Claude Fable 5.1, the Anthropic model in numbers

Claude Fable 5.1 at a glance

Anthropic released Claude Fable 5.1 on September 1, 2026. It has a reasoning mode: it can "think" before answering, which helps with multi-step problems but uses more tokens, and therefore more money. It is a closed model: you access it through an API, from Anthropic or one of the 38 providers listed on models.dev. It is a very recent release.

We file it under frontier models (24 models in this category on the site).

Measured performance

According to results published by Epoch AI (checked on October 22, 2026), Claude Fable 5.1 has a capability index of 92.6 out of 100, computed on 4 benchmarks from our basket.

That puts it at #6 in our weekly model ranking.

• OTIS Mock AIME (competition mathematics problems): 100% — the best tracked model, GPT-5.5 Pro, reaches 100%
• FrontierMath (very hard research-level mathematics): 90.2% — the best tracked model, GPT-6 Astra, reaches 93.7%
• FrontierCode (advanced programming tasks): 50.9% — the best tracked model, Claude Opus 5.5, reaches 54.6%
• APEX-Agents (professional tasks carried out by an agent): 68.6% — the best tracked model, Gemini 4 Argon, reaches 82.4%

Technical specifications

Its context window reaches 1,000,000 tokens, roughly 1,500 pages of text at 500 words per page: enough to load a whole book or a mid-sized codebase. A single answer can run to 128,000 tokens.

It accepts as input: text, image and PDF. Its knowledge stops at June 2026: beyond that, it needs web search or your documents.

It can call tools (functions, APIs) and return structured JSON output, which makes it a possible foundation for an AI agent.

How much does Claude Fable 5.1 cost?

Claude Fable 5.1 costs $10 per million input tokens and $50 per million output tokens (developer's price, checked on October 22, 2026).

To put it in perspective: reading 20,000 tokens (about thirty pages) and writing 2,000 costs roughly $0.30; a thousand of those tasks, $300.

That is 3.4 times the median output price of frontier models ($14.5).

Tokens read back from cache cost $0.25 per million, which helps an agent that keeps resending the same context.

To estimate your own budget, use the agent cost calculator.

Who is it for?

Given its category, Claude Fable 5.1 is mainly aimed at hard tasks where quality matters more than cost: deep reasoning, complex code, long analyses.

Its FrontierCode result (50.9%, above the median of tracked models) makes it a serious candidate for coding agents.

Its very large context window makes it worth considering for analysing large sets of documents in a single request.

Before adopting it, test it on your own examples: benchmarks measure standard tasks, not your job.

Alternatives

In the same category, the models closest to Claude Fable 5.1 are:

• Claude Opus 5 (Anthropic, index 92.7) — see the comparison
• GPT-6.1 Sol (OpenAI, index 92.1) — see the comparison
• Claude Fable 5 (Anthropic, index 90.8) — see the comparison
• GPT-6 Astra (OpenAI, index 94.7) — see the comparison