working-with-messages changedbuild-with-claude/working-with-messages
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353353### System role in messages
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355On Claude Fable 5.1, [Claude Mythos 5.1](https://platform.claude.com/docs/en/models/mythos-5-1/overview), Claude Fable 5, [Claude Mythos 5](https://platform.claude.com/docs/en/models/mythos-5/overview), Claude Opus 5.5, Claude Opus 4.8, Claude Opus 5, Claude Sonnet 5.5, and Claude Haiku 5.5, you can include messages with `"role": "system"` after a user turn (subject to [placement rules](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages#limitations)) to add a new system instruction partway through a conversation. A `system` message that carries content cannot be the first entry in `messages`. Use the top-level `system` field for instructions that apply from the start.
355You can include messages with `"role": "system"` after a user turn to add a new system instruction partway through a conversation, subject to [placement rules](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages#limitations). This works on Claude Fable 5.1, [Claude Mythos 5.1](https://platform.claude.com/docs/en/models/mythos-5-1/overview), Claude Fable 5, [Claude Mythos 5](https://platform.claude.com/docs/en/models/mythos-5/overview), Claude Opus 5.5, Claude Opus 4.8, Claude Opus 5, Claude Sonnet 5.5, and Claude Haiku 5.5. A `system` message that carries content cannot be the first entry in `messages`. Use the top-level `system` field for instructions that apply from the start.
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357357A mid-conversation system message has the same authority as the top-level `system` field, but because it is appended to the end of the message history, it does not invalidate any cached prefix that came before it. Use the top-level `system` field for instructions that should apply from the very first turn, and a mid-conversation system message for instructions that only become relevant later.
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