Most harnesses are not designed with conversations to be append-only from the start, which causes us all endless pain when caching is concerned.

When making a harness, it is so, so easy to do the "web thing" and have your system prompt be a template where you dynamically inject variables like the current date or the contents of the current working directory, and then just re-render that template on every request. THis is a bad, doubleplusungood, terrible, awful idea, and you should never do this under any circumstances. Anthropic will soon force you not to by literally refusing conversations that "break continuity" in this way and do anything but append new user messages at the end, but other providers at least appear to work.