Default to the latest stable protocol version

When starting an MCP server in Claude Code with `protocol_version` set to `DRAFT-2025-v3`,
it fails with the following error:

```console
2025-12-04T02:58:01.548Z [DEBUG] MCP server "example": Connection failed after 2093ms: Server's protocol version is not supported: DRAFT-2025-v3
2025-12-04T02:58:01.548Z [ERROR] MCP server "example" Connection failed: Server's protocol version is not supported: DRAFT-2025-v3
```

This PR changes the default behavior so that if `protocol_version` is not specified,
it uses the latest stable protocol version instead of the draft version.

This PR also updates the README to note that the latest stable protocol version includes
new features from the draft version.

Author: koic

README.md

@@ -349,7 +349,8 @@ 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.
+If no protocol version is specified, the latest stable version will be applied by default.
+The latest stable version includes new features from the [draft version](https://modelcontextprotocol.io/specification/draft).
 
 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`:
 

lib/mcp/configuration.rb

@@ -2,10 +2,8 @@
 
 module MCP
   class Configuration
-    # 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"]
+    LATEST_STABLE_PROTOCOL_VERSION = "2025-06-18"
+    SUPPORTED_STABLE_PROTOCOL_VERSIONS = [LATEST_STABLE_PROTOCOL_VERSION, "2025-03-26", "2024-11-05"]
 
     attr_writer :exception_reporter, :instrumentation_callback
 
@@ -35,7 +33,7 @@ def validate_tool_call_arguments=(validate_tool_call_arguments)
     end
 
     def protocol_version
-      @protocol_version || DRAFT_PROTOCOL_VERSION
+      @protocol_version || LATEST_STABLE_PROTOCOL_VERSION
     end
 
     def protocol_version?

test/mcp/configuration_test.rb

@@ -38,18 +38,19 @@ class ConfigurationTest < ActiveSupport::TestCase
     # 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::DRAFT_PROTOCOL_VERSION, config.protocol_version
+      assert_equal Configuration::LATEST_STABLE_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
+      assert_equal Configuration::LATEST_STABLE_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)
+        # DRAFT-2025-v3 is the latest draft protocol version:
+        # https://github.com/modelcontextprotocol/modelcontextprotocol/blob/14ec41c/schema/draft/schema.ts#L15
+        Configuration.new(protocol_version: "DRAFT-2025-v3")
       end
 
       assert_equal("protocol_version must be 2025-06-18, 2025-03-26, or 2024-11-05", exception.message)

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 Configuration::DRAFT_PROTOCOL_VERSION, body["result"]["protocolVersion"]
+          assert_equal Configuration::LATEST_STABLE_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: Configuration::DRAFT_PROTOCOL_VERSION,
+          protocolVersion: Configuration::LATEST_STABLE_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::DRAFT_PROTOCOL_VERSION, response[:result][:protocolVersion]
+      assert_equal Configuration::LATEST_STABLE_PROTOCOL_VERSION, response[:result][:protocolVersion]
     end
 
     test "server response does not include title when not configured" do