Question 1

What is Claude Fable 5.1?

Accepted Answer

Claude Fable 5.1 is Anthropic’s top-tier model, released on September 1, 2026. Anthropic recommends it for demanding reasoning and long-horizon agentic work. It has a 1M-token context window on Anthropic’s API and up to 128K output tokens.

Question 2

How do I use Fable 5.1 on Runable?

Accepted Answer

Sign in to Runable, start a new task and choose Fable 5.1 in the model menu next to the prompt box. Describe what you need and Runable runs the job on Claude Fable 5.1.

Question 3

Do I need a Claude subscription or API key?

Accepted Answer

No. Fable 5.1 runs on your Runable plan and credits, so you do not need an Anthropic account or API key.

Question 4

Fable 5.1 vs Opus 5.5: which should I use?

Accepted Answer

Anthropic suggests starting with Opus 5.5 for most work and moving to Fable 5.1 when a task still falls short. Opus 5.5 costs less than half as much per token, and in October 2026 Artificial Analysis ranked it above Fable 5.1. Use Fable 5.1 for the hardest reasoning and longest unattended runs.

Question 5

Fable 5.1 vs GPT-6 Astra: which is better?

Accepted Answer

They have the same API list price, $10 input and $50 output per million tokens. In October 2026 Artificial Analysis scored both 53 on its Intelligence Index. Fable 5.1 was faster, at about 69 tokens per second against 58, while Astra cost less per task in their runs, about $3.26 against $7.63. On Runable you can run both on the same task.

Question 6

What is Claude Mythos 5.1?

Accepted Answer

Mythos 5.1 is the same model as Fable 5.1 with different safeguards. Anthropic offers it only to vetted cyber-defence and life-science organisations. Fable 5.1 is the version that is generally available.

Question 7

What is the context window of Fable 5.1?

Accepted Answer

1 million tokens on Anthropic’s API, with up to 128K output tokens per response. On Runable, Fable 5.1 runs with a 150K-token context window.

Question 8

What can I make with Fable 5.1 on Runable?

Accepted Answer

Web apps and websites, research reports, financial models, slide decks, documents and spreadsheets. Runable gives the model a browser, a code sandbox and publishing tools to finish the job.

Model	Best for	Speed	API price in / out
Fable 5.1	Hardest reasoning, long agent runs	69 tokens/s	$10 / $50
Opus 5.5	Long, complex agent work	96 tokens/s	$4 / $20
Sonnet 5.5	Everyday tasks, best speed for the smarts	134 tokens/s	$2 / $10
Haiku 5.5	High-volume, quick tasks	235 tokens/s	$0.10 / $0.50 *

Claude Fable 5.1

Made on Runable

Fable 5.1 on Runable

Long jobs, unattended

Research with search

Switch models anytime

How to use Fable 5.1 on Runable

Select Fable 5.1

Describe the job

Get the finished work

One prompt, finished work

Why choose Fable 5.1

Science and research

Fixes root causes

Real knowledge work

Strong coder

Uses a computer

Cheaper than Fable 5

Fable 5.1 specs

Fable 5.1 vs Opus, Sonnet and Haiku 5.5

The only AI agent you need

Fable 5.1 FAQs

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