Claude Sonnet 5.5 or DeepSeek V4.1 Flash 1017: which to choose?

Side-by-side comparison

Criterion	Claude Sonnet 5.5	DeepSeek V4.1 Flash 1017
Developer	Anthropic	DeepSeek
Released	September 28, 2026	October 17, 2026
Capability index	95.9 / 100	90.6 / 100
Input price	$2	$0.15
Output price	$10	$0.60
Cost of a typical task	$0.06	$0.004
Context window	1,000,000 tokens	1,048,576 tokens
Maximum output	128,000 tokens	393,216 tokens
Reasoning mode	Yes	Yes
Tool calling	Yes	Yes
Accepted inputs	text, image, PDF	text, image
Open weights	No	Yes
Knowledge cutoff	June 2026	May 2025

Two models head to head

Claude Sonnet 5.5 and DeepSeek V4.1 Flash 1017 are the flagship models of two major developers, one from Anthropic, the other from DeepSeek. Claude Sonnet 5.5 was released on September 28, 2026, DeepSeek V4.1 Flash 1017 on October 17, 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 Sonnet 5.5 scores 95.9 out of 100 and DeepSeek V4.1 Flash 1017 90.6. Claude Sonnet 5.5 leads by 5.3 points.

On the 3 benchmarks reported for both, Claude Sonnet 5.5 wins 3 times and DeepSeek V4.1 Flash 1017 0:

GPQA Diamond (PhD-level science questions): Claude Sonnet 5.5 95.6%, DeepSeek V4.1 Flash 1017 93.2%
OTIS Mock AIME (competition mathematics problems): Claude Sonnet 5.5 100%, DeepSeek V4.1 Flash 1017 99.4%
FrontierMath (very hard research-level mathematics): Claude Sonnet 5.5 88.8%, DeepSeek V4.1 Flash 1017 71.2%

Which one costs less?

Claude Sonnet 5.5 costs $2 per million input tokens and $10 per million output tokens; DeepSeek V4.1 Flash 1017 costs $0.15 and $0.60.

For a typical agent task (read 20,000 tokens, write 2,000), expect $0.06 with Claude Sonnet 5.5 versus $0.004 with DeepSeek V4.1 Flash 1017. DeepSeek V4.1 Flash 1017 is 14.3 times cheaper, a gap that adds up quickly over 1,000 tasks: $55.8 difference.

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

Context and capabilities

DeepSeek V4.1 Flash 1017 reads more in one go: 1,048,576 tokens of context (roughly 1,573 pages), versus 1,000,000 for Claude Sonnet 5.5.

Reasoning mode: Claude Sonnet 5.5 yes, DeepSeek V4.1 Flash 1017 yes. Tool calling: Claude Sonnet 5.5 yes, DeepSeek V4.1 Flash 1017 yes. Image input: Claude Sonnet 5.5 yes, DeepSeek V4.1 Flash 1017 yes.

DeepSeek V4.1 Flash 1017 publishes its weights and can run on your servers; Claude Sonnet 5.5 is a closed model, available only through an API.

Knowledge cutoff: June 2026 for Claude Sonnet 5.5, May 2025 for DeepSeek V4.1 Flash 1017.

In short

Our reading of the data:

Choose Claude Sonnet 5.5 for a higher capability index.
Choose DeepSeek V4.1 Flash 1017 for a lower cost and open weights (you can host it yourself).

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

Frequently asked questions

Claude Sonnet 5.5 or DeepSeek V4.1 Flash 1017: which is better?

On our index (Epoch AI data), Claude Sonnet 5.5 scores 95.9 and DeepSeek V4.1 Flash 1017 90.6 out of 100. The best choice also depends on price and on your use case.

Which one is cheaper?

DeepSeek V4.1 Flash 1017. To read 20,000 tokens and write 2,000, expect $0.06 with Claude Sonnet 5.5 and $0.004 with DeepSeek V4.1 Flash 1017 (prices as of October 22, 2026).

Which one has the larger context window?

DeepSeek V4.1 Flash 1017, with 1,048,576 tokens versus 1,000,000.

Can I run Claude Sonnet 5.5 or DeepSeek V4.1 Flash 1017 on my own servers?

Only DeepSeek V4.1 Flash 1017, whose weights are published. Claude Sonnet 5.5 is used through an API.

Claude Sonnet 5.5 versus DeepSeek V4.1 Flash 1017