GPT-6 Astra vs Claude Fable 5.1: The Two Most Powerful AI Models of 2026, Compared

OpenAI and Anthropic shipped their top models three days apart, both at the same price. We break down what each one is best at, what they really cost, and which one is right for you, in plain English.

• On September 1, 2026, Anthropic released Claude Fable 5.1. Three days later, on September 3, OpenAI released GPT-6 Astra. For the first time in a while, the two most powerful Artificial Intelligence (AI) models in the world arrived in the same week, and both landed at exactly the same price: $10 per million input tokens and $50 per million output tokens. Tokens are the small chunks of text AI models read and write, roughly three quarters of a word each.
• This is the very top of the AI market right now. If you want the most capable model money can buy, one of these two is almost certainly it. So which should you choose?
• This guide breaks down what each model is, what it can do, what they really cost once you look past the sticker price, and how they score on the tests each company published. The one line answer: they cost the same, but they win at different things, so the right pick depends on the kind of work you do. We will show you exactly where each one pulls ahead.

Meet the Two Models

GPT-6 Astra is OpenAI's newest flagship and the first model in its GPT-6 family. It is a single large reasoning model, which means it is built to think through hard problems step by step. Unlike some past launches, there is no smaller "mini" or "nano" version at launch: Astra is the one model. On the OpenAI Application Programming Interface (API), the connection developers use to plug a model into their own software, its id is gpt-6-astra.

Claude Fable 5.1 is Anthropic's most capable widely released Claude model. It is the direct successor to Claude Fable 5 and sits at the top of the Claude line. On the Claude API its id is claude-fable-5-1.

Both are aimed at the same buyer: people and teams who want the strongest possible model for demanding work like coding, research, deep writing, and running AI agents (software that carries out multi step tasks on its own).

The Specs, Side by Side

Here is what each model can handle, in plain terms.

Context window (how much text the model can read at once, measured in tokens): GPT-6 Astra reads up to 1,050,000 tokens and can write up to 128,000 tokens in one reply, with room for about 922,000 tokens of input. Claude Fable 5.1 reads up to 1,000,000 tokens (often written 1M) and also writes up to 128,000 tokens. Both are enormous. Either can read a very long book or a large codebase in a single go.

What they can read and write: both take text and images as input and produce text as output. Neither generates video or audio here.

Thinking effort: GPT-6 Astra lets you pick from five reasoning effort levels (low, medium, high, xhigh, and max), so you can trade speed for depth. Claude Fable 5.1 always has adaptive thinking switched on, and it sets the effort for you: high by default on the API and in Claude Code, and medium on claude.ai and Cowork.

Speed: GPT-6 Astra runs at roughly 54 to 86 tokens per second. Claude Fable 5.1 is the slower model in its family, because that always on thinking takes time.

Knowledge cutoff (the date the model's training knowledge stops): GPT-6 Astra knows about the world up to April 30, 2026. Claude Fable 5.1 goes up to June 2026, the freshest of any Claude model. So on recent events and newer software libraries, Fable 5.1 has the more up to date knowledge.

What They Actually Cost

On paper, these two models cost exactly the same: $10 per million input tokens and $50 per million output tokens. If you only glanced at the sticker price, you would call it a tie.

The real difference is in the details, and it matters a lot for heavy users.

Cache reads: when you send the same big chunk of text again and again (say a codebase or a long document the model keeps referring to), providers let you "cache" it so you pay less on repeat reads. GPT-6 Astra charges $1.00 per million tokens for cache reads. Claude Fable 5.1 charges just $0.25, a quarter of the price. Cache writes are $12.50 for both, and Fable also offers a longer one hour cache write at $20.

Long context surcharge: with GPT-6 Astra, once your prompt goes above 272,000 tokens, the price jumps to $20 input and $75 output. Claude Fable 5.1 has no such surcharge: it stays $10 and $50 across the full 1M window.

Discount modes: GPT-6 Astra offers Batch and Flex modes at 50% off, and a Fast mode at twice the standard price. Claude Fable 5.1's Batch mode is $5 input and $25 output.

Put it together and the sticker price is identical, but Fable 5.1's cache reads are four times cheaper and it never charges extra for very long prompts. So on agentic work, and any job that re sends a large fixed prompt, Fable 5.1 tends to be cheaper in real use. Anthropic estimates roughly 25% lower cost on typical workloads, and up to 45% on highly agentic ones, compared with the older Fable 5.

Source: OpenAI API pricing (developers.openai.com/api/docs/pricing) and Anthropic Claude Fable 5.1 model page and release notes (platform.claude.com).

The Benchmarks, Honestly

Before any numbers, one honest warning: almost all of these scores are reported by the companies themselves, each using its own test setup. Treat them as a guide, not gospel. Where a number comes from one specific company, we say so.

Coding: on Terminal-Bench 4.0, a test of agentic coding (the model works through a real coding task in a terminal on its own), OpenAI's table shows GPT-6 Astra at 57.9%, Claude Fable 5.1 at 55.8%, and Claude Opus 5 at 52.6%. Very close at the top. But on CursorBench 3.2.0, a coding test run inside the Cursor code editor and reported by Anthropic, Fable 5.1 scores 73.4%, and Cursor called it their best scoring model on day one.

Math and science: this is where GPT-6 Astra pulls clearly ahead in OpenAI's tests. On FrontierMath Tier 4 v2, a set of very hard math problems, Astra scores 97.6% to Fable's 87.8%. On GPQA Diamond, a set of graduate level science questions, Astra scores 96.0% to Fable's 93.7%. On Terminal-Bench Science 0.1, an agentic research test, Astra leads 64.6% to 52.6%.

Reasoning and knowledge: on Humanity's Last Exam, a broad and very difficult reasoning test, run with tools, Claude Fable 5.1 wins with 65.0% to Astra's 57.2%. And independent aggregate rankings from Artificial Analysis, a third party that pools many tests, put Fable 5.1 at number 1 for agentic work, coding, and knowledge, and number 2 for reasoning, while GPT-6 Astra ranks number 1 for pure reasoning and lands in the top 5 on agentic, coding, and knowledge.

Hallucinations (when a model states something false as if it were true, where lower is better): OpenAI reports GPT-6 Astra at a 4.2% hallucination rate, far below the 12.2% it reports for its earlier GPT-5.6 Sol.

The fair summary: OpenAI's own tests put Astra ahead on most coding, math, and science, and on staying factual. Independent aggregate rankings put Fable 5.1 on top for agentic work, coding, and knowledge, with Astra strongest on pure reasoning.

Source: OpenAI GPT-6 Astra launch page (openai.com/index/gpt-6-astra), Anthropic Claude Fable 5.1 model page (platform.claude.com), and independent ranks from Artificial Analysis. All vendor scores are self reported by each company.

Where Each One Wins

Since they cost the same on paper, the smart choice is about fit. Here is the short version.

Pick GPT-6 Astra if you need: the hardest math and science work, where its FrontierMath and GPQA scores lead; heavy tool use and computer use agents, since it ships with web search, file search, a code interpreter, a hosted shell, computer use, and image generation built in; a home in the OpenAI and ChatGPT world your team already uses; or the lowest hallucination rate, which matters when being wrong is expensive.

Pick Claude Fable 5.1 if you need: long running agentic coding, where it holds the top independent rank and Cursor's best day one score; serious writing and knowledge work; the cheapest real cost on jobs that re send a big fixed prompt, thanks to cache reads that are four times cheaper and no long context surcharge; the freshest knowledge, up to June 2026; or the widest choice of platforms to run it on.

How to Get Each One

GPT-6 Astra: inside ChatGPT it appears as "GPT-6 Astra Pro" for people on the ChatGPT Pro, Business, and Enterprise plans. Developers reach it through the OpenAI API using the id gpt-6-astra.

Claude Fable 5.1: you can use it on claude.ai (Pro, Max, Team, and Enterprise plans) and through the Claude API. It is also available very widely elsewhere: Claude Code, Claude Cowork, Amazon Bedrock, Google Cloud (Vertex), Microsoft Foundry, DigitalOcean, and GitHub Copilot. If you want the same top model no matter which cloud you already pay for, Fable 5.1 is the easier one to reach.

One safety note worth knowing: OpenAI rates GPT-6 Astra at the "Critical" tier for cybersecurity under its Preparedness Framework, so the public model refuses advanced offensive security tasks. Anthropic ships Fable 5.1 with a text watermark and content credentials on generated media, requires 30 day data retention, and pairs it with a restricted twin, Claude Mythos 5.1, that is available for vetted research programs only.

Our Take

Here is the honest bottom line. GPT-6 Astra and Claude Fable 5.1 are the two best AI models you can buy right now, they launched three days apart, and they charge the same sticker price. There is no single winner. They are built to shine at different things.

If your work leans on the hardest reasoning, math, and science, or your team already lives inside ChatGPT, go with GPT-6 Astra. If you do long running agentic coding, deep writing, or you care about the real cost on big repeated prompts and the freshest knowledge, go with Claude Fable 5.1.

For most people the deciding factor will not be a benchmark. It will be the kind of work you do every day and the tools you already use.

Not sure which fits your work? Take our free 60 second AI Match quiz at aitoolsmentor.com/wizard.