Claude Sonnet 5.5 or GPT-5.5 Pro: which to choose?

Side-by-side comparison

Criterion	Claude Sonnet 5.5	GPT-5.5 Pro
Developer	Anthropic	OpenAI
Released	September 28, 2026	April 23, 2026
Capability index	95.9 / 100	96.9 / 100
Input price	$2	$30
Output price	$10	$180
Cost of a typical task	$0.06	$0.96
Context window	1,000,000 tokens	1,050,000 tokens
Maximum output	128,000 tokens	100,000 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	December 1, 2025

The match-up at a glance

This comparison pits two frontier models, the ones you pick when quality comes first against each other: Claude Sonnet 5.5 and GPT-5.5 Pro, one from Anthropic, the other from OpenAI. The first dates from September 28, 2026, the second from April 23, 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.

Which one is more capable?

On our capability index, Claude Sonnet 5.5 scores 95.9 out of 100 and GPT-5.5 Pro 96.9. GPT-5.5 Pro leads by 1 points.

On the 3 benchmarks reported for both, Claude Sonnet 5.5 wins 2 times and GPT-5.5 Pro 0:

GPQA Diamond (PhD-level science questions): Claude Sonnet 5.5 95.6%, GPT-5.5 Pro 93.9%
OTIS Mock AIME (competition mathematics problems): Claude Sonnet 5.5 100%, GPT-5.5 Pro 100%
FrontierMath (very hard research-level mathematics): Claude Sonnet 5.5 88.8%, GPT-5.5 Pro 87.7%

Budget

Claude Sonnet 5.5 costs $2 per million input tokens and $10 per million output tokens; GPT-5.5 Pro costs $30 and $180.

For a typical agent task (read 20,000 tokens, write 2,000), expect $0.06 with Claude Sonnet 5.5 versus $0.96 with GPT-5.5 Pro. Claude Sonnet 5.5 is 16 times cheaper, a gap that adds up quickly over 1,000 tasks: $900 difference.

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

Features and limits

GPT-5.5 Pro 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-5.5 Pro yes. Tool calling: Claude Sonnet 5.5 yes, GPT-5.5 Pro yes. Image input: Claude Sonnet 5.5 yes, GPT-5.5 Pro yes.

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

Knowledge cutoff: June 2026 for Claude Sonnet 5.5, December 1, 2025 for GPT-5.5 Pro.

In short

Our reading of the data:

Choose Claude Sonnet 5.5 for a lower cost and a more recent release.
Choose GPT-5.5 Pro for a higher capability index.

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-5.5 Pro: which is better?

On our index (Epoch AI data), Claude Sonnet 5.5 scores 95.9 and GPT-5.5 Pro 96.9 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 $0.96 with GPT-5.5 Pro (prices as of October 22, 2026).

Which one has the larger context window?

GPT-5.5 Pro, with 1,050,000 tokens versus 1,000,000.

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

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

Claude Sonnet 5.5 versus GPT-5.5 Pro