Move persistent memory from the user message into system blocks with
cache_control: ephemeral on the last block. The static prefix (system prompt +
memory, ~3-5K tokens typically) is identical between the two LLM calls of a
tool_use round-trip and stable across turns within the 5-minute cache TTL.
Without this, the tool-call retrieval architecture roughly doubled input
token cost on retrieval-needed turns (full context billed twice). With cache
reads at ~10% of standard input, the duplication cost drops by ~90% — the
"twice as expensive" hit becomes "slightly more expensive plus tool overhead."
client_time stays in the user message (per-turn dynamic, should not be in the
cached prefix).
aaron
84994f9282