mid-conversation-system-messages changedbuild-with-claude/mid-conversation-system-messages
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474474You can still set the top-level `system` field for instructions that should apply to the entire conversation. Reserve mid-conversation system messages for instructions that only become relevant later, or that you want to add without invalidating the cached prefix.
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476A `role: "system"` message can also carry `output_config.effort` to change the [effort](https://platform.claude.com/docs/en/build-with-claude/effort) level from the next `user` turn on. This is in beta on Claude Fable 5.1, Claude Mythos 5.1, and Claude Opus 5 on the Claude API and requires the `mid-conversation-output-config-2026-07-01` beta header. See [Per-message effort](https://platform.claude.com/docs/en/build-with-claude/effort#change-effort-mid-conversation-beta).
476A `role: "system"` message can also carry `output_config.effort` to change the [effort](https://platform.claude.com/docs/en/build-with-claude/effort) level from the next `user` turn on. This is in beta on Claude Fable 5.1, Claude Mythos 5.1, and Claude Opus 5 on the Claude API and Google Cloud, and requires the `mid-conversation-output-config-2026-07-01` beta header. See [Per-message effort](https://platform.claude.com/docs/en/build-with-claude/effort#change-effort-mid-conversation-beta).
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478478<CodeGroup>
479479 ```bash cURL
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