CS50 Duck (CS50.ai) — Harvard's Guardrailed AI Teaching Assistant for a Massive Introductory Programming Course

CS50, Harvard's introductory computing course (also delivered online as CS50x), launched the CS50 Duck in spring 2023 as part of CS50.ai. It is built on a commercial LLM API but wrapped in prompts and code checks intended to stop it giving outright solutions, plus a 'heart' system that limits questions per period.
The course team's SIGCSE 2024 paper (Liu, Zenke, Liu, Holmes, Thornton, Malan) reports student feedback in which roughly 75% used the tools frequently and 94% found them helpful; this is self-reported feedback, not a controlled learning-outcome study.
Harvard Gazette coverage records the team's own caveats: guardrails are not deterministic, the Duck can still emit code blocks in up to a quarter of responses, and some students asked around 200 questions. The course positions the tool as a net positive that eases demand on staff in a very large class, with explicit academic-honesty rules on permitted AI use.
Value for other institutions: a documented, openly described design for scaling tutor-like support, with honest reporting of failure modes; missing is causal evidence on learning gains.

Read the full analysis: https://cs50.harvard.edu/x/notes/ai/