What's new in Claude Fable 5.1

Claude Fable 5.1 keeps the same price as Claude Fable 5, reduces cache-read costs to 25%, and adds long-running agentic coding, multistep research, plus document, spreadsheet, and slide generation with a 1 million-token context window and 128 k max output. It enforces automatic tool selection (forced tool use returns a 400 error), always enables adaptive thinking controllable via an effort parameter, and retains thinking blocks only when advancing to newer models; editing prior turns invalidates those blocks. Claude Mythos 5.1 matches these specs and pricing but is restricted to Project Glasswing participants.