Claude Opus 5.5 or GPT-6.1 Sol: which to choose?

Side-by-side comparison

Criterion	Claude Opus 5.5	GPT-6.1 Sol
Developer	Anthropic	OpenAI
Released	September 22, 2026	September 29, 2026
Capability index	95.9 / 100	92.1 / 100
Input price	$4	$2
Output price	$20	$10
Cost of a typical task	$0.12	$0.06
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

What are we comparing?

This comparison pits two frontier models, the ones you pick when quality comes first against each other: Claude Opus 5.5 and GPT-6.1 Sol, one from Anthropic, the other from OpenAI. The first dates from September 22, 2026, the second from September 29, 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.

What the benchmarks say

On our capability index, Claude Opus 5.5 scores 95.9 out of 100 and GPT-6.1 Sol 92.1. Claude Opus 5.5 leads by 3.8 points.

On the 5 benchmarks reported for both, Claude Opus 5.5 wins 2 times and GPT-6.1 Sol 2:

GPQA Diamond (PhD-level science questions): Claude Opus 5.5 90.6%, GPT-6.1 Sol 95.4%
OTIS Mock AIME (competition mathematics problems): Claude Opus 5.5 100%, GPT-6.1 Sol 100%
FrontierMath (very hard research-level mathematics): Claude Opus 5.5 91.2%, GPT-6.1 Sol 93.7%
FrontierCode (advanced programming tasks): Claude Opus 5.5 54.6%, GPT-6.1 Sol 50.2%
APEX-Agents (professional tasks carried out by an agent): Claude Opus 5.5 73.5%, GPT-6.1 Sol 60%

Which one costs less?

Claude Opus 5.5 costs $4 per million input tokens and $20 per million output tokens; GPT-6.1 Sol costs $2 and $10.

For a typical agent task (read 20,000 tokens, write 2,000), expect $0.12 with Claude Opus 5.5 versus $0.06 with GPT-6.1 Sol. GPT-6.1 Sol is 2 times cheaper, a gap that adds up quickly over 1,000 tasks: $60 difference.

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

Context and capabilities

GPT-6.1 Sol reads more in one go: 1,050,000 tokens of context (roughly 1,575 pages), versus 1,000,000 for Claude Opus 5.5.

Reasoning mode: Claude Opus 5.5 yes, GPT-6.1 Sol yes. Tool calling: Claude Opus 5.5 yes, GPT-6.1 Sol yes. Image input: Claude Opus 5.5 yes, GPT-6.1 Sol yes.

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

Knowledge cutoff: June 2026 for Claude Opus 5.5, April 30, 2026 for GPT-6.1 Sol.

In short

Our reading of the data:

Choose Claude Opus 5.5 for a higher capability index.
Choose GPT-6.1 Sol for a lower cost.

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

Frequently asked questions

Claude Opus 5.5 or GPT-6.1 Sol: which is better?

On our index (Epoch AI data), Claude Opus 5.5 scores 95.9 and GPT-6.1 Sol 92.1 out of 100. The best choice also depends on price and on your use case.

Which one is cheaper?

GPT-6.1 Sol. To read 20,000 tokens and write 2,000, expect $0.12 with Claude Opus 5.5 and $0.06 with GPT-6.1 Sol (prices as of October 22, 2026).

Which one has the larger context window?

GPT-6.1 Sol, with 1,050,000 tokens versus 1,000,000.

Can I run Claude Opus 5.5 or GPT-6.1 Sol on my own servers?

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