Initial impressions of Claude Fable 5

<p>I didn't have early access to today's <a href="https://simonwillison.net/atom/everything/">Claude Fable 5</a> release, but I've spent the past ~5.5 hours putting it through its paces. My initial impressions are that this is something of a <em>beast</em>. It's slow, expensive and has been quite happily churning through everything I've thrown at it so far. As is frequently the case with current frontier models the challenge is finding tasks that it can't do.</p> <p>First, let's review the key characteristics.</p> <p>Anthropic claim that <a href="https://www.anthropic.com/news/claude-fable-5-mythos-5">Claude Fable 5</a> offers the same performance as Claude Mythos 5, except with much more strict guardrails in place to prevent it being used for harmful things. Those guardrails trigger often enough that the Claude API has new mechanisms for letting you know when you hit them, and the Claude API even has a <a href="https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback">new mechanism</a> where you can request it falls back to another model automatically if something gets rejected.</p> <p>Claude Mythos 5 is out today as well, <a href="https://platform.claude.com/docs/en/about-claude/models/introducing-claude-fable-5-and-claude-mythos-5">Anthropic say it</a> "Shares Claude Fable 5's capabilities without the safety classifiers".</p> <p>The models have a 1 million token context window, 128,000 maximum output tokens and a knowledge cut-off date of January 2026.</p> <p>They are priced at twice the price of Claude Opus 4.5/4.6/4.7/4.8: $10/million input tokens and $50/million output tokens. There's no increase in price for longer context usage.</p> <p>Other than that the <a href="https://platform.claude.com/docs/en/about-claude/models/introducing-claude-fable-5-and-claude-mythos-5">upgrade guide</a> is substantially thinner than <a href="https://platform.claude.com/docs/en/about-claude/models/whats-new-claude-4-8">the similar guide for Opus 4.8</a>.</p>