Free public beta until December 4, 2026: every course is open, no card needed. After that, the first chapter of each course stays free and full access is a one-time $39 ($19 for up to 100 founding members chosen for helpful feedback). Your progress is kept. How the beta works

← Challenges easy

bug hunt loops

Playing as a guest — your score won't be saved. Log in to keep it.

Exactly one line of this function has a bug, and changing that one line fixes it. Click that line. Beat the par time.

Found it! points.

Not quite — line was the first mistake.

+ XP

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