Grok 4 Fast Reasoning
A fast, cost-efficient reasoning model from xAI that delivers near-flagship performance with dramatically fewer thinking tokens and a massive 2 million token context window.
X.ai Fast reasoning with 2M token context window
Grok 4 Fast Reasoning is a text generation model developed by xAI, released in September 2025 as a cost-efficient counterpart to their flagship Grok 4 model. It is built using large-scale reinforcement learning and uses approximately 40% fewer thinking tokens on average compared to Grok 4, while achieving comparable benchmark results. The model supports a 2 million token context window, making it suitable for processing large documents, multi-file codebases, and extended conversations.
The model accepts both text and image inputs and outputs text, with a unified architecture that blends chain-of-thought reasoning with faster response modes depending on task complexity. It is trained end-to-end with tool-use reinforcement learning, enabling agentic web search, browsing X (Twitter), and real-time information synthesis. Grok 4 Fast Reasoning is well-suited for developers and users working on research, coding assistance, agentic workflows, and complex question answering where efficiency and speed are priorities.
What Grok 4 Fast Reasoning supports
Extended Context Window
Processes up to 2 million tokens in a single context, enabling analysis of large documents, multi-file codebases, and long conversation histories.
Chain-of-Thought Reasoning
Uses internal deliberation to work through complex problems, reaching high-quality answers with approximately 40% fewer thinking tokens than the flagship Grok 4 model.
Agentic Web Search
Trained end-to-end with tool-use reinforcement learning, allowing the model to autonomously browse the web and X (Twitter), follow links, and synthesize real-time findings.
Multimodal Input
Accepts both text and image inputs, outputting text responses — enabling tasks that require visual understanding alongside language reasoning.
Code Assistance
Supports coding tasks including generation, debugging, and review, scoring 80.0% on LiveCodeBench (January–May 2025 evaluation period).
Math and Science Reasoning
Achieves 92.0% on AIME 2025 and 93.3% on HMMT 2025, reflecting strong performance on competition-level mathematics 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 | 85.0% |
| GPQA Diamond | PhD-level science questions (biology, physics, chemistry) | 84.7% |
| LiveCodeBench | Real-world coding tasks from recent competitions | 83.2% |
| HLE | Questions that challenge frontier models across many domains | 17.0% |
| SciCode | Scientific research coding and numerical methods | 44.2% |
Common questions about Grok 4 Fast Reasoning
What is the context window size for Grok 4 Fast Reasoning?
Grok 4 Fast Reasoning supports a context window of 2 million tokens, which accommodates large documents, extended conversations, and multi-file codebases in a single request.
How does Grok 4 Fast Reasoning differ from the standard Grok 4 model?
Grok 4 Fast Reasoning is designed as a cost-efficient alternative to the flagship Grok 4. It uses approximately 40% fewer thinking tokens on average while achieving comparable benchmark performance, making it faster and less expensive to run.
What API identifiers are available for this model?
The model is available via the xAI API as 'grok-4-fast-reasoning' (with reasoning enabled) and 'grok-4-fast-non-reasoning' (without reasoning). Pricing details are available in the xAI API documentation.
What is the training data cutoff for Grok 4 Fast Reasoning?
Based on the available metadata, the model's training date is listed as September 2025. For precise knowledge cutoff details, refer to the official xAI API documentation.
What input and output types does Grok 4 Fast Reasoning support?
The model accepts both text and image inputs and produces text outputs. It also supports agentic tool use, including web browsing and real-time search via reinforcement learning-trained tool integration.
What people think about Grok 4 Fast Reasoning
Community discussion around Grok 4 Fast is generally positive, with users highlighting its cost efficiency relative to comparable models. One widely shared thread noted that Grok 4 Fast matches high-level performance benchmarks at a fraction of the cost of models like Claude Opus 4.1.
Discussions also touch on its competitive benchmark scores and real-world arena rankings, though some threads veer into broader AI capability debates rather than focusing specifically on Grok 4 Fast. Use cases mentioned include research, reasoning tasks, and cost-sensitive production deployments.
Poetiq Achieves SOTA on ARC-AGI 2 Public Eval
Grok 4 Fast matches same high-level performance as Claude Opus 4.1, at less than 1% of the cost
Documentation & links
Parameters & options
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