PRODUCTION SCENARIO
An operator console on Claude Opus 5 caches a large system prompt. Midway through a session the operator needs to switch the assistant into a terse mode, and editing the top-level system field would wipe the cached prefix.
How should the instruction be delivered?
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Show answer and explanation
Answer: Append a system-role message after the latest user turn instead of editing the field
A mid-conversation system message is appended where the instruction becomes relevant, so the cached prefix survives and the text still carries system-level priority. It has to immediately follow a user turn; editing the top-level field or restarting the session throws the cache away.