Is Claude Opus 5.5 better than GPT-6 Astra?

On most published benchmarks as of September 22, 2026, yes. Opus 5.5 leads Terminal-Bench 4.0 (66.4% vs 57.9% on Anthropic's table; Artificial Analysis measures them level at 59.6%), FrontierCode v1.1 (54.4% vs 53.3%), GDPval-AA v2.1 (1846 vs 1542 Elo) and Humanity's Last Exam (67.7% vs 57.2%), and scores 58 on the AA Intelligence Index versus 53 for Astra. GPT-6 Astra leads AutomationBench (41.4% vs 40.0%), Terminal-Bench-Science (64.6% vs 58.7%), and computer use per OpenAI, and uses far fewer output tokens per task (~27K vs ~119K at max). Astra costs $10/$50, Opus 5.5 $4/$20.

Should I use Claude Opus 5.5 or Claude Fable 5.1?

Use Opus 5.5 by default. Anthropic's own table shows it beating Fable 5.1 on every listed benchmark at 40% of the price ($4/$20 vs $10/$50), and Artificial Analysis scores it 58 versus 53. Fable 5.1 keeps two advantages: cache reads at $0.25 are nearly as cheap, and it did not over-think on Simon Willison's max-effort test where Opus 5.5 twice hit its 128K output limit. Anthropic says the real-world gap is narrower than the scores suggest, and Fable 5.1 remains the flagship for Max and Team Premium subscribers.

Which frontier model is cheapest per task in September 2026?

It depends on effort. Artificial Analysis puts four of Opus 5.5's five effort levels (medium, high, xhigh, max) on the intelligence-versus-cost Pareto frontier for models scoring 50+, ahead of GPT-6 Astra, Fable 5.1 and Opus 5. Anthropic says Opus 5.5 at default effort matches Astra on Terminal-Bench 4.0 for about 40% of the cost and beats Astra at max on GDPval for about a fifth of the cost. GPT-6 Astra's advantage is token efficiency: roughly 27,000 output tokens per AA task at max versus 119,000 for Opus 5.5 and 78,000 for Fable 5.1, so at high effort settings Astra's higher list price is partly offset.

Do Claude Opus 5.5 and Fable 5.1 share the same safeguards?

Largely yes. Because Opus 5.5 matches Claude Mythos 5.1 in biology and cybersecurity, it ships with Fable 5.1-class safeguards: most offensive-security tasks fall back transparently to Opus 4.8, biology and frontier-LLM-development tasks fall back to Opus 5, and preserved thinking blocks API users from editing prior context on accounts created on or after August 31, 2026. Vetted organizations can apply to the Life Sciences Verification Program now and the Cyber Verification Program in the coming weeks. GPT-6 Astra has its own Trusted Access tiers for cyber use.

Quick Answer

Claude Opus 5.5 vs Fable 5.1 vs GPT-6 Astra (Sep 2026)

Published: September 23, 2026

The short answer

Claude Opus 5.5 is the new default frontier pick. Released September 22, 2026 at $4/$20 per million tokens, it outscores both $10/$50 flagships, Claude Fable 5.1 and GPT-6 Astra, on most published benchmarks and takes the top spot on the Artificial Analysis Intelligence Index at 58 versus 53 for each of them. GPT-6 Astra still wins on business-workflow automation, scientific terminal work, computer use and token efficiency; Fable 5.1 wins on nothing decisive any more but remains a safer bet for max-effort runs. If you were choosing between the two $10/$50 models last week, the answer this week is usually neither.

Side by side

	Claude Opus 5.5	Claude Fable 5.1	GPT-6 Astra
Released	September 22, 2026	September 3, 2026	September 3, 2026
Price (in / out per MTok)	$4 / $20	$10 / $50	$10 / $50
Cache read	$0.20	$0.25	$1.00
Context / max output	1M / 128K	1M / 128K	1.05M / 128K
Long-prompt repricing	None	None	>272K input: 2x input, 1.5x output
Knowledge cutoff	June 2026	June 2026	April 30, 2026
Default effort	medium	high	medium
Thinking	Adaptive, always on	Adaptive, always on	Reasoning effort none → max
AA Intelligence Index	58	53	53
Output tokens per AA task (max)	~119K	~78K	~27K
Clouds	Anthropic, AWS, GCP, Azure	Anthropic, AWS, GCP, Azure	OpenAI, Azure, AWS (staged)
Cyber / bio gating	Fable-class safeguards, verification programs	Same	Trusted Access tiers

Benchmarks

Anthropic’s launch table (Opus 5.5 at max effort; Astra figures as reported by OpenAI):

Benchmark	Opus 5.5	Fable 5.1	GPT-6 Astra	Leader
Terminal-Bench 4.0	66.4%	55.8%	57.9%	Opus 5.5
FrontierCode v1.1 (Main)	54.4%	50.3%	53.3%	Opus 5.5
CursorBench 4.0	57.8%	51.8%	—	Opus 5.5
GDPval-AA v2.1 (Elo)	1846	1735	1542	Opus 5.5
AutomationBench (Zapier)	40.0%	31.4%	41.4%	Astra
Humanity’s Last Exam (tools)	67.7%	65.6%	57.2%	Opus 5.5
Terminal-Bench-Science 0.1	58.7%	52.6%	64.6%	Astra
OSWorld 2.0 (partial)	81.8%	80.7%	—	Opus 5.5 (Astra not listed)

Independent cross-check from Artificial Analysis: Opus 5.5 leads six of the ten Intelligence Index evaluations (HLE 61.4%, SciCode 66.9%, GDPval-AA, AA-Briefcase 1,822 Elo, AA-Omniscience, AutomationBench-AA); Terminal-Bench 4.0 is a tie with Astra at 59.6%; Opus 5.5 trails on CritPt, AA-LCR and GDP.pdf. OpenAI still describes Astra as “the world’s best model for computer use,” and Anthropic’s OSWorld row omits Astra, so treat computer use as Astra’s unless you benchmark your own tasks.

Two caveats on Anthropic’s numbers: Opus 5.5 was evaluated with production safeguards on, with cyber tasks falling back to Opus 4.8 and biology tasks to Opus 5, which Anthropic says likely lowers its scores; and Zapier’s AutomationBench run counted safeguard interventions as failures.

Cost per task, not per token

List price says Opus 5.5 is 60% cheaper than the flagships. Reality depends on how many tokens each model burns:

Opus 5.5 at max uses ~119K output tokens per AA task, 1.5x Fable 5.1 and 4.4x Astra. At max, its cost advantage over Fable 5.1 mostly holds (cheaper tokens), but against Astra it shrinks.
Opus 5.5 at medium (the default) is where Anthropic’s claims live: it matches Astra on Terminal-Bench 4.0 for about 40% of the cost, beats Astra on FrontierCode at roughly 20% of the cost per task, and beats Astra at max on GDPval for about a fifth of the cost.
Astra’s 272K threshold doubles input pricing on very long prompts; neither Claude model reprices. For 500K-token codebases, that alone can flip the comparison.
Cache reads: Opus 5.5 $0.20, Fable 5.1 $0.25, Astra $1.00. On agent loops where 90% of input is cached, Astra’s effective input price is roughly 4x the Claude models’.

Simon Willison’s max-effort warning applies: Opus 5.5 at max twice consumed its entire 128K output budget reasoning about an SVG and returned nothing, at $2.56 per attempt; Fable 5.1 at max produced his best-ever result. Run Opus 5.5 at medium or high unless you have measured otherwise.

Which to pick

Use case	Pick	Why
Agentic coding, migrations, terminal work	Opus 5.5	Leads Terminal-Bench 4.0, FrontierCode, CursorBench at 40% of the price
Reports, financial models, decks	Opus 5.5	GDPval +111 Elo over Fable, +304 over Astra; 16/18 fact-checked reports passed
Zapier-style multi-app business workflows	GPT-6 Astra (or GPT-6 Sol for cost)	AutomationBench 41.4% vs 40.0%; see GPT-6 Sol vs Luna vs Astra
Scientific research agents	GPT-6 Astra	Terminal-Bench-Science 64.6% vs 58.7%
Computer-use agents	Astra, test Opus 5.5	OpenAI claims the lead; Opus 5.5 posts 81.8% OSWorld partial
Max-effort, single-shot hard problems	Fable 5.1	Did not over-think to failure in independent testing
Long-context (>272K) prompts	Opus 5.5 or Fable 5.1	No repricing threshold
Offensive security research	Whichever program admits you	All three gate cyber behind verification/trusted access

Bottom line

For the first time, Anthropic’s mid-priced Opus is also its best-scoring model on most axes, and the $10/$50 tier has become a niche: Fable 5.1 for max-effort reliability, Astra for automation, science and computer use. Anthropic itself says “the gap between Opus 5.5 and Claude Fable 5.1 is narrower than these scores suggest,” so expect Fable 5.1 users to feel a smaller difference than the table implies. The practical move is to route default traffic to Opus 5.5 at medium effort and keep one flagship in the router for the workloads above.

Last verified: September 23, 2026.