Practice AI levels and agent-drafted assignments

What changed

• Systems practice runs now let you choose your help: no AI, a chat assistant that cannot edit your code, or a full AI agent. The choice is recorded with your score.
• When recruiters pick a question's AI mode, an AI agent is no longer offered on algorithm problems from the library, where it would solve them outright.
• The classroom agent can now draft assignments as well as quizzes. Teachers on a paid Classroom plan can also generate questions with AI in the assignment and quiz builders.