AI Radar

Grok 4 Fast

Retired · Language, Reasoning

Grok 4 Fast is an xAI reasoning model that xAI describes as cost-efficient, released on 19 September 2025 in the Grok apps, including for free users, and on the xAI API with a 2-million-token context window. One set of weights serves both reasoning and non-reasoning responses, offered on the API as two model ids; both were retired on 15 May 2026. [1] [2] [3] Primary source

Timeline of Grok 4 Fast →

Claims and evidence

  • Released Primary source[1]header; availability sections
  • Retired Effective 12:00 PM PT on the xAI API. Primary source[3]first paragraph
  • Status Retiredgrok-4-fast-reasoning and grok-4-fast-non-reasoning retired from the xAI API; requests are served by grok-4.3. Primary source[3]list of retired models
  • Replaced by Grok 4.3The reasoning slug redirects to grok-4.3 with low reasoning effort, the non-reasoning slug with reasoning effort none. Primary source[3]Recommended Replacements
  • Successor of Grok 3 mini EditorialEditorial link along the cost-efficient tier: the announcement compares Grok 4 Fast with Grok 3 Mini and says it builds on learnings from Grok 4, without naming a predecessor or a derivation method. Primary source[1]Advancing Cost-Efficient Intelligence
  • Change · Architecture Reasoning and non-reasoning modes run on the same model weights, steered by the system prompt, instead of separate models.Compared with Grok 3 mini Primary source[1]Unified Model: Reasoning and Non-Reasoning
  • Change · Tool use Trained end to end with tool-use reinforcement learning to decide when to call tools such as code execution and web or X search.Compared with Grok 3 mini Primary source[1]Native Tool Use with SOTA Search
  • Change · Availability Offered to all Grok users including free users; xAI says this gave free users its latest model without restrictions for the first time.Compared with Grok 3 mini Primary source[1]Grok 4 Fast in grok.com, iOS, and Android apps
  • Change · Efficiency xAI reports results comparable to Grok 4 with substantially fewer thinking tokens on average.Compared with Grok 4 Primary source[1]
  • Feature Reasoning modeOne set of weights handles reasoning and non-reasoning modes, steered by the system prompt; offered on the API as two model ids. Primary source[1]Unified Model: Reasoning and Non-Reasoning [2]1 Introduction
  • Feature Tool useTrained end to end with tool-use reinforcement learning, including web and X search. Primary source[1]Native Tool Use with SOTA Search
  • Context window 2M tokens Primary source[1]first paragraph; xAI API section
  • Parameters Not disclosedNot disclosed by xAI. No claim madeNo source
  • Access consumer app, API Primary source[1]Grok 4 Fast in grok.com, iOS, and Android apps; Grok 4 Fast on OpenRouter, Vercel AI Gateway, and the xAI API

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Not derived from another model.

Variants and derived

None recorded.

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None recorded.

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