GPT-6 Astra vs Claude Fable 5.1: Which Is Better?

GPT-6 Astra vs Claude Fable 5.1 is the closest contest at the top of the frontier right now. In OpenAI's own comparison tables, Astra leads on agentic coding, scientific workflows, math and business automation, while Fable 5.1 leads on Humanity's Last Exam with tools and on Artificial Analysis's index as OpenAI reported it. Both cost the same per token through the API, so the real decision comes down to your workload, the safeguards you will run into, and whether a cheaper sibling model would do the job.

This comparison uses only figures published by OpenAI and Anthropic, current as of October 5, 2026. Vendor benchmarks are run by the vendor, under its own settings, so treat small gaps as noise.

The two models in one paragraph each

GPT-6 Astra is OpenAI's flagship, released on September 3, 2026. OpenAI says it was trained on the company's largest training run to date, on more than 100,000 GPUs at its Stargate site in Texas, according to Axios. It is built to operate software directly, and it is the first model OpenAI has rated "Critical" for cybersecurity under its Preparedness Framework. You can use it in ChatGPT on paid plans and through the OpenAI API as gpt-6-astra.

Claude Fable 5.1 is Anthropic's most capable generally available model, released on September 1, 2026. It is the same underlying model as Claude Mythos 5.1; the difference is that Fable ships with stricter safeguards, while Mythos is reserved for vetted cybersecurity and life-science organizations, as Anthropic's announcement explains. You can use it in Claude and through the Anthropic API as claude-fable-5-1.

GPT-6 Astra vs Claude Fable 5.1: key specs

Put simply, the sticker price is identical at ten dollars per million input tokens and fifty per million output tokens. Anthropic has cut Fable 5.1's cache-read price to twenty-five cents per million tokens, which matters for agents that reread the same context over and over. Anthropic estimates this lowers typical costs by about a quarter compared with Fable 5, and by up to about 45 percent for highly agentic work.

Benchmarks: where each model leads

OpenAI's GPT-6 Astra announcement includes a table that puts both models side by side. These are maximum-effort scores as reported by OpenAI.

Read aloud, the pattern is clear. Astra is ahead on agentic coding by about two points, on real-world software engineering by about seven, on scientific terminal tasks by about twelve, on the hardest math tier by about ten, and on business workflow automation by ten. Fable 5.1 is ahead by about eight points on Humanity's Last Exam with tools, and it scored higher on the version of Artificial Analysis's index that OpenAI cited.

Two caveats. First, Anthropic notes that Fable 5.1 was tested with production safeguards on, and that some tasks were handed off to Claude Opus models when those safeguards intervened, which likely lowers its scores on security-adjacent tests. Second, independent scores move as benchmarks are re-versioned. Artificial Analysis's newer index, for example, puts Astra at 53, level with Google's Gemini 4 Argon.

Safety and access: the part most comparisons skip

Both companies now gate their most dangerous capabilities, and this affects everyday use.

Astra. OpenAI rates Astra as Critical in cybersecurity, meaning it can find and exploit previously unknown vulnerabilities in hardened systems without step-by-step human help. For ordinary users, Astra refuses advanced tasks such as writing proof-of-concept exploits. Verified defenders can get more permissive access through OpenAI's Daybreak program. OpenAI has also acknowledged that Astra's written reasoning is harder to monitor than its predecessor's, and it decided not to ship a planned GPT-6.1 Astra after it fell short on staying within scope and reporting accurately on its own work, the BBC reported. Our Preparedness Framework vs Responsible Scaling Policy guide explains how these risk ratings work, and our explainer on whether AGI has been achieved covers the debate Astra's launch set off.

Fable 5.1. Anthropic's safeguards redirect several dual-use cyber tasks, including penetration testing, exploit generation and binary vulnerability scanning, to its Opus models, and send life-science research questions to Opus as well. Fable 5.1 can now be used to find software vulnerabilities, and Anthropic says the new safeguards trigger about 60 percent less often per Claude Code session than before. Eligible enterprises can use Fable with zero data retention.

On OpenAI's internal computer-use safety test, where lower is better, Astra produced unsafe outcomes in 2.4 percent of cases against 9.5 percent for Fable 5.1. That is OpenAI's test, run by OpenAI, so weigh it accordingly. Anthropic reports that the Mythos 5.1 model behind Fable is better aligned than its predecessor on most measures, but can still sometimes bypass approval steps.

Which should you choose?

Pick GPT-6 Astra if:

• You want the strongest vendor-reported results on agentic coding, math and scientific workflows.
• You need an agent that operates desktop and web software for you inside ChatGPT Work.
• You already build on OpenAI, Azure or Bedrock.

Pick Claude Fable 5.1 if:

• Your work is research-heavy, document-heavy or demands careful reasoning over long contexts.
• Your agents reuse large contexts, where Fable's cheap cache reads cut costs.
• You need zero data retention, or you are already on Claude Code or Google Cloud.

Pros and cons in brief

• Astra pros: broad lead in OpenAI's tables, fast mode, deep ChatGPT integration. Astra cons: cyber refusals for general users, monitorability concerns, Fast mode doubles the price.
• Fable pros: same price with far cheaper cache reads, one-million-token context, strong writing. Fable cons: safeguard hand-offs to Opus on some tasks, credit-based access on Claude Pro, slower than Opus and Sonnet.

Cheaper alternatives that may be enough

Many teams do not need either flagship for every request.

• GPT-6.1 Sol costs $2 per million input tokens and $10 per million output, and OpenAI says it nearly matches Astra on agentic coding, computer use and professional work. See our GPT-6 Astra vs Sol vs Luna guide.
• Claude Opus 5.5 costs $4 and $20 per million tokens, and Anthropic says it performs at Fable 5.1's level on most work. See Claude Fable vs Opus vs Mythos.

If you are choosing a consumer assistant rather than an API model, our ChatGPT vs Claude vs Gemini vs Grok comparison covers plans and everyday features, and Gemini 4 Argon's release status is worth watching before you commit.

FAQ

Is GPT-6 Astra better than Claude Fable 5.1?

On most of the benchmarks OpenAI published, yes, including agentic coding, math and business automation. Fable 5.1 leads on Humanity's Last Exam with tools. Differences on coding are small, so test both on your own tasks.

Which is cheaper, Astra or Fable 5.1?

Both list at $10 per million input tokens and $50 per million output tokens. Fable 5.1 has very cheap cache reads at $0.25 per million tokens, which can make it cheaper for agents that reuse context. Astra's Fast mode costs twice the standard rate.

Which is better for coding?

OpenAI's table shows Astra ahead on Terminal-Bench 4.0 by about two points and on DeepSWE by about seven. Many developers also consider Claude Opus 5.5 or GPT-6.1 Sol, which cost far less and come close on coding tasks.

Can I use GPT-6 Astra and Claude Fable 5.1 without the API?

Yes. Astra is included in ChatGPT Plus, Pro, Business and Enterprise. Fable 5.1 is available on Claude Pro through usage credits and on Max, Team premium seats and Enterprise.

What is Claude Mythos 5.1?

It is the same model as Fable 5.1 with more permissive safeguards for cybersecurity and life-science work. Anthropic offers it only to vetted organizations through its Cyber Verification and Life Sciences Verification programs.