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GPT-6 Astra vs Claude Fable 5.1: which frontier API should you build on?

Published 2026-09-06 · Researched and price-checked on official sources

GPT-6 Astra vs Claude Fable 5.1 is not a price decision: OpenAI and Anthropic both list their new flagship APIs at $10 per million input and $50 per million output tokens as of September 6, 2026. Independent evaluations published this week split the pair, with Fable 5.1 leading the coding-agent index while Astra dominates cross-file code review.

Key facts

What matters

OpenAI's GPT-6 Astra and Anthropic's Claude Fable 5.1 both charge $10 per million input and $50 per million output tokens (standard tier), an exact price match verified on both official pricing pages on September 6, 2026.
Cached input reads are 4x cheaper on Claude Fable 5.1 ($0.25 vs $1.00 per million tokens), which compounds for long-running agents that re-read the same context.
Artificial Analysis (September 3, 2026): Claude Fable 5.1 leads the Coding Agent Index at 70 in Claude Code; GPT-6 Astra scores 67 in Codex, level with Claude Opus 5 and Fable 5.
CodeRabbit's code review evaluation (September 4, 2026): GPT-6 Astra caught 22% more actionable bugs than Claude Opus 5 overall and 33% more on cross-file reviews.
Zero data retention: OpenAI supports ZDR for eligible GPT-6 Astra API customers; Anthropic defaults Fable to 30-day retention with ZDR for eligible customers as Enterprise Frontier Safeguards rolls out (per CodeRabbit's policy rundown).

What is GPT-6 Astra?

GPT-6 Astra is OpenAI's most capable model, built for the hardest end-to-end work, according to OpenAI's model documentation. OpenAI announced the rollout on September 3, 2026; the model appeared on OpenRouter the next day, September 4.

Access is staged. CNBC reports that a limited group of companies in OpenAI's application-based cybersecurity program Daybreak went first, with ChatGPT Plus, Pro, Business, and Enterprise plans plus the OpenAI API and Amazon Web Services following. OpenAI disclosed that Astra is its first model to reach the internal Critical cybersecurity threshold and plans to limit its most advanced cyber capabilities to vetted customers.

Verified specifications from OpenAI's model card: a 1,050,000-token context window, 128,000 max output tokens, an April 30, 2026 knowledge cutoff, reasoning effort levels from low to max, and tools for functions, web search, file search, and computer use. It replaces GPT-5.6 Sol at the top of the lineup, where Sol's promotional $4/$20 pricing runs at least through November 21, 2026.

What is Claude Fable 5.1?

Claude Fable 5.1 is Anthropic's next-generation intelligence for long-running agents, per Anthropic's pricing page. It has been listed on OpenRouter since September 1, 2026, days before OpenAI's counter-launch.

The price card reads $10 per million input tokens and $50 per million output, with cached reads at $0.25 and cache writes at $12.50. OpenRouter lists its context window at 1,000,000 tokens. Anthropic's pricing page does not publish API context windows or knowledge cutoffs for any model, so those cells stay empty in the comparison below.

Fable 5.1 sits above Claude Opus 5 ($5/$25, positioned for complex agentic coding and enterprise work) and Claude Sonnet 5 ($2/$10) in Anthropic's September 2026 lineup.

Same $10/$50 sticker: where the two APIs actually differ

Both vendors charge $10 per million input and $50 per million output, and both bill cache writes at $12.50 per million. Batch processing is 50% off on each side. The differences hide in the details a heavy API user notices first.

Cached reads: GPT-6 Astra charges $1.00 per million cached input tokens against Claude Fable 5.1's $0.25. An agent that re-reads a large project context on every turn pays 4x less for that traffic on Anthropic.
Long context: OpenAI bills prompts over 272K input tokens at the long-context rate (2x input, 1.5x output). Anthropic publishes no equivalent threshold.
Task economics: CodeRabbit's illustrative task, 100,000 uncached input tokens plus 10,000 billable output tokens, costs $1.50 on either model, versus $0.60 on GPT-5.6 Sol at its promotional rate. OpenAI says its own evaluations show lower estimated task costs for Astra despite the higher token prices.

The price gap that matters is internal: GPT-6 Astra costs 2.5x GPT-5.6 Sol per token, so teams already paying Sol rates face a 150% bill increase if they switch every workload to Astra. Sol's promo runs at least through November 21, 2026.

Benchmarks: where GPT-6 Astra vs Claude Fable 5.1 splits

The independent numbers, all published September 3-5, 2026, do not crown one model. The winner depends on the workload and the harness it runs in.

Coding agents

Artificial Analysis' Coding Agent Index scores GPT-6 Astra at 67 inside Codex, approximately equal to Claude Opus 5 and Fable 5 inside Claude Code, and to Meta's Muse Spark 1.3 in Muse Code. Claude Fable 5.1 leads the same index at 70. Astra is dramatically more token-efficient, using about one third of GPT-5.6 Sol's tokens in the Codex harness, and at max effort it costs about the same as Sol max while scoring 2 points higher.

Reasoning and knowledge work

On the Artificial Analysis Intelligence Index, GPT-6 Astra ties GPT-5.6 Sol at 61, five points behind Claude Fable 5.1's 66. Astra cuts hallucination sharply, from a 92% hallucination rate to 51% at max effort on AA-Omniscience while accuracy rose 4 points. It gained roughly 80 Elo in AA-Briefcase long-horizon work but lost a similar amount on GDPval-AA v2, and 2-3 point regressions appeared on banking, SciCode, and long-context reasoning tasks.

Frontier reasoning

ARC Prize Foundation's September 3 evaluation scores GPT-6 Astra at 62.7% for $26K on ARC-AGI-3 Semi-Private with its Standard harness, and 99.9% for $19K with a Provider Adapter harness. Astra used fewer actions than the median tested human on 96% of levels.

Code review

CodeRabbit, which runs all models through its own review pipeline, reports Astra caught about 4% more labeled bugs than GPT-5.6 Sol and 22% more than Claude Opus 5. On harder cross-file reviews the gains reach 20% over Sol and 33% over Opus 5. CodeRabbit calls the result early and directional, and notes its evaluation does not establish an overall ranking.

Privacy and data handling: ZDR and retention policies

Data retention is a real differentiator for teams that review code or process regulated documents. CodeRabbit's September 4 policy rundown compares the two vendors directly: OpenAI supports zero data retention (ZDR) for GPT-6 Astra for eligible API customers, while Anthropic requires 30-day retention by default for Fable models for safety monitoring.

Anthropic's position is changing: eligible customers can now use Fable 5 and 5.1 with ZDR while Enterprise Frontier Safeguards is being introduced, and products serving other businesses need agreed terms. If retention policy decides your vendor, the Astra side is available today and the Fable side depends on your eligibility and contract.

GPT-6 Astra vs Claude Fable 5.1: which should you build on?

Start from the workload, not the headline benchmarks. Neither model wins everything, and each was measured inside its vendor's own coding harness, Astra in Codex and Fable 5.1 in Claude Code.

Pick GPT-6 Astra for the hardest end-to-end jobs: deep cross-file code review, computer use, visual work, and multi-step tasks where scattered evidence has to be connected. It is the only one of the two with a published computer-use toolset and a 128K max output.
Pick Claude Fable 5.1 when long-running agents re-read large cached contexts, where the $0.25 cached-read rate beats Astra's $1.00, and when you want the current Coding Agent Index leader in Claude Code. Its ZDR path exists but depends on eligibility.
Save money on routine work: Claude Opus 5 at $5/$25 and GPT-5.6 Sol at $4/$20 (through November 21, 2026) handle reliable everyday tasks at half or less of the flagship rate.

CodeRabbit's advice applies beyond code review: run both models side by side on your own tasks and compare answer quality, verification time, and total cost per completed outcome. Token price alone will not settle the choice, and neither will a single index. Our verified rate card for GPT-6 Astra and the earlier Claude Fable 5.1 vs GPT-5.6 Sol breakdown cover the pricing side in more depth.

At a glance

	GPT-6 Astra (OpenAI)	Claude Fable 5.1 (Anthropic)
Positioning	Most capable model for hard end-to-end work	Next-gen intelligence for long-running agents
Model ID	gpt-6-astra	claude-fable-5-1
Input price / 1M tokens	$10.00	$10.00
Output price / 1M tokens	$50.00	$50.00
Cached input read / 1M	$1.00	$0.25
Cache writes / 1M	$12.50	$12.50
Batch pricing	50% of standard	50% off (batch)
Context window	1,050,000 tokens	1,000,000 tokens (OpenRouter listing)
Max output tokens	128,000	Not stated on pricing page
Knowledge cutoff	Apr 30, 2026	Not stated on pricing page
ZDR (per CodeRabbit rundown)	Eligible API customers	Eligible customers, EFS rollout

GPT-6 Astra vs Claude Fable 5.1, verified September 6, 2026 (standard API tier; OpenAI short-context rates)

FAQ

Is GPT-6 Astra the same price as Claude Fable 5.1?

Yes on the headline rates: both charge $10 per million input and $50 per million output tokens, with identical $12.50 cache-write pricing. Cached reads differ, $1.00 on GPT-6 Astra versus $0.25 on Claude Fable 5.1, verified on the official pricing pages September 6, 2026.

Is GPT-6 Astra better than Claude Fable 5.1 for coding?

The indexes split. Artificial Analysis has Claude Fable 5.1 leading its Coding Agent Index at 70 (in Claude Code) while GPT-6 Astra scores 67 in Codex. CodeRabbit's code review evaluation found Astra caught 22% more actionable bugs than Claude Opus 5 and 33% more on cross-file reviews. Each model was measured in its vendor's own harness.

Does GPT-6 Astra support zero data retention?

OpenAI supports zero data retention for GPT-6 Astra for eligible API customers, per CodeRabbit's September 2026 rundown of both policies. Anthropic defaults Fable models to 30-day retention, with ZDR available to eligible customers for Fable 5 and 5.1 while Enterprise Frontier Safeguards is introduced.