# clm_0F5B0FB5DBDF

subject: Anthropic (/record/anthropic.md)  
predicate: released  
value: Claude Mythos 5.1  
valid_from: 2026-09-01  
valid_to: null  
published: 2026-09-01  
recorded: 2026-10-31  
confidence: 0.70  
status: valid  
assertable: true  
superseded_by: null  
last_updated: 2026-10-31  
checked: 2026-10-31 · its source page states the value and the date  

> September 1, 2026  We’ve launched Claude Fable 5.1 ( claude-fable-5-1 ), the successor to Claude Fable 5 for long-running agentic coding, knowledge work, and research, alongside Claude Mythos 5.1 ( claude-mythos-5-1 ) for Project Glasswing participants.

What https://platform.claude.com/docs/en/release-notes/overview#2026-09-01 says, word for word.

## Sources

- https://platform.claude.com/docs/en/release-notes/overview#2026-09-01 (primary, Anthropic, published 2026-09-01)

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