How does prefix caching work internally in an LLM server, and when does it actually help?
A shared system prompt is re-prefilled on every request unless the server remembers it. Prefix caching skips that work, but only when the blocks align exactly. Here is the hashing and eviction machinery underneath.
Updated Sep 2026 · Grounded in real GenAI, LLM, and AI/ML engineering interview loops and written to a senior-engineer editorial bar.
A shared system prompt is re-prefilled on every request unless the server remembers it. Prefix caching skips that work, but only when the blocks align exactly. Here is the hashing and eviction machinery underneath.
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.
