Anthropic says Claude Fable 5 and Claude Mythos 5 become available on June 9, 2026, with Fable 5 generally available through the Claude API, Amazon Bedrock, Claude Platform on AWS, Google Cloud, and Microsoft Foundry.
Anthropic describes Claude Fable 5 as its most capable widely released model for demanding reasoning and long-horizon agentic work, while Claude Mythos 5 shares Fable 5's capabilities and is offered only in limited availability through Project Glasswing.
The launch notes list the API model IDs as claude-fable-5 and claude-mythos-5, and they state that both models share a 1M-token context window, up to 128k output tokens per request, and pricing of $10 per million input tokens and $50 per million output tokens.
Anthropic says Claude Fable 5 includes safety classifiers that can return stop_reason: "refusal" in a successful HTTP 200 response, while Claude Mythos 5 does not include those classifiers.
The migration guide says adaptive thinking is always on for both Fable 5 and Mythos 5, that disabling thinking or setting manual extended-thinking budgets returns a 400 error, and that developers should use system-prompt instructions instead of assistant-message prefill.
Anthropic's prompting guide says Fable 5 and Mythos 5 require different scaffolding for effort, instruction following, long runs, memory, and API handling than earlier Claude models.
