Anthropic releases Claude Fable 5.1 and, for vetted organizations, Claude Mythos 5.1

Anthropic's release notes record that on September 1, 2026 it launched Claude Fable 5.1 (claude-fable-5-1), the successor to Claude Fable 5, alongside Claude Mythos 5.1 (claude-mythos-5-1) for Project Glasswing participants. Anthropic's announcement says the two are the same model with different levels of safeguards: Fable 5.1 is generally available, while Mythos 5.1 is offered only through trusted access programs for cybersecurity and life sciences work and is currently available only to a set of U.S. organizations. The release notes say text generated by both models carries Anthropic's text watermark, and the announcement says a watermark detection API is rolling out in private preview. Both are hosted models; no release of weights is described. USASI has not tested them.12

USASI. (2026, October 13). Anthropic releases Claude Fable 5.1 and, for vetted organizations, Claude Mythos 5.1. United States of America Superintelligence. https://unitedstatesofamericasuperintelligence.com/news/anthropic-releases-claude-fable-5-1-and-mythos-5-1/

@misc{usasi_news_anthropic_releases_claude_fable_5_1_and_mythos_5_1,
  author = {{USASI}},
  title = {Anthropic releases Claude Fable 5.1 and, for vetted organizations, Claude Mythos 5.1},
  year = {2026},
  month = oct,
  howpublished = {\url{https://unitedstatesofamericasuperintelligence.com/news/anthropic-releases-claude-fable-5-1-and-mythos-5-1/}},
  note = {United States of America Superintelligence. Last updated 2026-10-13}
}