Grok 4 Fast Reasoning is the speed-optimized reasoning variant of xAI's Grok 4 Fast. It combines chain-of-thought reasoning with faster inference than the full Grok 4, within a context window of 2M tokens.
import { streamText } from 'ai'
const result = streamText({ model: 'xai/grok-4-fast-reasoning', prompt: 'Why is the sky blue?'})Grok 4 Fast Reasoning is the reasoning-enabled configuration of xAI's Grok 4 Fast model, released July 9, 2025. It generates chain-of-thought reasoning traces before producing final answers, which helps on analytical and multi-step tasks compared to the non-reasoning variant, while staying faster than the full Grok 4.
The model operates with a context window of 2M tokens and excels at tasks where structured thinking improves accuracy: mathematical problem solving, code debugging, logical analysis, and complex question answering. The reasoning traces are visible in the API response, providing transparency into the model's analytical process.
Grok 4 Fast Reasoning is available at $0.2 per million input tokens and $0.5 per million output tokens through Vercel AI Gateway. It occupies the middle ground between the full Grok 4 (deepest reasoning, highest latency) and Grok 4 Fast Non-Reasoning (fastest, no reasoning overhead).