Omit icons from responses when empty or nil

Previously, the `icons` parameter was always included in responses (as an empty array)
even when not configured. This differs from the response representation in MCP Ruby SDK 0.4.0.

Since `icons` is optional, it should be omitted from responses when unused to reduce
unnecessary context window usage.
https://modelcontextprotocol.io/specification/2025-11-25/schema#implementation

This fix applies to `Tool#to_h`, `Prompt#to_h`, `Resource#to_h`, `ResourceTemplate#to_h`,
and `Server#server_info`.

It doesn't seem critical, but if there are no other issues, it would be good to cut 0.5.1 bugfix release soon.

Author: koic

lib/mcp/prompt.rb

@@ -20,7 +20,7 @@ def to_h
           name: name_value,
           title: title_value,
           description: description_value,
-          icons: icons&.map(&:to_h),
+          icons: icons_value&.then { |icons| icons.empty? ? nil : icons.map(&:to_h) },
           arguments: arguments_value&.map(&:to_h),
           _meta: meta_value,
         }.compact

lib/mcp/resource.rb

@@ -19,7 +19,7 @@ def to_h
         name: name,
         title: title,
         description: description,
-        icons: icons.map(&:to_h),
+        icons: icons&.then { |icons| icons.empty? ? nil : icons.map(&:to_h) },
         mimeType: mime_type,
       }.compact
     end

lib/mcp/resource_template.rb

@@ -19,7 +19,7 @@ def to_h
         name: name,
         title: title,
         description: description,
-        icons: icons.map(&:to_h),
+        icons: icons&.then { |icons| icons.empty? ? nil : icons.map(&:to_h) },
         mimeType: mime_type,
       }.compact
     end

lib/mcp/server.rb

@@ -290,7 +290,7 @@ def default_capabilities
     def server_info
       @server_info ||= {
         description: description,
-        icons: icons,
+        icons: icons&.then { |icons| icons.empty? ? nil : icons.map(&:to_h) },
         name: name,
         title: title,
         version: version,

lib/mcp/tool.rb

@@ -21,7 +21,7 @@ def to_h
           name: name_value,
           title: title_value,
           description: description_value,
-          icons: icons&.map(&:to_h),
+          icons: icons_value&.then { |icons| icons.empty? ? nil : icons.map(&:to_h) },
           inputSchema: input_schema_value.to_h,
           outputSchema: @output_schema_value&.to_h,
           annotations: annotations_value&.to_h,

test/mcp/prompt_test.rb

@@ -195,5 +195,24 @@ class NoArgumentsPrompt < Prompt
 
       assert_equal expected, prompt.to_h
     end
+
+    test "#to_h does not have `:icons` key when icons is empty" do
+      prompt = Prompt.define(
+        name: "prompt_without_icons",
+        description: "a prompt without icons",
+      )
+
+      refute prompt.to_h.key?(:icons)
+    end
+
+    test "#to_h does not have `:icons` key when icons is nil" do
+      prompt = Prompt.define(
+        name: "prompt_without_icons",
+        description: "a prompt without icons",
+        icons: nil,
+      )
+
+      refute prompt.to_h.key?(:icons)
+    end
   end
 end

test/mcp/resource_template_test.rb

@@ -0,0 +1,40 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+module MCP
+  class ResourceTemplateTest < ActiveSupport::TestCase
+    test "#to_h does not have `:icons` key when icons is empty" do
+      resource_template = ResourceTemplate.new(
+        uri_template: "file:///{path}",
+        name: "resource_template_without_icons",
+        description: "a resource template without icons",
+      )
+
+      refute resource_template.to_h.key?(:icons)
+    end
+
+    test "#to_h does not have `:icons` key when icons is nil" do
+      resource_template = ResourceTemplate.new(
+        uri_template: "file:///{path}",
+        name: "resource_template_without_icons",
+        description: "a resource template without icons",
+        icons: nil,
+      )
+
+      refute resource_template.to_h.key?(:icons)
+    end
+
+    test "#to_h includes icons when present" do
+      resource_template = ResourceTemplate.new(
+        uri_template: "file:///{path}",
+        name: "resource_template_with_icons",
+        description: "a resource template with icons",
+        icons: [Icon.new(mime_type: "image/png", sizes: ["48x48"], src: "https://example.com", theme: "light")],
+      )
+      expected_icons = [{ mimeType: "image/png", sizes: ["48x48"], src: "https://example.com", theme: "light" }]
+
+      assert_equal expected_icons, resource_template.to_h[:icons]
+    end
+  end
+end

test/mcp/resource_test.rb

@@ -0,0 +1,40 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+module MCP
+  class ResourceTest < ActiveSupport::TestCase
+    test "#to_h does not have `:icons` key when icons is empty" do
+      resource = Resource.new(
+        uri: "file:///test.txt",
+        name: "resource_without_icons",
+        description: "a resource without icons",
+      )
+
+      refute resource.to_h.key?(:icons)
+    end
+
+    test "#to_h does not have `:icons` key when icons is nil" do
+      resource = Resource.new(
+        uri: "file:///test.txt",
+        name: "resource_without_icons",
+        description: "a resource without icons",
+        icons: nil,
+      )
+
+      refute resource.to_h.key?(:icons)
+    end
+
+    test "#to_h includes icons when present" do
+      resource = Resource.new(
+        uri: "file:///test.txt",
+        name: "resource_with_icons",
+        description: "a resource with icons",
+        icons: [Icon.new(mime_type: "image/png", sizes: ["48x48"], src: "https://example.com", theme: "light")],
+      )
+      expected_icons = [{ mimeType: "image/png", sizes: ["48x48"], src: "https://example.com", theme: "light" }]
+
+      assert_equal expected_icons, resource.to_h[:icons]
+    end
+  end
+end

test/mcp/server_test.rb

@@ -895,6 +895,46 @@ class Example < Tool
       refute response[:result][:serverInfo].key?(:website_url)
     end
 
+    test "server response does not include icons when icons is empty" do
+      server = Server.new(name: "test_server")
+      request = {
+        jsonrpc: "2.0",
+        method: "initialize",
+        id: 1,
+      }
+      response = server.handle(request)
+
+      refute response[:result][:serverInfo].key?(:icons)
+    end
+
+    test "server response does not include icons when icons is nil" do
+      server = Server.new(name: "test_server", icons: nil)
+      request = {
+        jsonrpc: "2.0",
+        method: "initialize",
+        id: 1,
+      }
+      response = server.handle(request)
+
+      refute response[:result][:serverInfo].key?(:icons)
+    end
+
+    test "server response includes icons when icons is present" do
+      server = Server.new(
+        name: "test_server",
+        icons: [Icon.new(mime_type: "image/png", sizes: ["48x48"], src: "https://example.com", theme: "light")],
+      )
+      request = {
+        jsonrpc: "2.0",
+        method: "initialize",
+        id: 1,
+      }
+      response = server.handle(request)
+      expected_icons = [{ mimeType: "image/png", sizes: ["48x48"], src: "https://example.com", theme: "light" }]
+
+      assert_equal expected_icons, response[:result][:serverInfo][:icons]
+    end
+
     test "server uses default version when not configured" do
       server = Server.new(name: "test_server")
       request = {

test/mcp/tool_test.rb

@@ -53,6 +53,25 @@ def call(message:, server_context: nil)
       refute tool.to_h.key?(:title)
     end
 
+    test "#to_h does not have `:icons` key when icons is empty" do
+      tool = Tool.define(
+        name: "tool_without_icons",
+        description: "a tool without icons",
+      )
+
+      refute tool.to_h.key?(:icons)
+    end
+
+    test "#to_h does not have `:icons` key when icons is nil" do
+      tool = Tool.define(
+        name: "tool_without_icons",
+        description: "a tool without icons",
+        icons: nil,
+      )
+
+      refute tool.to_h.key?(:icons)
+    end
+
     test "#to_h includes annotations when present" do
       tool = TestTool
       expected_annotations = {