Claude Fable 5.1

What Anthropic says. The models overview tells readers to use Fable 5.1 "for demanding reasoning and long-horizon agentic work, or when your evals on Claude Opus 5.5 at higher effort still fall short." The choosing guide says it extends Claude Fable 5 with stronger long-running agentic coding, knowledge work and research at the same input and output prices, with cache reads at a quarter of the cost. Its selection matrix lists agent sessions that run for hours, multistep deep research, and analysis carried through to a finished document, spreadsheet or deck as typical uses. The guide also notes that Claude Mythos 5.1 offers the same capabilities, but only to organizations verified through Anthropic's verification programs.

Our editorial judgment. Fable 5.1 is a specialist tier for work where a failed run costs hours of human cleanup, such as a long research synthesis or an unsupervised multi-step agent. For everyday chat or drafting we would not pay a 2.5x premium over Opus 5.5 without evidence from your own tasks. If you are comparing assistants rather than API models, our AI chatbots guide and AI coding tools guide cover packaged products.

• Highest list price in the family. At USD 10 input and USD 50 output per 1M tokens, Fable 5.1 costs 2.5 times Opus 5.5 and 5 times Sonnet 5.5 per token.
• Thinking cannot be turned off. The overview lists adaptive thinking as always on, with high as the default effort.
• Breaking changes shared with Opus 5.5. Anthropic's Opus 5.5 page says three of its breaking changes also apply to Fable 5.1: thinking cannot be disabled, forced tool use returns an error, and thinking blocks are tied to the model and conversation that produced them.
• No fast mode. The pricing page lists fast mode only for Claude Opus 5.5, Claude Opus 5 and Claude Opus 4.8.
• Cache writes cost more than base input. A 5 minute write costs 1.25x base input and a 1 hour write costs 2x, so caching pays off only when a prefix is reused. Cache hits are 2.5% of the input price, the lowest ratio in the family.
• US-only inference costs more. Setting inference_geo to "us" applies a 1.1x multiplier to all token prices for Claude 4.6 and later models.

All four current models share a 1M token context window, 128K max output, text and image input and a June 2026 reliable knowledge cutoff, according to the models overview.

Against its predecessor, the pricing page lists Claude Fable 5 at the same USD 10 input and USD 50 output, but with cache hits at USD 1 per 1M tokens versus USD 0.25 for Fable 5.1.

1. Start from the tasks where Opus 5.5 already fails at xhigh or max effort; Anthropic's guide frames Fable 5.1 as the fix for that gap.
2. Run those tasks, plus a few Opus 5.5 already passes, on Fable 5.1 at the default high effort with identical prompts and tools, and score each against your own rubric.
3. Record input, cached and output tokens per task, then multiply by list prices to compare cost per completed task.
4. Route only the task types where Fable 5.1's pass rate covers its price premium, and leave the rest on Opus 5.5 or cheaper models.

We read Anthropic's models overview, pricing page, choosing a model guide and model deprecations page on 15 October 2026. We could not load the dedicated Fable 5.1 model page during this review, so platform availability, release date and batch output limits are not stated here. We have not run Claude Fable 5.1, and nothing here is a test result or ranking. See our ranking method for how HumanRank scores tools and the model directory for other profiles.

What is Claude Fable 5.1? It is the model Anthropic calls its most capable one open to all customers, aimed at demanding reasoning and long-horizon agentic work.

How much does Claude Fable 5.1 cost? It costs USD 10 per 1M input tokens and USD 50 per 1M output tokens at list price on 15 October 2026, with cache hits at USD 0.25 per 1M tokens.

What is the Claude Fable 5.1 context window? It is 1M tokens, with up to 128K output tokens per response.

Is Claude Fable 5.1 the same as Claude Mythos 5.1? No, they are separate model IDs, though Anthropic says Mythos 5.1 offers the same capabilities only to organizations verified through its verification programs.

Can I use fast mode with Claude Fable 5.1? No, the pricing page lists fast mode only for Claude Opus 5.5, Claude Opus 5 and Claude Opus 4.8.

Should I start with Fable 5.1 or Opus 5.5? Start with Opus 5.5, because Anthropic's own guide recommends moving to Fable 5.1 only when Opus 5.5 at high effort still falls short.