Claude Sonnet 5.5 vs GPT-6 Astra: a data-driven comparison

Side-by-side comparison

Criterion	Claude Sonnet 5.5	GPT-6 Astra
Developer	Anthropic	OpenAI
Released	September 28, 2026	September 4, 2026
Capability index	95.9 / 100	94.7 / 100
Input price	$2	$10
Output price	$10	$50
Cost of a typical task	$0.06	$0.30
Context window	1,000,000 tokens	1,050,000 tokens
Maximum output	128,000 tokens	128,000 tokens
Reasoning mode	Yes	Yes
Tool calling	Yes	Yes
Accepted inputs	text, image, PDF	text, image, PDF
Open weights	No	No
Knowledge cutoff	June 2026	April 30, 2026

The match-up at a glance

Claude Sonnet 5.5 and GPT-6 Astra are two frontier models, the ones you pick when quality comes first, one from Anthropic, the other from OpenAI. Claude Sonnet 5.5 was released on September 28, 2026, GPT-6 Astra on September 4, 2026.

All figures below come from public sources checked on October 22, 2026: models.dev and OpenRouter for prices and capabilities, Epoch AI for scores.

Performance

On our capability index, Claude Sonnet 5.5 scores 95.9 out of 100 and GPT-6 Astra 94.7. Claude Sonnet 5.5 leads by 1.2 points.

On the 6 benchmarks reported for both, Claude Sonnet 5.5 wins 1 time and GPT-6 Astra 4:

GPQA Diamond (PhD-level science questions): Claude Sonnet 5.5 95.6%, GPT-6 Astra 95.8%
OTIS Mock AIME (competition mathematics problems): Claude Sonnet 5.5 100%, GPT-6 Astra 100%
FrontierMath (very hard research-level mathematics): Claude Sonnet 5.5 88.8%, GPT-6 Astra 93.7%
FrontierCode (advanced programming tasks): Claude Sonnet 5.5 52.1%, GPT-6 Astra 53.3%
APEX-Agents (professional tasks carried out by an agent): Claude Sonnet 5.5 75.5%, GPT-6 Astra 64.7%
Humanity's Last Exam (multidisciplinary expert questions): Claude Sonnet 5.5 46.1%, GPT-6 Astra 54.8%

Budget

Claude Sonnet 5.5 costs $2 per million input tokens and $10 per million output tokens; GPT-6 Astra costs $10 and $50.

For a typical agent task (read 20,000 tokens, write 2,000), expect $0.06 with Claude Sonnet 5.5 versus $0.30 with GPT-6 Astra. Claude Sonnet 5.5 is 5 times cheaper, a gap that adds up quickly over 1,000 tasks: $240 difference.

Run the numbers with your own volumes in the agent cost calculator.

Features and limits

GPT-6 Astra reads more in one go: 1,050,000 tokens of context (roughly 1,575 pages), versus 1,000,000 for Claude Sonnet 5.5.

Reasoning mode: Claude Sonnet 5.5 yes, GPT-6 Astra yes. Tool calling: Claude Sonnet 5.5 yes, GPT-6 Astra yes. Image input: Claude Sonnet 5.5 yes, GPT-6 Astra yes.

Neither publishes its weights: both are used through an API.

Knowledge cutoff: June 2026 for Claude Sonnet 5.5, April 30, 2026 for GPT-6 Astra.

In short

Our reading of the data:

Choose Claude Sonnet 5.5 for a higher capability index and a lower cost.

This verdict relies on public figures; it does not replace a trial on your own tasks.

Frequently asked questions

Claude Sonnet 5.5 or GPT-6 Astra: which is better?

On our index (Epoch AI data), Claude Sonnet 5.5 scores 95.9 and GPT-6 Astra 94.7 out of 100. The best choice also depends on price and on your use case.

Which one is cheaper?

Claude Sonnet 5.5. To read 20,000 tokens and write 2,000, expect $0.06 with Claude Sonnet 5.5 and $0.30 with GPT-6 Astra (prices as of October 22, 2026).

Which one has the larger context window?

GPT-6 Astra, with 1,050,000 tokens versus 1,000,000.

Can I run Claude Sonnet 5.5 or GPT-6 Astra on my own servers?

No: both are closed models, available through an API.

Claude Sonnet 5.5 vs GPT-6 Astra