We've launched Claude Fable 5.1 (claude-fable-5-1)
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…
Anthropic has 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 it, Anthropic released Claude Mythos 5.1 (claude-mythos-5-1) for Project Glasswing participants.
- Both models support a 1M token context window by default, 128k max output tokens, and always-on adaptive thinking.
- Pricing stays at $10 / $50 USD per MTok, the same as Claude Fable 5, but prompt cache reads drop to $0.25 per MTok, 0.025x the base input price instead of the usual 0.1x.
- Claude Fable 5.1 is available on the Claude API, Claude in Amazon Bedrock, Claude Platform on AWS, Claude on Google Cloud, and Claude in Microsoft Foundry.
- The tool_choice types any and tool are no longer supported on either model and now return a 400 error; auto and none are unchanged.
- Thinking blocks from Claude Fable 5.1 and Claude Mythos 5.1 are preserved only for the model that produced them or a newer one; earlier models can't read them.
Text generated by both models carries Anthropic's text watermark, and supported image, video and audio files produced through the code execution tool carry C2PA Content Credentials when retrieved via the Files API. Both models require 30-day data retention and aren't available under zero data retention unless Anthropic authorises it. Full details are in the release notes.