Function reply_prefix_tokens 

pub fn reply_prefix_tokens(history: &[SharedMessage]) -> u64

Conservative token estimate of the part of the history’s last message (the turn’s final reply) that the cached prefix will hold: its content minus thinking blocks, which the API strips when the next request re-sends the reply (they were billed as output, never cached). Ping 1 may write at most this (plus the last request’s uncached input) without counting as a partial miss.