Claude Sonnet 5.5 | Epoch AI

ECI#3

165

Anthropic

Sep. 28, 2026

Closed weights

Claude Sonnet 5.5 is a closed-weights model from Anthropic, released September 28, 2026. It has an ECI score of 165 and ranks 3rd of 253 tracked models.

Claude Sonnet 5.5 vs select alternatives

Domain	Benchmark	Claude Sonnet 5.5 Anthropic Sep. 28, 2026 Closed weights	Claude Opus 5.5 Anthropic Sep. 22, 2026 Closed weights	GPT-6 Astra OpenAI Sep. 3, 2026 Closed weights	Kimi K3 Moonshot Jul. 16, 2026 Open weights
	ECI scoreBest score 167	165	167	167	158
	Ranking	#3	#1 – Best	#2	#13
Games	Mystery Game PuzzlesBest score 84%	65%	71%	84%	26%
Mathematics	FrontierMath Tiers 1-3 (v2)Best score 94%	89%	91%	94%	72%
	FrontierMath Tier 4 (v2)Best score 100%	80%	95%	98%	39%
	OTIS Mock AIME 2024-2025Best score 100%	100%	100%	100%	97%
Multimodal	Furniture Assembly BenchmarkBest score 83%	75%	83%	80%	34%
Science	SciCodeBest score 67%	61%	67%	56%	59%
	GPQA DiamondBest score 96%	96%	91%	96%	93%
Software engineering	Text Arena (Coding)Best score 1820	1699	1820	1800	1674
World knowledge	SimpleQA VerifiedBest score 76%	47%	72%	76%	51%

Model metadata

General
Organization	Anthropic
Release date	Sep. 28, 2026
Model accessibility	Closed weights
Country	United States of America
Knowledge cutoff	Unknown
Provider notes	Claude Sonnet 5.5

Pricing
Input price	$2 / 1M tokens
Output price	$10 / 1M tokens

Training
Parameters	Unknown
Training compute	Unknown
Training hardware	Unknown
Training time	Unknown
Training dataset size	Unknown