Choose classic system design if the hard part is scale, state, or failure handling. Choose ML system design if the hard part includes data, model, evaluation, or serving drift.
Spend 35 to 45 minutes writing the design. The prompt gives you the expected sections; it does not give a reference answer.
Mark each criterion with evidence from your design. Missing evidence is a repair target, not a vibe check.
Rewrite the lowest-scoring section first: requirements, scale, data model, failure modes, observability, cost, or ML-specific concerns.