Claude by Anthropic

Fable 5 runs on Anthropic's Mythos-class transformer architecture with adaptive thinking, giving it a 1M-token input context and up to 128k tokens of output — which means a codebase migration or a multi-document research synthesis fits in a single pass without chunking hacks. The vendor positions this explicitly for autonomous agent work: chained tool use, multi-step reasoning, and tasks where the model needs to hold complex state across many turns. Where it breaks is cost — per-token billing is paid-only, and at the rates the validator documents, teams running high-volume pipelines will feel it fast. Vision-dependent scientific analysis and complex software engineering are the use cases the vendor calls out directly. Teams doing commodity summarization or single-turn Q&A will pay a premium they cannot justify.