Anthropic has released two new flagship models: Claude Fable 5.1, available broadly through the Claude API, and Claude Mythos 5.1, which remains restricted to a select group of vetted users inside the company’s Project Glasswing program. Both models target the same class of demanding work that has become the real battleground in enterprise AI: long-horizon coding tasks, multistep research, and dense document processing.
The release landed on September 1, 2026, and builds directly on the Fable 5 and Mythos 5 lineage that Anthropic first shipped on June 9, 2026.
What the models actually do differently
Fable 5.1 and Mythos 5.1 each support one million tokens in context, with output capacity reaching 128,000 tokens per response.
Fable 5.1 adds two notable beta features that weren’t available in its predecessor. The first is mid-conversation effort adjustment, letting users dial up or down how hard the model works on a given step without starting a new session. The second is content provenance tracking, which tags outputs so downstream systems can verify where generated content originated.
