GPT-6 Astra vs Claude Fable 5.1: Which to Use (2026)

GPT-6 Astra and Claude Fable 5.1 are the two newest flagship AI models, and the difference between them is what each was built to do: Astra is built to drive your screen, Fable 5.1 is built to work on one problem for hours.

This comparison covers what each model is, what the launch benchmarks say, what the two cost once you are using them daily, and which one to reach for depending on the work in front of you.

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Both companies published their own numbers on launch day. Each set shows the model doing the thing it was optimised for, which is why Astra leads on maths and Fable 5.1 leads on reasoning. Read side by side they tell you what each lab built for, not which one you should use.

The question that decides it is narrower: what job are you handing the model. The rest of this article answers that.

GPT-6 Astra is OpenAI's flagship, released on 3 September 2026. It lives inside ChatGPT (Plus, Pro, Business and Enterprise) and Codex, and it is built to drive your computer: hand it your screen and it fills forms, updates your CRM, and turns a brief into documents, spreadsheets and presentations in your own templates.

Claude Fable 5.1 is Anthropic's top model, released on 1 September 2026. It is the same model as Mythos 5.1, which only approved organisations can access, with extra safeguards for everyone else. It is built for long, sustained work: research, code review, and tasks that run for hours without you watching.

Both give you a 1 million token context window, roughly 1,500 pages of text in one conversation. Both cost the same at list price for developers, $10 in and $50 out per million tokens. On paper that is a tie. The differences show up in use.

Benchmarks are the ones each company published at launch, so treat them as a direction, not a verdict.

Astra is the stronger of the two at taking over your screen: forms, CRM updates, a deck in your brand template built from one brief. It scores 97.6% on FrontierMath Tier 4, and OpenAI reports the hallucination rate dropped to 4.2% from 12.2% on GPT-5.6 Sol. If you already pay for ChatGPT, it is included in your plan.

It costs more in long sessions. Cached reads cost four times what Claude charges, and there is a surcharge above 272K tokens, so an agent that runs all day adds up. Its launch also carried two caveats: OpenAI says it is the first model to hit the "Critical" cybersecurity threshold in its own safety framework, and the release landed in the same week US senators proposed a pause on advanced AI development.

Fable 5.1 scores higher on reasoning and on work that runs long. It takes 65.0% on Humanity's Last Exam with tools against Astra's 57.2%, agentic coding went from 42% on Fable 5 to 55.8%, and agentic scientific research more than doubled. Cache reads now cost 75% less than before, so Anthropic says a workload that runs for hours costs about 25% to 45% less than it did a month earlier.

It is behind Astra on maths, 87.8% against 97.6%, and on screen-driving benchmarks, and it is not the one to reach for when you want a slide deck built inside your own template. It also thinks longer at full effort, so a single task can cost more, $3.76 against $1.67 on the Artificial Analysis Intelligence Index tasks, even though the list price is the same.

If you are still choosing your first assistant, this comparison does not decide anything for you. Pick one and move on, because either will carry you through everything a beginner needs to learn. Our setup guide for ChatGPT, Claude and Copilot walks through the first configuration.

If you already have one, stay with it. The gap between these two models is smaller than the gap between a reader who briefs a model well and one who does not, and briefing is the part you control. The 26 principles of prompt engineering is where to start on that.

If you want to know what we do: Claude Fable 5.1 for thinking, writing, research and anything that runs long. GPT-6 Astra for the admin, the forms, the CRM, the deck that has to match the template. That is the one job we hand to Astra.

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Neither is better across the board. On the numbers each company published at launch, Astra leads on maths (97.6% against 87.8% on FrontierMath Tier 4) and coding (57.9% against 55.8% on Terminal-Bench 4.0), and Fable 5.1 leads on reasoning (65.0% against 57.2% on Humanity's Last Exam with tools). Astra is the stronger one at driving your screen. Fable 5.1 is the stronger one at work that runs for hours.

List price is identical for developers: $10 per million tokens in, $50 out. The difference shows up in use. Cached input costs $1.00 per million on Astra against $0.25 on Fable 5.1, and Astra adds a surcharge above 272K tokens. Fable 5.1 thinks longer at full effort, so a single task can cost more, $3.76 against $1.67 on Artificial Analysis Intelligence Index tasks.

They are the same model. Mythos 5.1 is available only to approved organisations. Fable 5.1 is the version everyone else gets, with extra safeguards.

No. One is enough for almost everyone. Paying for both makes sense only if you have work in both shapes, long research on one side and screen-driving admin on the other, and you are already using your current model close to its limit.

Either. At beginner level the model you choose changes less than how you brief it. Pick the one attached to a subscription you already have, and spend the time on the briefing instead. The AI Bootcamp covers that across 36 free chapters.