Anthropic releases Claude Fable 5.1 alongside limited-access Claude Mythos 5.1

Anthropic has launched Claude Fable 5.1 and Claude Mythos 5.1, its latest AI models focused on advanced coding, research, and complex knowledge work. Both models arrived on September 1, 2026.

Claude Fable 5.1 is available to general users, while Claude Mythos 5.1 is offered by invitation through Anthropic's Project Glasswing. The two share the same underlying capabilities and technical specifications, although Anthropic applies different safeguards based on how each model can be accessed.

Claude Fable 5.1 brings major performance gains

Anthropic says Claude Fable 5.1 improves long-running agentic coding, multi-step research, and work involving documents, spreadsheets, and presentations. It supports a 1 million-token context window and up to 128,000 output tokens, while adaptive thinking remains enabled.

The model also posted a large improvement in scientific tasks. Fable 5.1 scored 52.6% on Terminal-Bench-Science 0.1, compared with 24.7% for Fable 5. On Terminal-Bench 4.0, its score increased from 42% to 55.8%.

Anthropic has also adjusted its safeguards. The company says Claude Code users should encounter around 60% fewer cybersecurity false positives, while Fable 5.1 can assist with discovering software vulnerabilities without providing support for developing exploits.

Fable 5.1 is cheaper for many workloads

Standard API pricing remains $10 per million input tokens and $50 per million output tokens. However, cache-read pricing has fallen to $0.25 per million tokens, one-quarter of the previous cost.

Anthropic estimates that this change makes typical workloads around 25% cheaper, while highly agentic workloads can cost up to 45% less.

Claude Fable 5.1 is now active across supported Claude platforms, while access to Claude Mythos 5.1 remains limited to approved Project Glasswing participants.