PRODUCTION SCENARIO
An operator console on Claude Opus 5 caches a large system prompt. Midway through a session the operator must switch the assistant into terse mode, the instruction has to outrank anything the end user asks for afterwards, and editing the top-level system field would wipe the cached prefix.
How should the instruction be delivered?
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Answer: As a system-role message appended after the latest user turn in messages
A mid-conversation system message is appended at the point the instruction becomes relevant, so the cached prefix is untouched and the text still carries operator priority over the user turns that follow. Claude does follow instructions that arrive in user turns, but a user message is treated as coming from the end user and loses precedence when the two conflict. Editing the top-level system field applies the rule correctly and discards the whole cache, which the scenario rules out. And restarting the session throws away the cache and the verbatim conversation state together.