diff --git a/lib/mcp/annotations.rb b/lib/mcp/annotations.rb
index 88bead5..50eb0eb 100644
--- a/lib/mcp/annotations.rb
+++ b/lib/mcp/annotations.rb
@@ -1,18 +1,13 @@
 # frozen_string_literal: true
 
 module MCP
-  class Annotations
-    attr_reader :audience, :priority
-
+  Annotations = Data.define(:audience, :priority) do
     def initialize(audience: nil, priority: nil)
-      @audience = audience
-      @priority = priority
-
-      freeze
+      super(audience:, priority:)
     end
 
     def to_h
-      { audience:, priority: }.compact.freeze
+      super.compact.freeze
     end
   end
 end
diff --git a/lib/mcp/content.rb b/lib/mcp/content.rb
index 65fc11f..e9cf283 100644
--- a/lib/mcp/content.rb
+++ b/lib/mcp/content.rb
@@ -3,30 +3,24 @@
 
 module MCP
   module Content
-    class Text
-      attr_reader :text, :annotations
-
+    Text = Data.define(:text, :annotations) do
       def initialize(text, annotations: nil)
-        @text = text
-        @annotations = annotations
+        super(text:, annotations:)
       end
 
       def to_h
-        { text:, annotations:, type: "text" }.compact
+        { **super, type: "text" }.compact.freeze
       end
     end
 
-    class Image
-      attr_reader :data, :mime_type, :annotations
-
+    Image = Data.define(:data, :mime_type, :annotations) do
       def initialize(data, mime_type, annotations: nil)
-        @data = data
-        @mime_type = mime_type
-        @annotations = annotations
+        super(data:, mime_type:, annotations:)
       end
 
+      # FIXME: mime_type should probably turn into mimeType
       def to_h
-        { data:, mime_type:, annotations:, type: "image" }.compact
+        { **super, type: "image" }.compact.freeze
       end
     end
   end
diff --git a/lib/mcp/prompt.rb b/lib/mcp/prompt.rb
index 1a260f7..1a6ce6a 100644
--- a/lib/mcp/prompt.rb
+++ b/lib/mcp/prompt.rb
@@ -2,8 +2,8 @@
 # frozen_string_literal: true
 
 module MCP
-  class Prompt
-    attr_reader :name, :description, :arguments, :to_h
+  Prompt = Data.define(:name, :description, :arguments) do
+    attr_reader :to_h
 
     class << self
       def define(...) = new(...)
@@ -20,18 +20,12 @@ def inherited(subclass)
     def initialize(name:, description:, arguments:, &block)
       arguments = arguments.map { |arg| Hash === arg ? Argument.new(**arg) : arg }
 
-      @name = name
-      @description = description
-      @arguments = arguments
       @block = block
-
       @to_h = { name:, description:, arguments: arguments.map(&:to_h) }.compact.freeze
 
-      freeze
+      super(name:, description:, arguments:)
     end
 
-    def call(args, server_context:)
-      @block.call(args, server_context:)
-    end
+    def call(args, server_context:) = @block.call(args, server_context:)
   end
 end
diff --git a/lib/mcp/prompt/argument.rb b/lib/mcp/prompt/argument.rb
index 307099a..57ec050 100644
--- a/lib/mcp/prompt/argument.rb
+++ b/lib/mcp/prompt/argument.rb
@@ -3,21 +3,17 @@
 
 module MCP
   class Prompt
-    class Argument
-      attr_reader :name, :description, :required, :to_h
+    Argument = Data.define(:name, :description, :required) do
+      attr_reader :to_h
 
       def initialize(name:, description: nil, required: false)
-        @name = name
-        @description = description
-        @required = required
-
         @to_h = {
           name:,
           description:,
           required:,
         }.compact.freeze
 
-        freeze
+        super(name:, description:, required:)
       end
     end
   end
diff --git a/lib/mcp/prompt/message.rb b/lib/mcp/prompt/message.rb
index b5c94f9..f0d2ebc 100644
--- a/lib/mcp/prompt/message.rb
+++ b/lib/mcp/prompt/message.rb
@@ -3,16 +3,13 @@
 
 module MCP
   class Prompt
-    class Message
-      attr_reader :role, :content
-
+    Message = Data.define(:role, :content) do
       def initialize(role:, content:)
-        @role = role
-        @content = content
+        super(role:, content:)
       end
 
       def to_h
-        { role:, content: content.to_h }.compact
+        { role:, content: content.to_h }.compact.freeze
       end
     end
   end
diff --git a/lib/mcp/prompt/result.rb b/lib/mcp/prompt/result.rb
index 693885f..8007558 100644
--- a/lib/mcp/prompt/result.rb
+++ b/lib/mcp/prompt/result.rb
@@ -3,16 +3,13 @@
 
 module MCP
   class Prompt
-    class Result
-      attr_reader :description, :messages
-
+    Result = Data.define(:description, :messages) do
       def initialize(description: nil, messages: [])
-        @description = description
-        @messages = messages
+        super(description:, messages:)
       end
 
       def to_h
-        { description:, messages: messages.map(&:to_h) }.compact
+        { description:, messages: messages.map(&:to_h) }.compact.freeze
       end
     end
   end
diff --git a/lib/mcp/resource.rb b/lib/mcp/resource.rb
index 95c7db8..143d1e7 100644
--- a/lib/mcp/resource.rb
+++ b/lib/mcp/resource.rb
@@ -2,23 +2,13 @@
 # frozen_string_literal: true
 
 module MCP
-  class Resource
-    attr_reader :uri, :name, :description, :mime_type
-
+  Resource = Data.define(:uri, :name, :description, :mime_type) do
     def initialize(uri:, name:, description: nil, mime_type: nil)
-      @uri = uri
-      @name = name
-      @description = description
-      @mime_type = mime_type
+      super(uri:, name:, description:, mime_type:)
     end
 
     def to_h
-      {
-        uri: @uri,
-        name: @name,
-        description: @description,
-        mimeType: @mime_type,
-      }.compact
+      { uri:, name:, description:, mimeType: mime_type }.compact.freeze
     end
   end
 end
diff --git a/lib/mcp/resource/contents.rb b/lib/mcp/resource/contents.rb
index f49d18f..3adf280 100644
--- a/lib/mcp/resource/contents.rb
+++ b/lib/mcp/resource/contents.rb
@@ -3,42 +3,36 @@
 
 module MCP
   class Resource
-    class Contents
-      attr_reader :uri, :mime_type
+    # FIXME: Should mime_type to_h as mimeType?
+    # FIXME: Is it a problem that we lose inheritance here?
 
+    Contents = Data.define(:uri, :mime_type) do
       def initialize(uri:, mime_type: nil)
-        @uri = uri
-        @mime_type = mime_type
+        super(uri:, mime_type:)
       end
 
       def to_h
-        { uri:, mime_type: }.compact
+        { uri:, mime_type: }.compact.freeze
       end
     end
 
-    class TextContents < Contents
-      attr_reader :text
-
+    TextContents = Data.define(:text, :uri, :mime_type) do
       def initialize(text:, uri:, mime_type:)
-        super(uri:, mime_type:)
-        @text = text
+        super(text:, uri:, mime_type:)
       end
 
       def to_h
-        super.merge(text:)
+        { text:, uri:, mime_type: }.compact.freeze
       end
     end
 
-    class BlobContents < Contents
-      attr_reader :data
-
+    BlobContents = Data.define(:data, :uri, :mime_type) do
       def initialize(data:, uri:, mime_type:)
-        super(uri:, mime_type:)
-        @data = data
+        super(data:, uri:, mime_type:)
       end
 
       def to_h
-        super.merge(data:)
+        { data:, uri:, mime_type: }.compact.freeze
       end
     end
   end
diff --git a/lib/mcp/resource/embedded.rb b/lib/mcp/resource/embedded.rb
index 66a76d8..94a838b 100644
--- a/lib/mcp/resource/embedded.rb
+++ b/lib/mcp/resource/embedded.rb
@@ -3,16 +3,13 @@
 
 module MCP
   class Resource
-    class Embedded
-      attr_reader :resource, :annotations
-
+    Embedded = Data.define(:resource, :annotations) do
       def initialize(resource:, annotations: nil)
-        @resource = resource
-        @annotations = annotations
+        super(resource:, annotations:)
       end
 
       def to_h
-        { resource: resource.to_h, annotations: }.compact
+        { resource: resource.to_h, annotations: }.compact.freeze
       end
     end
   end
diff --git a/lib/mcp/resource_template.rb b/lib/mcp/resource_template.rb
index ab6b46b..8186335 100644
--- a/lib/mcp/resource_template.rb
+++ b/lib/mcp/resource_template.rb
@@ -2,15 +2,10 @@
 # frozen_string_literal: true
 
 module MCP
-  class ResourceTemplate
-    attr_reader :uri_template, :name, :description, :mime_type, :to_h
+  ResourceTemplate = Data.define(:uri_template, :name, :description, :mime_type) do
+    attr_reader :to_h
 
     def initialize(uri_template:, name:, description: nil, mime_type: nil)
-      @uri_template = uri_template
-      @name = name
-      @description = description
-      @mime_type = mime_type
-
       @to_h = {
         uriTemplate: @uri_template,
         name: @name,
@@ -18,7 +13,7 @@ def initialize(uri_template:, name:, description: nil, mime_type: nil)
         mimeType: @mime_type,
       }.compact.freeze
 
-      freeze
+      super(uri_template:, name:, description:, mime_type:)
     end
   end
 end
diff --git a/lib/mcp/tool.rb b/lib/mcp/tool.rb
index 334e704..12f3e79 100644
--- a/lib/mcp/tool.rb
+++ b/lib/mcp/tool.rb
@@ -1,8 +1,8 @@
 # frozen_string_literal: true
 
 module MCP
-  class Tool
-    attr_reader :name, :description, :input_schema, :annotations, :to_h
+  Tool = Data.define(:name, :description, :input_schema, :annotations) do
+    attr_reader :to_h
 
     class << self
       def define(...) = new(...)
@@ -17,16 +17,11 @@ def inherited(subclass)
     end
 
     def initialize(name:, description: nil, input_schema: nil, annotations: nil, &block)
-      input_schema = InputSchema.new(**input_schema) if Hash === input_schema
-      annotations  = Annotations.new(**annotations)  if Hash === annotations
+      input_schema = Tool::InputSchema.new(**input_schema) if Hash === input_schema
+      annotations  = Tool::Annotations.new(**annotations)  if Hash === annotations
       raise ArgumentError, "Tool definition requires a block" unless block
 
-      @name         = name
-      @description  = description
-      @input_schema = input_schema
-      @annotations  = annotations
-      @block        = block
-
+      @block = block
       @to_h = {
         name:,
         description:,
@@ -34,7 +29,7 @@ def initialize(name:, description: nil, input_schema: nil, annotations: nil, &bl
         annotations: annotations&.to_h,
       }.compact.freeze
 
-      freeze
+      super(name:, description:, input_schema:, annotations:)
     end
 
     def call(args, server_context:)
diff --git a/lib/mcp/tool/annotations.rb b/lib/mcp/tool/annotations.rb
index dfdafa1..cecbf07 100644
--- a/lib/mcp/tool/annotations.rb
+++ b/lib/mcp/tool/annotations.rb
@@ -2,16 +2,10 @@
 
 module MCP
   class Tool
-    class Annotations
-      attr_reader :title, :read_only_hint, :destructive_hint, :idempotent_hint, :open_world_hint, :to_h
+    Annotations = Data.define(:title, :read_only_hint, :destructive_hint, :idempotent_hint, :open_world_hint) do
+      attr_reader :to_h
 
       def initialize(title: nil, read_only_hint: nil, destructive_hint: nil, idempotent_hint: nil, open_world_hint: nil)
-        @title = title
-        @read_only_hint = read_only_hint
-        @destructive_hint = destructive_hint
-        @idempotent_hint = idempotent_hint
-        @open_world_hint = open_world_hint
-
         @to_h = {
           title:,
           readOnlyHint: read_only_hint,
@@ -20,7 +14,7 @@ def initialize(title: nil, read_only_hint: nil, destructive_hint: nil, idempoten
           openWorldHint: open_world_hint,
         }.compact.freeze
 
-        freeze
+        super(title:, read_only_hint:, destructive_hint:, idempotent_hint:, open_world_hint:)
       end
     end
   end
diff --git a/lib/mcp/tool/input_schema.rb b/lib/mcp/tool/input_schema.rb
index dca3a43..dbf225e 100644
--- a/lib/mcp/tool/input_schema.rb
+++ b/lib/mcp/tool/input_schema.rb
@@ -2,12 +2,12 @@
 
 module MCP
   class Tool
-    class InputSchema
-      attr_reader :properties, :required, :to_h
+    InputSchema = Data.define(:properties, :required) do
+      attr_reader :to_h
 
       def initialize(properties: {}, required: [])
-        @properties = properties.transform_keys(&:to_sym)
-        @required = required.map(&:to_sym)
+        properties = properties.transform_keys(&:to_sym)
+        required = required.map(&:to_sym)
 
         @to_h = {
           type: "object",
@@ -15,7 +15,7 @@ def initialize(properties: {}, required: [])
           required:,
         }.compact.freeze
 
-        freeze
+        super(properties:, required:)
       end
     end
   end
diff --git a/lib/mcp/tool/response.rb b/lib/mcp/tool/response.rb
index 690f360..3ae9c84 100644
--- a/lib/mcp/tool/response.rb
+++ b/lib/mcp/tool/response.rb
@@ -2,16 +2,13 @@
 
 module MCP
   class Tool
-    class Response
-      attr_reader :content, :is_error
-
+    Response = Data.define(:content, :is_error) do
       def initialize(content, is_error: false)
-        @content = content
-        @is_error = is_error
+        super(content:, is_error:)
       end
 
       def to_h
-        { content:, isError: is_error }.compact
+        { content:, isError: is_error }.compact.freeze
       end
     end
   end