A large amount of our curriculum and programming tools at Code.org have been designed around Blockly, which has allowed us to create customized programming environments that are accessible to young students. Creating Blockly-based environments, however, has always been a slow process, requiring a great deal of engineering time. To reduce the cost of iteration and allow our curriculum team greater flexibility in exploring new programming contexts, Code.org’s engineering team developed a web-based toolkit for developing new blockly libraries allowing much more rapid prototyping of new block-based programming environments.