How do you choose top-k and the context budget for RAG, given recall, noise, and cost all pull against each other?

More chunks means higher recall but more noise, more cost, and more lost-in-the-middle. The right k is an empirical tradeoff, not a default of 5. Here is how to find it.

Updated Aug 2026 · Grounded in real Applied AI Engineer interview loops and written to a senior-engineer editorial bar.