I’m trying to understand prompt caching beyond the basic advice to put stable instructions first. If I change a small part of a long system prompt, does that prevent the rest of the prefix from being reused, or can the cache still help with the unchanged portion?

I’m also curious how people handle dynamic values such as dates or user IDs without needlessly breaking the cached prompt prefix. If you’ve measured this in production, what changes made a noticeable difference to cache hits?