On June 9, 2026, Anthropic shipped Claude Fable 5, a model in a new tier that sits above Opus. I have been building on the Claude API for over a year, and this is the first release that made me stop and re-read my whole prompt stack before touching the model string. Here is what actually changed and what it means if you ship software.
The short version
Fable 5 is the public release of the Mythos line, the family that earlier in the year unsettled the security world with how well it found and exploited vulnerabilities. The version you and I get is the same underlying model with safeguards bolted on. Anthropic calls the safe one Fable and the unrestricted one Mythos, and only a small group of cyberdefenders gets Mythos.
The numbers, for context:
1M token context window, 128K max output, knowledge cutoff January 2026.
