It sounds like you're saying that /prompt changes the system message part of the session. Doesn't that cause a cache break and result in higher usage/cost?
I took a quick look at the source code and it looks like, yes, using /prompt during a session will rebuild the session with a new preamble/system prompt, causing a full cache miss on the next turn.
So in that way it's not like skills at all, neither of those result in paying full read price on the entire session, just the skill prompt itself.
Something else I noticed... In the Anthropic implementation it doesn't seem to be using 'cache_control' in the body. Assuming my understanding is current, without that the Anthropic API won't do any caching at all (unlike most other APIs that do some level of automatic caching without it being requested). So that would result in paying full read price on every turn.
Of course I could be missing something, it was a quick look. Can you clarify?
