Practice the skill tutorials skip

Get better at finding bugs — not just writing code.

Choose a language, trace a realistic failure, check your fix and see exactly where your reasoning holds. Start with the core challenge loop for free.

Start a free challenge Guides

const theory = inspect(symptom);
const change = test(theory);
assert(change.explains(symptom));

From code to learning

Choose your first language

Nine verified programming languages. The interface and feedback support five languages, while editorial documentation initially publishes in English and Brazilian Portuguese.

1Choose a language
2Investigate the bug
3Check the solution
4Read explainable feedback
5Track your progress

PythonPractice now →JavaScriptPractice now →TypeScriptPractice now →JavaPractice now →C#Practice now →SQLPractice now →KotlinPractice now →SwiftPractice now →C++Practice now →

Interface and feedback: English, Português, Español, العربية, 한국어. Published docs: EN / PT-BR.

Bugs that look like real work

Build debugging instinct with recognizable failure patterns in Python, JavaScript, SQL and six more languages — not riddles disguised as engineering.

A verdict you can learn from

Check your solution and get contextual feedback on the bug, the expected behavior and the edge cases that matter.

Progress you can explain

See attempts and solutions separately, follow skill growth and let challenge difficulty respond to your results.