Anthropic launches Claude Fable 5 with Mythos-class power at half the price

Anthropic unveiled Claude Fable 5 as its most capable model yet, generally available with cyber safeguards built in, while a sibling, Mythos 5, removes those guardrails and is restricted to vetted cyber defenders and critical-infrastructure operators because of its ability to find and exploit software vulnerabilities. Both are priced at $10 per million input tokens and $50 per million output tokens, less than half the prior Mythos Preview rate, and Anthropic added a 30-day data-retention requirement for Mythos-class defensive use.

The headline capability is endurance: Anthropic says Fable 5 sustained 9.5-hour autonomous runs from a single brief, including a 50-million-line Ruby migration completed in one day. AWS's Matt Wood described it as built for 'long-running, autonomous agent work' that can 'drive complex projects for days,' understanding diagrams, charts, and nested tables and proactively verifying its own output. Independent developer Simon Willison called it 'relentlessly proactive,' describing it spinning up custom CORS Python servers and using pyobjc to capture screenshots from a single bug report.

Competitively, Fable 5 lands as OpenAI weighs price cuts and Google ships cheaper Gemini variants, sharpening a frontier price war. LlamaIndex's Jerry Liu cautioned that for document parsing Fable 5 is 'roughly equivalent to Gemini 3 Flash at 10-15x the token cost' and falls short of specialized OCR providers, while excelling on reasoning benchmarks like SWE-Bench Pro and FrontierCode. Anthropic itself later conceded it 'made the wrong tradeoff' on guardrails after researchers complained Fable 5 over-refuses biology and AI-research questions — a controversy that quickly merged with the bigger export-control storm engulfing the launch.