Build with Mythos: Claude Fable 5 is now in Augment Code

Claude Fable 5 (Mythos family) is now available in the Augment Code model picker. At roughly 2x the cost of Opus 4.8, Fable 5 is intended for long, multi-step tasks that require deep reasoning.

In early testing, it caught every known issue in our toughest internal code review benchmark and flagged additional potential problems we’re still validating. In practice, it has started to expose the limits of benchmarks we've relied on to differentiate frontier models.

If your day-to-day work is mostly coding, existing models continue to offer tremendous value. For longer running, more complex work involving deep reasoning and multi-turn workflows, Fable 5 offers an advantage in quality.

Fable 5 is at its best when it can operate with full context and follow-through, not just generate an answer. Augment’s unified agent platform, Cosmos, unlocks that.

With Cosmos, Fable 5 can pull the right context from across your tools, coordinate work across the full SDLC (triage → spec → implementation → review → test → deploy), and keep durable memory of decisions and patterns so progress compounds over time. Instead of a single prompt in a single surface, you get a coordinated system where agents run in the background, trigger off real workflow events, and bring your team in when judgment matters, so you can apply Fable 5’s premium capability to the hardest, highest-leverage engineering work.

One caveat: Early Mythos model family reports focused on the extensive cybersecurity capabilities of this model class. Anthropic applies hard restrictions on this use case, and will fall back to the Opus family for those tasks.

Fable 5 is powerful, and that capability comes at a premium. We’ll soon be adding it to our model router, Prism, to help teams access right-sized models for the tasks at hand, saving cost without sacrificing quality.

If you try Fable 5, let us know: we’re eager to hear how you use it and where you find the most value with this new model family.