Claude Fable 5.1

Verdict

The top of the Anthropic lineup, above Opus. Fable 5.1 succeeds Fable 5 at the same $10/$50 per million tokens, with a 1M context and 128K output. Thinking is always on and you steer depth with an effort setting rather than a token budget; forced tool choice is rejected, so tool use has to be steered from the prompt. Reach for it on architecture review and long-horizon agent work, and expect single requests on hard tasks to run for minutes. For most work, Opus 5.5 is the better cost-to-quality pick.