Implement a cross-encoder reranker and prove it improves nDCG@10.

Anyone can call a rerank endpoint. The word doing the work in this question is 'prove': you have to implement DCG, ideal DCG, and nDCG@k from graded labels and report the before-and-after. Plus the ceiling nobody mentions until it bites them in production.

Updated Sep 2026 · Grounded in real GenAI, LLM, and AI/ML engineering interview loops and written to a senior-engineer editorial bar.