mid-conversation-system-messages

 
 Mid-conversation system messages close that gap. You append a `{"role": "system"}` message at the point in the conversation where the new instruction becomes relevant, instead of editing the top-level `system` field. The cached prefix stays the same, so the next request still reads it from cache, and the new instruction is still applied as a system instruction rather than as ordinary user text.
 
-This page covers two features: mid-conversation system messages, which are generally available, and [mid-conversation tool changes](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages#mid-conversation-tool-changes), a beta introduced with Claude Opus 5 that applies the same approach to the `tools` array.
+This page covers two features: mid-conversation system messages, and [mid-conversation tool changes](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages#mid-conversation-tool-changes), a beta introduced with Claude Opus 5 that applies the same approach to the `tools` array.
 
 <Note>
   Mid-conversation system messages are available on the Claude API, [Claude in Amazon Bedrock](https://platform.claude.com/docs/en/build-with-claude/claude-in-amazon-bedrock), and [Google Cloud](https://platform.claude.com/docs/en/build-with-claude/claude-on-vertex-ai).
 
-  This feature is available on Claude Fable 5, [Claude Mythos 5](https://anthropic.com/glasswing), Claude Opus 4.8, Claude Opus 5, and Claude Sonnet 5. No beta header is required for mid-conversation system messages.
+  This feature is available on Claude Fable 5, [Claude Mythos 5](https://anthropic.com/glasswing), Claude Opus 4.8, and Claude Opus 5. No beta header is required for mid-conversation system messages. This feature is not available on Claude Sonnet 5; use the top-level `system` field instead.
 
   Mid-conversation tool changes are in beta and require the `mid-conversation-tool-changes-2026-07-01` beta header. They are available on Claude Fable 5, Claude Mythos 5, Claude Opus 4.8, and Claude Opus 5, on the Claude API, Amazon Bedrock, and Google Cloud. They are not available on Claude Sonnet 5.
 </Note>