", prompt.name_value
       assert_equal "a mock prompt for testing", prompt.description
+      assert_equal([{ mimeType: "image/png", sizes: ["48x48", "96x96"], src: "https://example.com", theme: "light" }], prompt.icons)
       assert_equal "test_argument", prompt.arguments.first.name
       assert_equal "Test argument", prompt.arguments.first.description
       assert prompt.arguments.first.required
@@ -112,6 +115,7 @@ module MCP
         name: "mock_prompt",
         title: "Mock Prompt",
         description: "a mock prompt for testing",
+        icons: [{ mimeType: "image/png", sizes: ["48x48", "96x96"], src: "https://example.com", theme: "light" }],
         arguments: [
           Prompt::Argument.new(
             name: "test_argument",
@@ -135,6 +139,7 @@ module MCP
 
       assert_equal "mock_prompt", prompt.name_value
       assert_equal "a mock prompt for testing", prompt.description
+      assert_equal([{ mimeType: "image/png", sizes: ["48x48", "96x96"], src: "https://example.com", theme: "light" }], prompt.icons)
       assert_equal "test_argument", prompt.arguments.first.name
       assert_equal "Test argument title", prompt.arguments.first.title
       assert_equal "This is a test argument description", prompt.arguments.first.description

test/mcp/server_test.rb

@@ -52,6 +52,7 @@ module MCP
         name: "test-resource",
         title: "Test Resource",
         description: "Test resource",
+        icons: [{ mimeType: "image/png", sizes: ["48x48", "96x96"], src: "https://example.com", theme: "light" }],
         mime_type: "text/plain",
       )
 
@@ -60,6 +61,7 @@ module MCP
         name: "test-resource",
         title: "Test Resource",
         description: "Test resource",
+        icons: [{ mimeType: "image/png", sizes: ["48x48", "96x96"], src: "https://example.com", theme: "light" }],
         mime_type: "text/plain",
       )
 
@@ -69,6 +71,7 @@ module MCP
 
       @server = Server.new(
         description: "Test server",
+        icons: [{ mimeType: "image/png", sizes: ["48x48", "96x96"], src: "https://example.com", theme: "light" }],
         name: @server_name,
         title: "Example Server Display Name",
         version: "1.2.3",
@@ -142,6 +145,7 @@ module MCP
           },
           serverInfo: {
             description: "Test server",
+            icons: [{ mimeType: "image/png", sizes: ["48x48", "96x96"], src: "https://example.com", theme: "light" }],
             name: @server_name,
             title: "Example Server Display Name",
             version: "1.2.3",

test/mcp/tool_test.rb

@@ -7,6 +7,7 @@ module MCP
     class TestTool < Tool
       tool_name "test_tool"
       description "a test tool for testing"
+      icons [Icon.new(mime_type: "image/png", sizes: ["48x48", "96x96"], src: "https://example.com", theme: "light")]
       input_schema({ properties: { message: { type: "string" } }, required: ["message"] })
       annotations(
         destructive_hint: false,
@@ -26,13 +27,22 @@ module MCP
       end
     end
 
-    test "#to_h returns a hash with name, description, and inputSchema" do
+    test "#to_h returns a hash including name, description, icons, and inputSchema" do
+      expected = {
+        name: "mock_tool",
+        title: "Mock Tool",
+        description: "a mock tool for testing",
+        icons: [{ mimeType: "image/png", sizes: ["48x48", "96x96"], src: "https://example.com", theme: "light" }],
+        inputSchema: { type: "object" },
+      }
       tool = Tool.define(
         name: "mock_tool",
         title: "Mock Tool",
         description: "a mock tool for testing",
+        icons: [Icon.new(mime_type: "image/png", sizes: ["48x48", "96x96"], src: "https://example.com", theme: "light")],
       )
-      assert_equal({ name: "mock_tool", title: "Mock Tool", description: "a mock tool for testing", inputSchema: { type: "object" } }, tool.to_h)
+
+      assert_equal(expected, tool.to_h)
     end
 
     test "#to_h does not have `:title` key when title is omitted" do
@@ -308,19 +318,22 @@ module MCP
     end
 
     test "#to_h includes outputSchema when present" do
-      tool = Tool.define(
-        name: "mock_tool",
-        title: "Mock Tool",
-        description: "a mock tool for testing",
-        output_schema: { properties: { result: { type: "string" } }, required: ["result"] },
-      )
       expected = {
         name: "mock_tool",
         title: "Mock Tool",
         description: "a mock tool for testing",
+        icons: [{ mimeType: "image/png", sizes: ["48x48", "96x96"], src: "https://example.com", theme: "light" }],
         inputSchema: { type: "object" },
         outputSchema: { type: "object", properties: { result: { type: "string" } }, required: ["result"] },
       }
+      tool = Tool.define(
+        name: "mock_tool",
+        title: "Mock Tool",
+        description: "a mock tool for testing",
+        icons: [Icon.new(mime_type: "image/png", sizes: ["48x48", "96x96"], src: "https://example.com", theme: "light")],
+        output_schema: { properties: { result: { type: "string" } }, required: ["result"] },
+      )
+
       assert_equal expected, tool.to_h
     end