Claude Opus 5.5 system prompt published in official docs, confirming Mythos tier
npinto · x · 2026-09-24
Anthropic's official docs publish the full system prompt for Claude Opus 5.5 (claude.ai and mobile apps), with big reveals:
- Current lineup: Claude Fable 5.1, Opus 5.5, Sonnet 5, Haiku 4.5, with API model strings listed.
- Above Opus sits a new Mythos tier: Claude Mythos Preview isn't public and is used by a small set of trusted organizations under Project Glasswing; Mythos 5.1 and Fable 5.1 share the same underlying model, with Fable adding extra safeguards for biology, cybersecurity, and LLM R&D.
- The docs also log an unusual episode: Fable 5 / Mythos 5 launched June 9, 2026, were suspended June 12 under U.S. Commerce Department export controls, and restored July 1 after the controls were lifted June 30.
The poster jokes this is a system prompt "leaked without knowing" — the behavioral constraints make it worth a close read for alignment and prompt-engineering folks.
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