Add "Draft" protocol version to supported protocol versions

README.md

@@ -343,6 +343,8 @@ configuration = MCP::Configuration.new(protocol_version: "2024-11-05")
 MCP::Server.new(name: "test_server", configuration: configuration)
 ```
 
+If no protocol version is specified, the [Draft version](https://modelcontextprotocol.io/specification/draft) will be applied by default.
+
 This will make all new server instances use the specified protocol version instead of the default version. The protocol version can be reset to the default by setting it to `nil`:
 
 ```ruby

lib/mcp/configuration.rb

@@ -2,8 +2,10 @@
 
 module MCP
   class Configuration
-    DEFAULT_PROTOCOL_VERSION = "2025-06-18"
-    SUPPORTED_PROTOCOL_VERSIONS = [DEFAULT_PROTOCOL_VERSION, "2025-03-26", "2024-11-05"]
+    # DRAFT-2025-v3 is the latest draft protocol version:
+    # https://github.com/modelcontextprotocol/modelcontextprotocol/blob/14ec41c/schema/draft/schema.ts#L15
+    DRAFT_PROTOCOL_VERSION = "DRAFT-2025-v3"
+    SUPPORTED_STABLE_PROTOCOL_VERSIONS = ["2025-06-18", "2025-03-26", "2024-11-05"]
 
     attr_writer :exception_reporter, :instrumentation_callback
 
@@ -33,7 +35,7 @@ def validate_tool_call_arguments=(validate_tool_call_arguments)
     end
 
     def protocol_version
-      @protocol_version || DEFAULT_PROTOCOL_VERSION
+      @protocol_version || DRAFT_PROTOCOL_VERSION
     end
 
     def protocol_version?
@@ -93,8 +95,8 @@ def merge(other)
     private
 
     def validate_protocol_version!(protocol_version)
-      unless SUPPORTED_PROTOCOL_VERSIONS.include?(protocol_version)
-        message = "protocol_version must be #{SUPPORTED_PROTOCOL_VERSIONS[0...-1].join(", ")}, or #{SUPPORTED_PROTOCOL_VERSIONS[-1]}"
+      unless SUPPORTED_STABLE_PROTOCOL_VERSIONS.include?(protocol_version)
+        message = "protocol_version must be #{SUPPORTED_STABLE_PROTOCOL_VERSIONS[0...-1].join(", ")}, or #{SUPPORTED_STABLE_PROTOCOL_VERSIONS[-1]}"
         raise ArgumentError, message
       end
     end

lib/mcp/server.rb

@@ -106,6 +106,8 @@ def define_tool(name: nil, title: nil, description: nil, input_schema: nil, anno
     def define_prompt(name: nil, title: nil, description: nil, arguments: [], &block)
       prompt = Prompt.define(name:, title:, description:, arguments:, &block)
       @prompts[prompt.name_value] = prompt
+
+      validate!
     end
 
     def define_custom_method(method_name:, &block)
@@ -171,6 +173,21 @@ def prompts_get_handler(&block)
     private
 
     def validate!
+      # NOTE: The draft protocol version is the next version after 2025-03-26.
+      if @configuration.protocol_version <= "2025-03-26"
+        if server_info.key?(:title)
+          message = "Error occurred in server_info. `title` is not supported in protocol version 2025-03-26 or earlier"
+          raise ArgumentError, message
+        end
+
+        primitive_titles = [@tools.values, @prompts.values, @resources, @resource_templates].flatten.map(&:title)
+
+        if primitive_titles.any?
+          message = "Error occurred in #{primitive_titles.join(", ")}. `title` is not supported in protocol version 2025-03-26 or earlier"
+          raise ArgumentError, message
+        end
+      end
+
       if @configuration.protocol_version == "2024-11-05"
         if @instructions
           message = "`instructions` supported by protocol version 2025-03-26 or higher"

test/mcp/configuration_test.rb

@@ -35,9 +35,24 @@ class ConfigurationTest < ActiveSupport::TestCase
       assert_equal test_context, reported_context
     end
 
+    # https://github.com/modelcontextprotocol/modelcontextprotocol/blob/14ec41c/schema/draft/schema.ts#L15
     test "initializes with default protocol version" do
       config = Configuration.new
-      assert_equal Configuration::DEFAULT_PROTOCOL_VERSION, config.protocol_version
+      assert_equal Configuration::DRAFT_PROTOCOL_VERSION, config.protocol_version
+    end
+
+    test "uses the draft protocol version when protocol_version is set to nil" do
+      config = Configuration.new(protocol_version: nil)
+      assert_equal Configuration::DRAFT_PROTOCOL_VERSION, config.protocol_version
+    end
+
+    test "raises ArgumentError when setting the draft protocol version" do
+      exception = assert_raises(ArgumentError) do
+        # To use the draft version externally, either omit `protocol_version` or set it to nil.
+        Configuration.new(protocol_version: Configuration::DRAFT_PROTOCOL_VERSION)
+      end
+
+      assert_equal("protocol_version must be 2025-06-18, 2025-03-26, or 2024-11-05", exception.message)
     end
 
     test "raises ArgumentError when protocol_version is not a supported protocol version" do

test/mcp/server/transports/streamable_http_transport_test.rb

@@ -81,7 +81,7 @@ class StreamableHTTPTransportTest < ActiveSupport::TestCase
           body = JSON.parse(response[2][0])
           assert_equal "2.0", body["jsonrpc"]
           assert_equal "123", body["id"]
-          assert_equal "2025-06-18", body["result"]["protocolVersion"]
+          assert_equal Configuration::DRAFT_PROTOCOL_VERSION, body["result"]["protocolVersion"]
         end
 
         test "handles GET request with valid session ID" do

test/mcp/server_test.rb

@@ -133,7 +133,7 @@ class ServerTest < ActiveSupport::TestCase
         jsonrpc: "2.0",
         id: 1,
         result: {
-          protocolVersion: "2025-06-18",
+          protocolVersion: Configuration::DRAFT_PROTOCOL_VERSION,
           capabilities: {
             prompts: { listChanged: true },
             resources: { listChanged: true },
@@ -776,7 +776,7 @@ def call(message:, server_context: nil)
       }
 
       response = @server.handle(request)
-      assert_equal Configuration::DEFAULT_PROTOCOL_VERSION, response[:result][:protocolVersion]
+      assert_equal Configuration::DRAFT_PROTOCOL_VERSION, response[:result][:protocolVersion]
     end
 
     test "server response does not include title when not configured" do
@@ -852,6 +852,67 @@ def call(message:, server_context: nil)
       assert_equal("Error occurred in defined_tool. `annotations` are supported by protocol version 2025-03-26 or higher", exception.message)
     end
 
+    test "raises error if `title` of `server_info` is used with protocol version 2025-03-26" do
+      configuration = Configuration.new(protocol_version: "2025-03-26")
+
+      exception = assert_raises(ArgumentError) do
+        Server.new(name: "test_server", title: "Example Server Display Name", configuration: configuration)
+      end
+      assert_equal("Error occurred in server_info. `title` is not supported in protocol version 2025-03-26 or earlier", exception.message)
+    end
+
+    test "raises error if `title` of tool is used with protocol version 2025-03-26" do
+      configuration = Configuration.new(protocol_version: "2025-03-26")
+      server = Server.new(name: "test_server", configuration: configuration)
+
+      exception = assert_raises(ArgumentError) do
+        server.define_tool(
+          title: "Test tool",
+        )
+      end
+      assert_equal("Error occurred in Test tool. `title` is not supported in protocol version 2025-03-26 or earlier", exception.message)
+    end
+
+    test "raises error if `title` of prompt is used with protocol version 2025-03-26" do
+      configuration = Configuration.new(protocol_version: "2025-03-26")
+      server = Server.new(name: "test_server", configuration: configuration)
+
+      exception = assert_raises(ArgumentError) do
+        server.define_prompt(
+          title: "Test prompt",
+        )
+      end
+      assert_equal("Error occurred in Test prompt. `title` is not supported in protocol version 2025-03-26 or earlier", exception.message)
+    end
+
+    test "raises error if `title` of resource is used with protocol version 2025-03-26" do
+      configuration = Configuration.new(protocol_version: "2025-03-26")
+
+      resource = Resource.new(
+        uri: "https://test_resource.invalid",
+        name: "test-resource",
+        title: "Test resource",
+      )
+      exception = assert_raises(ArgumentError) do
+        Server.new(name: "test_server", resources: [resource], configuration: configuration)
+      end
+      assert_equal("Error occurred in Test resource. `title` is not supported in protocol version 2025-03-26 or earlier", exception.message)
+    end
+
+    test "raises error if `title` of resource template is used with protocol version 2025-03-26" do
+      configuration = Configuration.new(protocol_version: "2025-03-26")
+
+      resource = Resource.new(
+        uri: "https://test_resource.invalid",
+        name: "test-resource",
+        title: "Test resource template",
+      )
+      exception = assert_raises(ArgumentError) do
+        Server.new(name: "test_server", resources: [resource], configuration: configuration)
+      end
+      assert_equal("Error occurred in Test resource template. `title` is not supported in protocol version 2025-03-26 or earlier", exception.message)
+    end
+
     test "#define_tool adds a tool to the server" do
       @server.define_tool(
         name: "defined_tool",