Every cache these providers keep is a prefix cache, and the working
directory of a terminal is the most volatile value the app has. Putting
one inside the other meant a single `cd` between turns changed byte zero
of the request and missed the entire replayed conversation — on a BYOK
key, in an app whose whole purpose is moving around a filesystem.
Nothing sets `cache_control` yet, so Anthropic was unaffected, but
OpenAI and Google cache prefixes automatically: this was already
costing money, and it was a trap laid for the day explicit caching lands.
The context now rides the tail of the message list, immediately before
the turn it describes. History the model has already seen is immutable,
so `[system + history]` keeps hitting and the divergence is the newest
turn, which was never cacheable anyway. The system prompt goes back to
being byte-identical across every run, conversation, and user.
It is sent but not kept. `snapshot.messages` is the durable transcript,
and a block added there would be persisted, replayed, and — since
restore projects user rows into the visible list — eventually shown in
the user's own chat bubble. Leaving it out costs the next run one
uncached message at the tail and nothing before it.
Two consecutive user messages are already routine here: `.tool` rows
become `user` rows on all three adapters, so any parallel tool call
produces them.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
