diff --git a/lib/mcp.rb b/lib/mcp.rb
index 84ccce4..6e8f144 100644
--- a/lib/mcp.rb
+++ b/lib/mcp.rb
@@ -19,6 +19,7 @@
 require_relative "mcp/tool"
 require_relative "mcp/tool/input_schema"
 require_relative "mcp/tool/response"
+require_relative "mcp/tool/error_response"
 require_relative "mcp/tool/annotations"
 require_relative "mcp/transport"
 require_relative "mcp/version"
diff --git a/lib/mcp/tool/error_response.rb b/lib/mcp/tool/error_response.rb
new file mode 100644
index 0000000..fbf5022
--- /dev/null
+++ b/lib/mcp/tool/error_response.rb
@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+module MCP
+  class Tool
+    class ErrorResponse
+      def initialize(content) = super(content, error: true)
+    end
+  end
+end
diff --git a/lib/mcp/tool/response.rb b/lib/mcp/tool/response.rb
index abf6ff4..32e6e82 100644
--- a/lib/mcp/tool/response.rb
+++ b/lib/mcp/tool/response.rb
@@ -3,16 +3,18 @@
 module MCP
   class Tool
     class Response
-      attr_reader :content, :is_error
+      attr_reader :content, :error
 
-      def initialize(content, is_error = false)
+      def initialize(content, error: false)
         @content = content
-        @is_error = is_error
+        @error = error
       end
 
       def to_h
-        { content:, isError: is_error }.compact
+        { content:, isError: error? }.compact
       end
+
+      alias_method :error?, :error
     end
   end
 end
diff --git a/test/mcp/tool/error_response.rb b/test/mcp/tool/error_response.rb
new file mode 100644
index 0000000..33181e3
--- /dev/null
+++ b/test/mcp/tool/error_response.rb
@@ -0,0 +1,38 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+module MCP
+  class Tool
+    class ErrorResponseTest < ActiveSupport::TestCase
+      test "#initialize with content" do
+        content = [{
+          type: "text",
+          text: "Unauthorized",
+        }]
+        response = ErrorResponse.new(content)
+
+        assert_equal content, response.content
+        assert response.error?
+      end
+
+      test "#to_h" do
+        content = [{
+          type: "text",
+          text: "Unauthorized",
+        }]
+        response = ErrorResponse.new(content)
+        actual = response.to_h
+
+        assert_equal [:content, :isError].sort, actual.keys.sort
+        assert_equal content, actual[:content]
+        assert actual[:isError]
+      end
+
+      test "#error?" do
+        response = ErrorResponse.new(nil)
+        assert response.error?
+      end
+    end
+  end
+end
diff --git a/test/mcp/tool/response_test.rb b/test/mcp/tool/response_test.rb
new file mode 100644
index 0000000..e68f93b
--- /dev/null
+++ b/test/mcp/tool/response_test.rb
@@ -0,0 +1,55 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+module MCP
+  class Tool
+    class ResponseTest < ActiveSupport::TestCase
+      test "#initialize with content and error flag" do
+        content = [{
+          type: "text",
+          text: "Unauthorized",
+        }]
+        response = Response.new(content, error: true)
+
+        assert_equal content, response.content
+        assert response.error?
+
+        response = Response.new(content, error: false)
+        assert_equal content, response.content
+        refute response.error?
+
+        response = Response.new(content)
+        assert_equal content, response.content
+        refute response.error?
+      end
+
+      test "#to_h" do
+        content = [{
+          type: "text",
+          text: "Unauthorized",
+        }]
+        response = Response.new(content)
+        actual = response.to_h
+
+        assert_equal [:content, :isError].sort, actual.keys.sort
+        assert_equal content, actual[:content]
+        refute actual[:isError]
+
+        response = Response.new(content, error: true)
+        actual = response.to_h
+        assert_equal [:content, :isError].sort, actual.keys.sort
+        assert_equal content, actual[:content]
+        assert actual[:isError]
+      end
+
+      test "#error?" do
+        response = Response.new(nil, error: true)
+        assert response.error?
+
+        response = Response.new(nil, error: false)
+        refute response.error?
+      end
+    end
+  end
+end
diff --git a/test/mcp/tool_test.rb b/test/mcp/tool_test.rb
index abd3fcb..76c9544 100644
--- a/test/mcp/tool_test.rb
+++ b/test/mcp/tool_test.rb
@@ -45,7 +45,7 @@ def call(message:, server_context: nil)
       tool = TestTool
       response = tool.call(message: "test")
       assert_equal response.content, [{ type: "text", content: "OK" }]
-      assert_equal response.is_error, false
+      refute response.error?
     end
 
     test "allows declarative definition of tools as classes" do
@@ -250,7 +250,7 @@ def call(message:, server_context: nil)
       tool = TypedTestTool
       response = tool.call(message: "test")
       assert_equal response.content, [{ type: "text", content: "OK" }]
-      assert_equal response.is_error, false
+      refute response.error?
     end
 
     class TestToolWithoutServerContext < Tool