Claude API — Mid-conversation system messages now GA on Claude Fable 5, Mythos 5, and Opus 4.8

What's new

• Mid-conv system messages GA: available without anthropic-beta header on Fable 5, Mythos 5, Opus 4.8
• Cache-friendly: updating system instructions mid-conversation no longer invalidates cache prefixes
• Availability: Claude API, Amazon Bedrock, Google Cloud (Vertex AI)
• Prior state: feature was in beta since Q4 2025, requiring anthropic-beta: mid-conv-sys-msgs header

Why it matters

Critical for agentic and multi-turn applications where the assistant context or role changes during a conversation. Cache preservation reduces costs for long conversations.

How to try it

Simply omit the beta header in your API call. Available to all API customers on supported models.