Claude API adds turn-scoped system messages in beta
BetaVerifiedAdded Sep 22, 2026
Setting clear_at next_user_message on a mid-conversation system message renders it for the current turn only, then keeps it in history at no token cost. Per-turn reminders no longer accumulate or invalidate the prompt cache and later thinking blocks.
Topics: Pricing
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More Claude Platform releases
Every Claude Platform release| Date | Release | Type |
|---|---|---|
| Sep 1 | Claude Fable 5.1 launches alongside Claude Mythos 5.1 with 1M token context GA | GA |
| Sep 1 | Prompt cache reads on Claude Fable 5.1 and Mythos 5.1 drop to $0.25 per MTok Pricing | Pricing |
| Sep 1 | Claude Fable 5.1 and Mythos 5.1 reject tool_choice any and tool Update | Update |
| Sep 1 | Claude Fable 5.1 thinking blocks only replay to the same or newer models Update | Update |
| Sep 1 | Per-message effort changes enter beta on the Claude API Beta | Beta |
| Sep 1 | Claude API thinking.display adds an updates mode in beta Beta | Beta |
| Sep 1 | Claude Fable 5.1 output carries text watermarks and C2PA Content Credentials Update | Update |
| Sep 1 | Claude Fable 5.1 and Mythos 5.1 require 30-day data retention Update | Update |
