Anthropic launches Claude Fable 5 and Mythos 5

What's new

• Claude Fable 5 publicly available through Claude API and Enterprise consumption plans
• Mythos 5 rolled out to approved organisations (same runtime, looser guardrails)
• Benchmarks: 10+ % above Claude Opus 4.8 on coding/knowledge work
• Input tokens: $10 / 1M, output: $50 / 1M (2× the price of Opus 4.8)
• Hard safety blocks: cybersec/biology/chemistry/distillation refuse and fall back to Opus 4.8

Why it matters

Fable 5 is Anthropic's answer to the Mythos safety panic of April 2026. A publicly controlled release with declared Mythos-minus-high-risk capability. For devs it means: a new top model for coding agents (Claude Code, Cursor agents) but also a new over-refusal threshold to calibrate around in prompts.

How to try it

``bash
export ANTHROPIC_API_KEY=...
curl https://api.anthropic.com/v1/messages \
-H "content-type: application/json" \
-d '{"model":"claude-fable-5","max_tokens":1024,"messages":[{"role":"user","content":"Refactor this Python function..."}]}'
``
On Enterprise plans Mythos 5 requires an opt-in.