How does prompt caching work, and when does it actually pay off?
The KV-cache mechanics behind the discount, the prefix rule that silently breaks caching for most teams, and the workloads where caching cuts bills 50-90% versus the ones where it does nothing.
Updated Sep 2026 · Grounded in real Forward Deployed Engineer interview loops and written to a senior-engineer editorial bar.
The KV-cache mechanics behind the discount, the prefix rule that silently breaks caching for most teams, and the workloads where caching cuts bills 50-90% versus the ones where it does nothing.
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.
