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Claude Opus 4.8: "a modest but tangible improvement"

simonwillison.net·May 28, 2026

Anthropic has released Claude Opus 4.8, which offers modest improvements over its predecessor, particularly in its ability to flag uncertainties and avoid unsupported claims, making it more reliable. The update also introduces features like mid-conversation system messages and a lower minimum prompt cache length, while maintaining the same pricing structure as previous versions.

Claude Opus 4.8 introduces mid-conversation system messages that allow for dynamic instruction updates without restating the full system prompt, enhancing prompt caching efficiency and reducing input costs in agentic loops. This feature could significantly improve AI workflow and productivity by enabling more flexible and cost-effective interactions in long-running conversations.

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