No timestamps, request IDs, localized dates, or A/B copy in the system prompt. Put those in the last user turn if you need them.
Canonical JSON key order for tools. Some languages shuffle maps and change the prefix without changing meaning.
History only grows at the end. Editing turn 2 of a 20-turn chat invalidates every token after it.
If you sit just under 1,024 tokens, expand static instructions. Silent skip is the usual failure mode, not an error.
A hit refreshes the clock. Quiet gaps kill the entry. Use 1-hour Claude TTL, or pre-warm before users arrive.
Claude writes only at breakpoints. Automatic mode on a changing last block writes forever and never reads.
Wait until the first response starts before parallel twins. A write is not visible to siblings that launched with it.
Use prompt_cache_key / user when offered so related traffic lands on the replica still holding that KV prefix.