Add "Draft" protocol version to supported protocol versions

For example, the `title` introduced in #109 is not supported in the latest stable protocol version "2025-06-18":
https://modelcontextprotocol.io/specification/2025-06-18

It is supported in the Draft of the next version:
https://modelcontextprotocol.io/specification/draft

In other words, the fact that the Ruby SDK treats "2025-06-18" as the default protocol version while `title` is
only supported in the Draft results in an inconsistency.

Because of this, a new version `"Draft"` has been added to the list of supported versions.
Its value follows the schema below and is defined as `DRAFT-2025-v3`:
https://github.com/modelcontextprotocol/modelcontextprotocol/blob/14ec41c/schema/draft/schema.ts#L15

It was also considered to continue using the latest stable version "2025-06-18" as the Ruby SDK default instead of the Draft.
However, in that case, users would need to explicitly specify `MCP::Server.new(protocol_version: 'DRAFT-2025-v3')`,
which adds extra burden. In this PR, Draft is made the default in order to provide the following benefits:

- Users are likely to prefer using the Draft version over the latest stable version
- No need to specify the `protocol_version` option when initializing `MCP::Server`
- Users generally do not need to know the internal development value `DRAFT-2025-v3`

The primary case for specifying `protocol_version` explicitly is when strict adherence to a particular version is required.
However, it seems more practical to prioritize use cases where backward-compatible features, including those from the Draft,
are desirable. This is the rationale behind making the Draft version the default.

In addition, making the Draft version the default protocol version preserves backward compatibility for `MCP::Server`.

NOTE: `DRAFT-2025-v3` is a development code, so its value cannot be set externally.
It is applied either when `protocol_version` is omitted or when `nil` is set explicitly,
such as with `MCP::Configuration.new(protocol_version: nil)`.
This prevents users from creating implementations that depend on development codes like `DRAFT-2025-v3`.

Author: koic

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",