Fix: Disallow $ref in tool input schemas

test/mcp/tool/input_schema_test.rb

@@ -61,6 +61,18 @@ module MCP
           end
         end
       end
+
+      test "rejects schemas with $ref references" do
+        assert_raises(ArgumentError) do
+          InputSchema.new(properties: { foo: { "$ref" => "#/definitions/bar" } }, required: [:foo])
+        end
+      end
+
+      test "rejects schemas with symbol $ref references" do
+        assert_raises(ArgumentError) do
+          InputSchema.new(properties: { foo: { :$ref => "#/definitions/bar" } }, required: [:foo])
+        end
+      end
     end
   end
 end

test/mcp/tool_test.rb

@@ -249,7 +249,7 @@ module MCP
         properties: {
           foo: { "$ref" => "#/definitions/bar" },
         },
-        required: [],
+        required: ["foo"],
         definitions: {
           bar: { type: "string" },
         },