Grok 4 Fast
xAI's cost-efficient reasoning model that delivers frontier-level intelligence with exceptional speed and token efficiency.
X.ai Cost-efficient reasoning with 2M token context
Grok 4 Fast is a text generation model developed by xAI, the AI division of X. It is built on learnings from Grok 4 and is designed to deliver high-quality reasoning at lower computational cost, using approximately 40% fewer thinking tokens on average compared to its full counterpart. The model features a 2 million token context window and supports both reasoning and non-reasoning modes within a single unified architecture.
Grok 4 Fast is trained end-to-end with tool-use reinforcement learning, enabling it to handle agentic tasks such as web browsing, code execution, and real-time information synthesis. It accepts both text and image inputs and produces text output. The model is well-suited for developers and enterprises that need multi-step reasoning, long-context document processing, and real-time web research without the computational overhead of a full frontier model.
What Grok 4 Fast supports
Long Context Window
Supports a 2 million token context window, enabling processing of very long documents, codebases, or multi-turn conversations in a single request.
Reasoning Modes
Offers both reasoning and non-reasoning modes in one unified architecture, allowing developers to choose the appropriate inference style per task.
Token Efficiency
Uses approximately 40% fewer thinking tokens on average than Grok 4, achieved through large-scale reinforcement learning optimized for intelligence density.
Agentic Tool Use
Trained end-to-end with tool-use reinforcement learning, supporting web browsing, code execution, and real-time information synthesis across multi-step tasks.
Multimodal Input
Accepts both text and image inputs, producing text output, making it usable for tasks that involve visual content alongside natural language.
Web & Search Integration
The search-enabled variant (grok-4-fast-search) supports real-time web and X (Twitter) browsing, and ranked first on LMArena's Search Arena with an Elo score of 1163.
Code Generation
Scored 80.0% on LiveCodeBench (January–May evaluation window), reflecting strong performance on competitive programming and code synthesis tasks.
Math & Science Reasoning
Achieved 92.0% on AIME 2025 and 93.3% on HMMT 2025 without tools, demonstrating strong performance on formal mathematical reasoning benchmarks.
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Get Started FreeBenchmark scores
Scores represent accuracy — the percentage of questions answered correctly on each test.
| Benchmark | What it tests | Score |
|---|---|---|
| MMLU-Pro | Expert knowledge across 14 academic disciplines | 73.0% |
| GPQA Diamond | PhD-level science questions (biology, physics, chemistry) | 60.6% |
| LiveCodeBench | Real-world coding tasks from recent competitions | 40.1% |
| HLE | Questions that challenge frontier models across many domains | 5.0% |
| SciCode | Scientific research coding and numerical methods | 32.9% |
Common questions about Grok 4 Fast
What is the context window size for Grok 4 Fast?
Grok 4 Fast supports a 2 million token context window, which allows it to process very long documents, extended conversations, or large codebases within a single request.
How does Grok 4 Fast differ from Grok 4?
Grok 4 Fast is a cost-efficient variant built on learnings from Grok 4. It uses approximately 40% fewer thinking tokens on average, making it less computationally expensive while targeting comparable reasoning quality.
What input types does Grok 4 Fast support?
Grok 4 Fast accepts both text and image inputs and produces text output. It also supports tool use including web browsing, X (Twitter) browsing, and code execution.
What is the training data cutoff for Grok 4 Fast?
Based on the available metadata, the training date for Grok 4 Fast is listed as September 2025.
Where can I access the Grok 4 Fast API?
Grok 4 Fast is available through the xAI API. You can find API documentation and access details at x.ai/api. On MindStudio, no separate API key is required to use the model.
Does Grok 4 Fast support reasoning mode?
Yes. Grok 4 Fast supports both reasoning and non-reasoning modes within a single unified architecture, allowing developers to select the appropriate mode depending on the task.
What people think about Grok 4 Fast
Community discussions around Grok 4 Fast have focused on its benchmark performance and token efficiency, with users noting its scores on AIME, HMMT, and LiveCodeBench as concrete evidence of its reasoning capabilities. The search-enabled variant's first-place ranking on LMArena's Search Arena has also drawn attention in AI-focused communities.
Some threads in the r/singularity community have broadened into discussions about general progress on benchmarks like ARC-AGI 2, which are not directly tied to Grok 4 Fast but reflect the wider context in which the model is being evaluated. Developers appear most interested in the model's cost-to-performance ratio and its suitability for agentic and long-context use cases.
xAI releases details and performance benchmarks for Grok 4 Fast
ARC-AGI 2 is Solved
Poetiq Achieves SOTA on ARC-AGI 2 Public Eval
Parameters & options
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