Set default values for ToolAnnotations hints

According to the specification, `destructive_hint`, `idempotent_hint`, `open_world_hint`,
and `read_only_hint` of `ToolAnnotations` are optional with default values:
https://modelcontextprotocol.io/specification/2025-06-18/schema#toolannotations

This PR sets their default values in accordance with the specification.

Additionally, for readability, the hints have been reordered to follow the specification.

Author: koic

README.md

@@ -420,10 +420,10 @@ e.g. around authentication state.
 
 Tools can include annotations that provide additional metadata about their behavior. The following annotations are supported:
 
-- `destructive_hint`: Indicates if the tool performs destructive operations
-- `idempotent_hint`: Indicates if the tool's operations are idempotent
-- `open_world_hint`: Indicates if the tool operates in an open world context
-- `read_only_hint`: Indicates if the tool only reads data (doesn't modify state)
+- `destructive_hint`: Indicates if the tool performs destructive operations. Defaults to true
+- `idempotent_hint`: Indicates if the tool's operations are idempotent. Defaults to false
+- `open_world_hint`: Indicates if the tool operates in an open world context. Defaults to true
+- `read_only_hint`: Indicates if the tool only reads data (doesn't modify state). Defaults to false
 - `title`: A human-readable title for the tool
 
 Annotations can be set either through the class definition using the `annotations` class method or when defining a tool using the `define` method.

lib/mcp/tool/annotations.rb

@@ -3,9 +3,9 @@
 module MCP
   class Tool
     class Annotations
-      attr_reader :title, :read_only_hint, :destructive_hint, :idempotent_hint, :open_world_hint
+      attr_reader :destructive_hint, :idempotent_hint, :open_world_hint, :read_only_hint, :title
 
-      def initialize(title: nil, read_only_hint: nil, destructive_hint: nil, idempotent_hint: nil, open_world_hint: nil)
+      def initialize(destructive_hint: true, idempotent_hint: false, open_world_hint: true, read_only_hint: false, title: nil)
         @title = title
         @read_only_hint = read_only_hint
         @destructive_hint = destructive_hint
@@ -15,11 +15,11 @@ def initialize(title: nil, read_only_hint: nil, destructive_hint: nil, idempoten
 
       def to_h
         {
-          title:,
-          readOnlyHint: read_only_hint,
           destructiveHint: destructive_hint,
           idempotentHint: idempotent_hint,
           openWorldHint: open_world_hint,
+          readOnlyHint: read_only_hint,
+          title:,
         }.compact
       end
     end

test/mcp/tool/annotations_test.rb

@@ -7,31 +7,31 @@ class Tool
     class AnnotationsTest < ActiveSupport::TestCase
       test "Tool::Annotations initializes with all properties" do
         annotations = Tool::Annotations.new(
-          title: "Test Tool",
-          read_only_hint: true,
           destructive_hint: false,
           idempotent_hint: true,
           open_world_hint: false,
+          read_only_hint: true,
+          title: "Test Tool",
         )
 
-        assert_equal "Test Tool", annotations.title
-        assert annotations.read_only_hint
         refute annotations.destructive_hint
         assert annotations.idempotent_hint
         refute annotations.open_world_hint
+        assert annotations.read_only_hint
+        assert_equal "Test Tool", annotations.title
       end
 
       test "Tool::Annotations initializes with partial properties" do
         annotations = Tool::Annotations.new(
-          title: "Test Tool",
           read_only_hint: true,
+          title: "Test Tool",
         )
 
-        assert_equal "Test Tool", annotations.title
+        assert annotations.destructive_hint
+        refute annotations.idempotent_hint
+        assert annotations.open_world_hint
         assert annotations.read_only_hint
-        assert_nil annotations.destructive_hint
-        assert_nil annotations.idempotent_hint
-        assert_nil annotations.open_world_hint
+        assert_equal "Test Tool", annotations.title
       end
 
       test "Tool::Annotations#to_h omits nil values" do
@@ -41,34 +41,37 @@ class AnnotationsTest < ActiveSupport::TestCase
         )
 
         expected = {
-          title: "Test Tool",
+          destructiveHint: true,
+          idempotentHint: false,
+          openWorldHint: true,
           readOnlyHint: true,
+          title: "Test Tool",
         }
         assert_equal expected, annotations.to_h
       end
 
       test "Tool::Annotations#to_h handles all properties" do
         annotations = Tool::Annotations.new(
-          title: "Test Tool",
-          read_only_hint: true,
           destructive_hint: false,
           idempotent_hint: true,
           open_world_hint: false,
+          read_only_hint: true,
+          title: "Test Tool",
         )
 
         expected = {
-          title: "Test Tool",
-          readOnlyHint: true,
           destructiveHint: false,
           idempotentHint: true,
           openWorldHint: false,
+          readOnlyHint: true,
+          title: "Test Tool",
         }
         assert_equal expected, annotations.to_h
       end
 
-      test "Tool::Annotations#to_h returns empty hash when all values are nil" do
+      test "Tool::Annotations#to_h returns hash with default hint values" do
         annotations = Tool::Annotations.new
-        assert_empty annotations.to_h
+        assert_equal({ destructiveHint: true, idempotentHint: false, openWorldHint: true, readOnlyHint: false }, annotations.to_h)
       end
     end
   end

test/mcp/tool_test.rb

@@ -10,11 +10,11 @@ class TestTool < Tool
       description "a test tool for testing"
       input_schema({ properties: { message: { type: "string" } }, required: ["message"] })
       annotations(
-        title: "Test Tool",
-        read_only_hint: true,
         destructive_hint: false,
         idempotent_hint: true,
         open_world_hint: false,
+        read_only_hint: true,
+        title: "Test Tool",
       )
 
       class << self
@@ -36,11 +36,11 @@ def call(message:, server_context: nil)
     test "#to_h includes annotations when present" do
       tool = TestTool
       expected_annotations = {
-        title: "Test Tool",
-        readOnlyHint: true,
         destructiveHint: false,
         idempotentHint: true,
         openWorldHint: false,
+        readOnlyHint: true,
+        title: "Test Tool",
       }
       assert_equal expected_annotations, tool.to_h[:annotations]
     end
@@ -142,15 +142,15 @@ class InputSchemaTool < Tool
       assert_equal "Mock Tool", tool.title
       assert_equal "a mock tool for testing", tool.description
       assert_equal tool.input_schema, Tool::InputSchema.new
-      assert_equal({ readOnlyHint: true, title: "Mock Tool" }, tool.annotations_value.to_h)
+      assert_equal({ destructiveHint: true, idempotentHint: false, openWorldHint: true, readOnlyHint: true, title: "Mock Tool" }, tool.annotations_value.to_h)
     end
 
     test "Tool class method annotations can be set and retrieved" do
       class AnnotationsTestTool < Tool
         tool_name "annotations_test"
         annotations(
-          title: "Annotations Test",
           read_only_hint: true,
+          title: "Annotations Test",
         )
       end