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Grok 4 Fast (Reasoning)

Retired May 2026

Reasoning mode of Grok 4 Fast, released September 2025, sharing unified weights with the non-reasoning variant and comparable to Grok 4 while using fewer thinking tokens.

Context
2M tokens
Released
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Capabilities

What Grok 4 Fast (Reasoning) accepts and can do, as published by xAI.

  • Text Accepts and generates natural-language text.
  • Image Accepts images as input alongside text.
  • Reasoning Thinks before it answers, always on or as a switchable mode.
  • Function calling Connect to external tools, APIs, and systems.
  • Structured output Return responses in structured formats like JSON.

Common questions

What is Grok 4 Fast (Reasoning) good for?

Multi-step reasoning where thinking spend mattered: xAI said it matched Grok 4 while using fewer thinking tokens. It took text and images.

When is Grok 4 Fast (Reasoning) not a good fit?

Any new work: xAI retired it in May 2026, and its migration guide points to Grok 4.3 with low reasoning effort. No built-in web search or code execution, and closed weights.

How does Grok 4 Fast (Reasoning) compare with Grok 4.1 Fast (Reasoning)?

xAI says Grok 4.1 Fast (Reasoning) was trained with long-horizon reinforcement learning in simulated tool environments across dozens of domains, holds up better over long multi-turn agent runs, and hallucinates much less than Grok 4 Fast (Reasoning). It launched alongside xAI's Agent Tools API. Same 2M-token context and text and image input. Grok 4 Fast (Reasoning) came out 2 months earlier.

Can I self-host Grok 4 Fast (Reasoning)?

No. xAI does not publish the weights; Grok 4 Fast (Reasoning) is available only through hosted APIs.

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