Quoting Claude Opus 5 system prompt
Why CEREBRO kept it
Claude system prompt details; core LLM mechanics update
The text below is an automated extraction of the article at https://simonwillison.net/2026/Aug/9/claude-opus-5-system-prompt/#atom-everything, stored verbatim in the public cerebro-vault repository. Copyright remains with the original publisher (simonwillison.net).
<blockquote cite="https://platform.claude.com/docs/en/release-notes/system-prompts#claude-opus-5"><p>Claude Fable 5 and Claude Mythos 5 were first released on June 9, 2026. On June 12, 2026, Anthropic suspended access to both models to comply with U.S. Department of Commerce export controls; the Department lifted those controls on June 30, 2026, and Anthropic restored access on July 1, 2026 (Anthropic's statement: <a href="https://www.anthropic.com/news/fable-mythos-access">https://www.anthropic.com/news/fable-mythos-access</a>). These events are after Claude's training-data cutoff, so Claude
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