A budget-constrained design problem, not an ideology question. The per-call math that rules an LLM in or out, the cascade that keeps quality while cutting cost 90%, and the SLO arithmetic that decides where the LLM is even allowed to sit.
Updated Aug 2026 · Grounded in real Forward Deployed Engineer interview loops and written to a senior-engineer editorial bar.
A budget-constrained design problem, not an ideology question. The per-call math that rules an LLM in or out, the cascade that keeps quality while cutting cost 90%, and the SLO arithmetic that decides where the LLM is even allowed to sit.
The screen is whether you do the arithmetic before picking a side: convert the monthly budget and daily volume into a per-call ceiling, compare it to realistic LLM and classical costs and latencies, and only then design. The reserved follow-up is the cascade, a cheap high-coverage model that handles the easy 90% and escalates the uncertain tail to the LLM, which is how you respect both constraints at once; candidates who answer 'LLM' or 'classical' without the budget math fail the question on contact.
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