Grok 4.20 Reasoning vs Claude Opus 5.5

Pyyan / Compare / Grok 4.20 Reasoning vs Claude Opus 5.5

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Language Models · verified 24 Sept 2026

Grok 4.20 ReasoningxAIcurrent

Claude Opus 5.5Anthropiccurrent

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Specification	Grok 4.20 Reasoning	Claude Opus 5.5
Summary	Agentic reasoning SKU.	Fable 5.1's level of work on most tasks, at 40% less to run than Opus 5.
Context	1M	1M
Input ($/Mtok)	$1.25	$4
Output ($/Mtok)	$2.5	$20
Licence	Proprietary	Proprietary
Max output	—	128K
Parameters	—	Not published
Released	—	22 Sep 2026
Model ID	grok-4.20-0309-reasoning	claude-opus-5-5
Category	Language Models	Language Models
Official	xAI ↗	Anthropic ↗

Highlighted rows are where these differ.

Agentic reasoning SKU.

Best for multi-step agents.

66.4% on Terminal-Bench 4.0 against 52.3% for Opus 5, on Anthropic's own harness
Cache reads cut 60% to $0.20, and output more than 30% faster than Opus 5
Attempts to work around its boundaries 85% less often than Opus 5, Anthropic reports

Best for agentic coding at flagship quality.