Implement a GPU credit allocation manager: issue credits, track usage, enforce limits and expiry.

OpenAI's signature build screen: a credit ledger that grants GPU-hours, spends them, and expires unused grants in issue order. The trap is spending against the right grant first. Here is the FIFO-by-expiry design that survives every follow-up.

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