Grok 4.20 Reasoning vs Claude Fable 5.1

Pyyan / Compare / Grok 4.20 Reasoning vs Claude Fable 5.1

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Language Models · verified 2 Sept 2026

Grok 4.20 ReasoningxAIcurrent

Claude Fable 5.1Anthropiccurrent

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Specification	Grok 4.20 Reasoning	Claude Fable 5.1
Summary	Agentic reasoning SKU.	Same price as Fable 5, and cache reads cost a quarter of what they did.
Context	1M	1M
Input ($/Mtok)	$1.25	$10
Output ($/Mtok)	$2.5	$50
Licence	Proprietary	Proprietary
Max output	—	128K
Parameters	—	Not published
Released	—	1 Sep 2026
Model ID	grok-4.20-0309-reasoning	claude-fable-5-1
Category	Language Models	Language Models
Official	xAI ↗	—

Highlighted rows are where these differ.

Agentic reasoning SKU.

Best for multi-step agents.

Matches or beats Fable 5 at low and medium effort, and pulls clear at high effort
Cache reads cut to 25% of the old price, so re-reading a long context got much cheaper
Identical model to Mythos 5.1 underneath; the difference is the safeguards

Best for coding and long-running work.