LLM Agents Interview Questions, issue 14, Mar 8, 2026
The Synthetic Dataset Trap
Senior Engineer interview at Anthropic, and the interviewer asks:
“You found an open-source synthetic reasoning dataset that claims a 15% bump on MMLU and GSM8K. You have the compute budget to run an SFT pass. Before adding it to your mix, what is the exact programmatic validation step you run against your eval suites to ensure that 15% isn’t a lie?”
Frontier models generate reasoning data that often already contains benchmark knowledge, so without overlap detection you’re silently training on your test set.
The full answer, with the mechanism and the arithmetic, is for paid subscribers on Substack.