Anthropic ·
Anthropic: Claude Fable 5.1 and Claude Mythos 5.1
Published by Nova3 AI Research
What shipped
On 2026-09-01, Anthropic launched Claude Fable 5.1 (`claude-fable-5-1`) and Claude Mythos 5.1 (`claude-mythos-5-1`) with a 1M token context window by default, 128k max output tokens, always-on adaptive thinking, $10 / $50 USD per MTok pricing, and $0.25 per MTok cache reads.
Vendor source headline: Claude Fable 5.1 and Claude Mythos 5.1
Anthropic release notes: Claude Fable 5.1What this means for your business
Teams using long-context agentic coding or research workflows should benchmark Claude Fable 5.1 against current prompts and cache patterns before migration. Cost models should separate base input, output, and cache-read volume because Anthropic changed cache-read pricing for these models.
Still open
General availability and region limits for Claude Mythos 5.1 outside Project Glasswing participants are not established.
Go deeper
- AI operations, explained
- Nova3 implementation and advisory rates
- Anthropic: Claude Sonnet cache pricing: calculate the cache-read saving only
- Anthropic OpenTelemetry: what operations teams should log
- Anthropic: Claude scheduled agents on AWS: define missed-run and retry controls
- Anthropic: The Compliance API now returns the text each organization member saved as Instructions for Claude
- Who is behind Nova3 AI