On a long agent loop, per-turn nudges appended as ordinary mid-conversation system messages pile up and re-bill the whole growing stack as input on every subsequent turn; rewriting the top-level system prompt to carry the current nudge is worse, re-processing the entire cached history. Anthropic's turn-scoped system messages fix this: set clear_at:"next_user_message" on a role:"system" message and it renders only until the next user turn, then stays in the array rendering nothing and costing zero input tokens - the model sees only the newest reminder and you stop paying for the stale ones, all without editing history or busting the cache.
Fire a Per-Turn Reminder That Clears Itself Instead of Re-Billing Every Later Turn
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