Design token-based metering and billing for an LLM product, and explain why it must be decoupled from the call path.
The LLM billing round has three trapdoors: retries double-charging customers, metering latency leaking into inference, and month-end numbers that don't match the provider invoice. The design that survives all three is the answer.
Updated Aug 2026 · Grounded in real Forward Deployed Engineer interview loops and written to a senior-engineer editorial bar.
The LLM billing round has three trapdoors: retries double-charging customers, metering latency leaking into inference, and month-end numbers that don't match the provider invoice. The design that survives all three is the answer.
Lead with where the obvious approach breaks, because that is the judgment they are screening for — most candidates jump straight to the happy path and lose the room.
Then walk the failure back through the pipeline in order, naming the one metric the customer's exec sponsor actually cares about before you propose the fix.
