Block-based environments such as Scratch are increasingly popular in programming education. While block syntax reduces surface errors, semantic bugs remain common and challenging for novices to resolve. Existing debugging workflows typically show the correct program directly to learners, a strategy that may fix errors but undermines the development of problem-solving skills. We present Stitch, an interactive tutoring system that replaces "showing the answer" with step-by-step scaffolding. The system's Diff-Analyze module contrasts a student's project with a reference implementation, identifies the most critical differences, and uses a large language model to explain why these changes matter. Learners inspect highlighted blocks through a custom rendering engine, understand the explanations, and selectively apply partial fixes. This iterative process continues until the intended functionality is achieved. We evaluate Stitch in an empirical study, comparing it against a state-of-the-art automated feedback generation tool for Scratch. Our key insight is that simply presenting the correct program is pedagogically ineffective. In contrast, our interactive, step-by-step guided system promotes a more effective learning experience. More broadly, what constitutes effective feedback in block-based programming remains an open question. Our evaluation provides new evidence that step-by-step tutoring significantly enhances learning outcomes, outperforming both direct-answer approaches and current automated feedback generation tools.

翻译：Scratch等积木式编程环境在编程教育中日益普及。虽然积木语法减少了表面错误，但语义错误仍然常见且对初学者具有挑战性。现有的调试流程通常直接向学习者展示正确程序，这种策略虽能修正错误，却削弱了问题解决能力的培养。本文提出Stitch——一个用分步式脚手架替代“直接展示答案”的交互式教学系统。系统的Diff-Analyze模块通过对比学生项目与参考实现，识别关键差异，并利用大型语言模型解释这些修改的重要性。学习者通过定制渲染引擎查看高亮标注的积木块，理解解释说明，并有选择地应用局部修正。这一迭代过程持续进行直至实现预期功能。我们通过实证研究评估Stitch，并与当前最先进的Scratch自动反馈生成工具进行对比。核心发现表明：直接呈现正确程序的教学效果有限；相比之下，我们这种交互式分步引导系统能显著提升学习体验。更广泛而言，积木式编程中有效反馈的构成标准仍是开放问题。本研究提供的新证据表明：分步式教学显著提升学习成效，其效果优于直接答案呈现方式和现有自动反馈生成工具。