Anthropic ships Claude Fable 5.1 and Mythos 5.1

Anthropic released Claude Fable 5.1 and Claude Mythos 5.1 on September 1, 2026. Fable 5.1 holds Fable 5's $10 per million input and $50 per million output, but cache reads drop to $0.25 per million, 75% less, which Anthropic estimates cuts typical workloads by about 25% and highly agentic ones by up to roughly 45%. Anthropic reports 55.8% on Terminal-Bench 4.0 for Fable 5.1 and 60.9% for Mythos 5.1, which is the same model with different safeguards, limited to vetted organizations.

Why it matters

An agent loop re-reads its context on every turn, so cache reads, not fresh input, are where a long-running agent's bill actually accumulates. A 75% cut there changes the economics of leaving an agent running without changing the sticker price anyone quotes.

Who should care

Anyone running agents on Claude, and teams budgeting long-context workloads

What you can do

If you already run cache-heavy agent loops, re-run last month's bill against the new cache-read rate before assuming your model choice still holds. Point the API at claude-fable-5-1 to pick it up.