mid-conversation-system-messages changedbuild-with-claude/mid-conversation-system-messages
Nearest release: v2.1.252, published 4 hours before this site recorded the change. Shown because the two are within 24 hours of each other. Nothing here says the release caused the edit.
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77<Note>
8 For how zero data retention (ZDR) applies to this feature, see [API and data retention](https://platform.claude.com/docs/en/manage-claude/api-and-data-retention).
8 To learn how zero data retention (ZDR) applies to this feature, see [API and data retention](https://platform.claude.com/docs/en/manage-claude/api-and-data-retention).
99</Note>
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1111System instructions normally live in the top-level `system` field, ahead of every message in the conversation. That position is great for [prompt caching](https://platform.claude.com/docs/en/build-with-claude/prompt-caching): the system prompt is part of the stable prefix, so subsequent turns hit the cache. It is a poor position for instructions you only discover you need partway through a session, because editing the top-level `system` field changes the very beginning of the prompt and invalidates the cache for everything that follows.
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