Claude Fable 5.1 / Mythos 5.1 repo
Anthropic's next model generation, released September 1, 2026. Fable 5.1 is generally available as claude-fable-5-1. Mythos 5.1 stays restricted to vetted cybersecurity and life sciences organizations. Both carry a 1 million token context window, 128,000 max output tokens, and adaptive thinking always on.
What it does for you: Axion and Hermes both already run on Claude models, and this release cuts cache read pricing 75 percent, from 1.00 to 0.25 dollars per million tokens, which Anthropic measures as roughly 25 percent lower cost on typical workloads and up to 45 percent lower on context heavy agentic workloads, exactly the shape of Axion's multi tool sessions. On Terminal Bench Science 0.1, an agentic research benchmark, it scores 52.6 percent against 24.7 percent for the prior Fable 5 and 29.0 percent for Opus 5, a real capability jump for the long running research and build tasks YY already delegates.
In practice: A quiet, business like release. Anthropic disclosed a standard error range on its own benchmark numbers instead of just posting a leaderboard win, which reads as more trustworthy than most model launch posts.
For: Both runtimes. It is a model id reachable through the Claude API, so any harness that calls that API, Claude Code today and a reconfigured Hermes tomorrow, can point at claude-fable-5-1.
Verdict: build now. It requires no new code, only pointing sessions at claude-fable-5-1, and the cache pricing cut alone pays for the evaluation effort within days given how much of Axion's cost sits in repeated cache reads across long agent sessions.
Build #1 Claude Fable 5.1 / Mythos 5.1: use the ai-implementation-build-intake skill to build this safely. Source: https://www.marktechpost.com/2026/09/01/anthropic-releases-claude-fable-5-1-and-claude-mythos-5-1-52-6-on-terminal-bench-science-and-75-cheaper-cache-reads/. Save canonical skill/agent under AXION\Skills and AXION\Agents.
Source · Released September 1, 2026. 1 million token context window, 128,000 max output tokens. Cache read price cut 75 percent, from 1.00 to 0.25 dollars per million tokens. Terminal Bench Science 0.1 score of 52.6 percent versus 24.7 percent for Fable 5 and 29.0 percent for Opus 5. Base pricing unchanged at 10 and 50 dollars per million input and output tokens.