When should a request hit a reasoning model, and how do you stop it from overthinking?

Most candidates answer 'use the reasoning model for hard problems' and stop. The interviewer wants it framed as an eval and a budget problem: how you prove the extra thinking tokens paid off, and what you do when the model talks itself out of a correct answer.

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