Claude Fable 5.1 launches alongside Claude Mythos 5.1 with 1M token context

GASep 1, 2026VerifiedAdded Sep 22, 2026

Claude Fable 5.1 succeeds Claude Fable 5 for long-running agentic coding, knowledge work and research, with Claude Mythos 5.1 for Project Glasswing participants. Both have a 1M context, 128k output and adaptive thinking at $10 / $50 per MTok, with cache reads cut to $0.25 per MTok.

AI agents LLMs Developer tools

Read Anthropic's release notes

https://platform.claude.com/docs/en/release-notes/overview#september-1-2026

Summaries of vendors' own notes. Product names and logos belong to their owners; logos via logo.dev.

More Claude Platform releases

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Claude PlatformSep 1, 2026

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Also shipped on Sep 1, 2026

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