Add validation to MCP::Configuration setters

I noticed through the existing tests that `MCP::Configuration#protocol_version=`
allows setting an arbitrary protocol version.

Since the official MCP Ruby SDK is an implementation of the MCP specification,
it should not support arbitrary protocol versions. This change adds validation
to the setter, consistent with `Configuration#initialize`.

The same validation has also been added to `MCP::Configuration#validate_tool_call_arguments=`.

Author: koic

lib/mcp/configuration.rb

@@ -5,20 +5,29 @@ class Configuration
     DEFAULT_PROTOCOL_VERSION = "2025-06-18"
     SUPPORTED_PROTOCOL_VERSIONS = [DEFAULT_PROTOCOL_VERSION, "2025-03-26", "2024-11-05"]
 
-    attr_writer :exception_reporter, :instrumentation_callback, :protocol_version, :validate_tool_call_arguments
+    attr_writer :exception_reporter, :instrumentation_callback
 
     def initialize(exception_reporter: nil, instrumentation_callback: nil, protocol_version: nil,
       validate_tool_call_arguments: true)
       @exception_reporter = exception_reporter
       @instrumentation_callback = instrumentation_callback
       @protocol_version = protocol_version
-      if protocol_version && !SUPPORTED_PROTOCOL_VERSIONS.include?(protocol_version)
-        message = "protocol_version must be #{SUPPORTED_PROTOCOL_VERSIONS[0...-1].join(", ")}, or #{SUPPORTED_PROTOCOL_VERSIONS[-1]}"
-        raise ArgumentError, message
-      end
-      unless validate_tool_call_arguments.is_a?(TrueClass) || validate_tool_call_arguments.is_a?(FalseClass)
-        raise ArgumentError, "validate_tool_call_arguments must be a boolean"
+      if protocol_version
+        validate_protocol_version!(protocol_version)
       end
+      validate_value_of_validate_tool_call_arguments!(validate_tool_call_arguments)
+
+      @validate_tool_call_arguments = validate_tool_call_arguments
+    end
+
+    def protocol_version=(protocol_version)
+      validate_protocol_version!(protocol_version)
+
+      @protocol_version = protocol_version
+    end
+
+    def validate_tool_call_arguments=(validate_tool_call_arguments)
+      validate_value_of_validate_tool_call_arguments!(validate_tool_call_arguments)
 
       @validate_tool_call_arguments = validate_tool_call_arguments
     end
@@ -83,6 +92,19 @@ 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]}"
+        raise ArgumentError, message
+      end
+    end
+
+    def validate_value_of_validate_tool_call_arguments!(validate_tool_call_arguments)
+      unless validate_tool_call_arguments.is_a?(TrueClass) || validate_tool_call_arguments.is_a?(FalseClass)
+        raise ArgumentError, "validate_tool_call_arguments must be a boolean"
+      end
+    end
+
     def default_exception_reporter
       @default_exception_reporter ||= ->(exception, server_context) {}
     end

test/mcp/configuration_test.rb

@@ -40,11 +40,21 @@ class ConfigurationTest < ActiveSupport::TestCase
       assert_equal Configuration::DEFAULT_PROTOCOL_VERSION, config.protocol_version
     end
 
-    test "allows setting a custom protocol version" do
+    test "raises ArgumentError when protocol_version is not a supported protocol version" do
       config = Configuration.new
-      custom_version = "2025-03-27"
-      config.protocol_version = custom_version
-      assert_equal custom_version, config.protocol_version
+      exception = assert_raises(ArgumentError) do
+        custom_version = "2025-03-27"
+        config.protocol_version = custom_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 boolean value" do
+      config = Configuration.new
+      exception = assert_raises(ArgumentError) do
+        config.validate_tool_call_arguments = "true"
+      end
+      assert_equal("validate_tool_call_arguments must be a boolean", exception.message)
     end
 
     test "merges protocol version from other configuration" do