It's usually not as binary as "hit" or "miss" with a prefix cache, and you need to know the token boundaries to know where the cache hit ends.

The current structures used for KV-caching in vLLM and SGLang work by chunking the KV-cache tokens into prefix trees, and you need to hash chunks of tokens in order to look up in these, meaning you need to be able to slice up your tokens by token count.

Again I have no idea what proprietary engines are doing, but this is why open source stuff needs to tokenize before cache lookup at least.