Validate ToolCall Args against Schema

(cherry picked from commit eeb287117535dfb43785704171625323f01d5bda)

lib/mcp/tool/input_schema.rb

@@ -1,13 +1,18 @@
 # frozen_string_literal: true
 
+require "json-schema"
+
 module MCP
   class Tool
     class InputSchema
+      class ValidationError < StandardError; end
+
       attr_reader :properties, :required
 
       def initialize(properties: {}, required: [])
         @properties = properties
         @required = required.map(&:to_sym)
+        validate_schema!
       end
 
       def to_h
@@ -21,6 +26,32 @@ module MCP
       def missing_required_arguments(arguments)
         (required - arguments.keys.map(&:to_sym))
       end
+
+      def validate_arguments(arguments)
+        errors = JSON::Validator.fully_validate(to_h, arguments)
+        if errors.any?
+          raise ValidationError, "Invalid arguments: #{errors.join(", ")}"
+        end
+      end
+
+      private
+
+      def validate_schema!
+        schema = to_h
+        begin
+          schema_reader = JSON::Schema::Reader.new(
+            accept_uri: false,
+            accept_file: ->(path) { path.to_s.start_with?(Gem.loaded_specs["json-schema"].full_gem_path) },
+          )
+          metaschema = JSON::Validator.validator_for_name("draft4").metaschema
+          errors = JSON::Validator.fully_validate(metaschema, schema, schema_reader: schema_reader)
+          if errors.any?
+            raise ArgumentError, "Invalid JSON Schema: #{errors.join(", ")}"
+          end
+        rescue StandardError => e
+          raise ArgumentError, "Invalid JSON Schema: #{e.message}"
+        end
+      end
     end
   end
 end