Claude Fable 5.1 (and Claude Mythos 5.1 for Project Glasswing participants) launched at the same $10 / $50 per million tokens as Fable 5, with cache reads cut to $0.25 per million tokens.
On Fable 5.1 and Mythos 5.1, tool_choice any and tool return a 400 error. For accounts created on or after August 31, 2026, replaying a thinking block after the system prompt, tools or earlier messages changed also returns a 400 error.
Action needed:
Use tool_choice auto with strict tool use, and stop editing context before thinking blocks.
Fable 5 returned on July 1 on the Claude Platform, Claude.ai, Claude Code and Claude Cowork. Paid plans could use it for up to 50% of weekly limits through July 7. Mythos 5 returned for a set of US organizations.
A US government export control directive, prompted by a reported method of bypassing Fable 5 safeguards found by Amazon researchers, led Anthropic to disable Fable 5 and Mythos 5 for all customers. Other models were not affected.
Claude Fable 5 launched as a generally available Mythos-class model at $10 / $50 per million tokens, with safety classifiers that route some requests to Claude Opus 4.8. Claude Mythos 5 is the same model with fewer safeguards, limited to Project Glasswing partners.
The model deprecations table lists claude-mythos-preview as deprecated on June 9, 2026, the day Mythos 5 launched as its upgrade. No retirement date has been set.
