GPT-6 Astra vs Claude Fable 5.1: Which Frontier Model Wins in 2026?
Introduction
September 2026 delivered the two most significant frontier model releases of the year within 48 hours of each other. OpenAI launched GPT-6 Astra on September 3 — an agentic model that saturated ARC-AGI-3 at 99.9% and posts the strongest alignment results OpenAI has reported. Anthropic answered with Claude Fable 5.1 on September 1 — the same base model as the restricted Mythos 5.1, with cache pricing cut 75% and Terminal-Bench results that make the coding race genuinely close again.
Both cost $10/$50 per 1M tokens. Both target agentic work. So this comparison comes down to execution details: benchmark by benchmark, dollar by dollar. All numbers below come from the vendors' official model cards and announcements.
If you want a deeper look at either model individually, see our GPT-6 Astra review and our Claude review.
Overview
GPT-6 Astra
OpenAI's flagship, announced September 3, 2026:
- 99.9% on ARC-AGI-3 (Sol scored 7.8%) — the benchmark is effectively saturated
- State of the art on computer use, browsing, and software engineering
- 1M token context with 96.3% recall at 512K–1M (MRCR v2)
- First OpenAI model to cross the Critical cybersecurity threshold
- Available in ChatGPT (Plus and up), API, Azure, and Amazon Bedrock
Claude Fable 5.1
Anthropic's update to Fable 5, available September 1, 2026 on all platforms:
- Same base model as Mythos 5.1, with production-grade safeguards
- Terminal-Bench 4.0 at 55.8% (Mythos variant: 60.9%)
- Cache reads cut 75% to $0.25 per 1M tokens — ~25-45% cheaper workloads than Fable 5
- 77.9% OSWorld 2.0 partial completion
- Available via Claude apps, API (
claude-fable-5-1), AWS, Google Cloud, and Azure
Features Comparison
| Feature | GPT-6 Astra | Claude Fable 5.1 |
|---|---|---|
| Coding | Excellent | Excellent |
| Computer use | Excellent (fastest) | Excellent (most reliable) |
| Long context | 1M tokens | Very long (unspecified) |
| Async agentic work | Yes (Codex) | Yes (Claude Code, background computer use) |
| Security work | Restricted (Critical threshold) | Discovery allowed, exploitation blocked |
| Cache pricing | Separate rate | $0.25/M (75% cut) |
| Open availability | All ChatGPT paid plans | All platforms |
Benchmark Comparison
Coding
| Benchmark | GPT-6 Astra | Claude Fable 5.1 | Claude Mythos 5.1 |
|---|---|---|---|
| Terminal-Bench 4.0 | 57.9% | 55.8% | 60.9% |
| DeepSWE v1.1 | 74.1% | — | — |
| CursorBench 3.2.0 | — | 73.4% | — |
Terminal-Bench 4.0 is the cleanest head-to-head: Astra wins by 2.1 points, but Mythos 5.1 — the same weights as Fable with lighter safeguards — actually beats Astra. The practical takeaway: at the frontier, coding differences are within noise, and workflow fit matters more than leaderboard position.
Agentic and Computer Use
| Benchmark | GPT-6 Astra | Claude Fable 5.1 |
|---|---|---|
| OSWorld 2.0 (partial) | 72.6% | 77.9% |
| OSWorld 2.0 (strict) | — | 41.7% |
| Agents' Last Exam | 59.3% | — |
| AutomationBench | 41.4% | 31.4% |
| ScreenSpot-Pro | 92.7% | — |
Astra's speed advantage is real — OSWorld tasks in ~40 minutes vs Sol's 75 — and AutomationBench shows a 10-point gap on long automation pipelines. But Fable 5.1 is the stronger screen-grounded performer on OSWorld partial completion, and Browserbase measured it finishing 82% of browser tasks at ~10 minutes each. Astra moves faster; Fable 5.1 finishes more reliably.
Reasoning
| Benchmark | GPT-6 Astra | Claude Fable 5.1 |
|---|---|---|
| ARC-AGI-3 | 99.9% | — |
| Humanity's Last Exam (no tools) | — | 60.9% |
| Humanity's Last Exam (with tools) | 57.2% | 65.0% |
| GPQA Diamond | 96.0% | — |
| FrontierMath Tier 4 | 97.6% | — |
Astra dominates abstract reasoning and formal math. Fable 5.1 posts the better Humanity's Last Exam scores, particularly with tools. These are different test philosophies — novel puzzle-solving (ARC) versus broad expert knowledge (HLE) — and the models genuinely split them.
Pricing Comparison
| Cost Item | GPT-6 Astra | Claude Fable 5.1 |
|---|---|---|
| Input / 1M | $10.00 | $10.00 |
| Output / 1M | $50.00 | $50.00 |
| Cache read / 1M | Separate rate | $0.25 (75% cut) |
| Typical workloads vs predecessor | — | ~25% cheaper |
| Highly agentic workloads vs predecessor | — | ~45% cheaper |
| Speed mode | 2x price for up to 2x speed | Effort levels (high default in Claude Code) |
Headline pricing is identical. The cache-read difference is the story: agentic workloads re-read long contexts on every step. At $0.25 per 1M cached tokens, a Fable 5.1 agent looping over a 200K context pays a fraction of what an equivalent Astra workflow costs. Anthropic explicitly engineered Fable 5.1's pricing for agent economics.
Safety and Restrictions
| Aspect | GPT-6 Astra | Claude Fable 5.1 |
|---|---|---|
| Unauthorized actions | 0% (Sol: 48%) | — |
| Hallucination rate | 4.2% (Sol: 12.2%) | — |
| Cybersecurity posture | Critical threshold; refuses exploit PoCs | Discovery allowed, exploitation blocked |
| Unrestricted variant | Astra Pro (paid plans) | Mythos 5.1 (verified US orgs only) |
| Watermarking | — | Invisible watermark per EU AI Act |
Both vendors now gate their strongest variants: OpenAI restricts by request type, Anthropic by organization verification. Notably, Astra found and disclosed two zero-day vulnerabilities during evaluation, and Fable 5.1 cut security false positives by 60% — safety tuning that unblocks legitimate work is the actual differentiator here.
Which Should You Choose?
| Your situation | Pick | Why |
|---|---|---|
| Long-running agent loops on a budget | Fable 5.1 | $0.25 cache reads reshape agent economics |
| Speed-critical computer automation | Astra | ~40 min/task on OSWorld, 1.9x faster on Mind2Web |
| Formal math and abstract reasoning | Astra | 97.6% FrontierMath, 99.9% ARC-AGI-3 |
| Coding inside an existing IDE workflow | Fable 5.1 | CursorBench lead; Claude Code maturity |
| Broad expert knowledge with tools | Fable 5.1 | HLE 65.0% with tools |
| Cybersecurity (defensive) | Fable 5.1 or Mythos | More permissive for verified defenders |
| Already on ChatGPT Plus/Business | Astra | Included in plan; no extra subscription |
Summary
These models are close enough that neither dominates. GPT-6 Astra takes abstract reasoning, agent speed, and raw benchmark ceilings. Claude Fable 5.1 takes computer-use reliability, cache economics, and the practical cost of running agents all day. Both cost $10/$50 per 1M tokens, both restrict security capabilities in different ways, and both represent the 2026 frontier.
The pragmatic advice: match the model to the workload's cost profile. Cache-heavy agent loops favor Fable 5.1's pricing; compute-heavy one-shot reasoning favors Astra's ceiling. And if you're on either vendor's subscription already, the included tier of that ecosystem's flagship is the path of least resistance — see our ChatGPT vs Claude breakdown for the assistant-level view.
Frequently Asked Questions
Is GPT-6 Astra better than Claude Fable 5.1?+
How do the prices of GPT-6 Astra and Claude Fable 5.1 compare?+
Which model is better at coding?+
Which is better for computer use and agents?+
Are there safety restrictions on either model?+
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