Matthew Berman examines Anthropic's Fable 5.1 and Mythos 5.1 releases, focusing on performance, effective cost and enterprise data controls. Cache reads are 75% cheaper, which Anthropic says can reduce typical workloads by about 25% and long agent runs by as much as 45%, while base input and output token prices remain unchanged.
The benchmark picture is strongest for scientific terminal work, agentic coding, computer use and professional tasks. Fable 5.1 improves sharply over Fable 5 on Terminal Bench Science and posts smaller gains on Cursor Bench and Humanity's Last Exam. Mythos 5.1 often scores slightly higher under looser guardrails, while new safeguards target reward hacking, context manipulation and model distillation.
Independent cost data complicates the headline savings. Artificial Analysis ranks Fable 5.1 at the top of its intelligence index, but reports that it uses about 1.7 times as many output tokens and therefore costs more per completed task than Fable 5 in that evaluation. Matthew Berman's coding, simulation and website tests show detailed, capable results, but he still rates GPT 5.6 Sol as a much stronger value and sometimes a better visual performer.
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