Claude Opus 5.5 | Epoch AI

ECI#1 – Best

167

Anthropic

Sep. 22, 2026

Closed weights

Claude Opus 5.5 is a closed-weights model from Anthropic, released September 22, 2026. It has an ECI score of 167 and ranks 1st of 257 tracked models.

Claude Opus 5.5 vs select alternatives

Domain	Benchmark	Claude Opus 5.5 Anthropic Sep. 22, 2026 Closed weights	Claude Fable 5.1 Anthropic Sep. 1, 2026 Closed weights	GPT-6 Astra OpenAI Sep. 3, 2026 Closed weights	Kimi K3 Moonshot Jul. 16, 2026 Open weights
	ECI scoreBest score 167	167	165	166	157
	Ranking	#1 – Best	#5	#2	#15
Games	Earthborne Rangers (EBR-bench)Best score 76%	71%	57%	76%	n/a
	Mystery Game PuzzlesBest score 84%	71%	58%	84%	26%
Mathematics	FrontierMath Tiers 1-3 (v2)Best score 94%	91%	90%	94%	72%
	FrontierMath Tier 4 (v2)Best score 100%	95%	88%	98%	39%
	OTIS Mock AIME 2024-2025Best score 100%	100%	100%	100%	97%
Multimodal	Furniture Assembly BenchmarkBest score 83%	83%	70%	80%	34%
Science	GPQA DiamondBest score 96%	91%	n/a	96%	93%
Software engineering	MirrorCodeBest score 77%	77%	73%	47%	n/a
World knowledge	SimpleQA VerifiedBest score 76%	72%	71%	76%	51%

Model metadata

General
Organization	Anthropic
Release date	Sep. 22, 2026
Model accessibility	Closed weights
Country	United States of America
Knowledge cutoff	Unknown
Provider notes	Introducing Claude Opus 5.5

Pricing
Input price	$4 / 1M tokens
Output price	$20 / 1M tokens

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