Grok 4 Fast Reasoning

Grok 4 Fast Reasoning is an advanced variant of xAI’s Grok 4 model, optimized for ultra-fast reasoning and extensive context handling. It supports a massive 2,000,000 token context window, enabling sophisticated long-horizon text understanding and multi-step inference with high efficiency. This version balances speed and reasoning depth, making it ideal for large-scale, real-time applications.

Technical Specification

Performance Benchmarks

• Context Window: 2,000,000 tokens
• Max Output: ~4,096 tokens
• Training Regime: Enhanced for fast inference with optimized compute pathways
• Tool Use: Integrated native support with streamlined multi-step execution

Performance Metrics

• Superior performance in long-context tasks requiring rapid comprehension
• High accuracy in complex text-to-text scenarios with intricate dependencies

Key Capabilities

• Ultra-long context understanding up to 2 million tokens for deep document comprehension
• Accelerated reasoning providing faster turnaround on multi-step tasks
• Deterministic outputs optimized for stable responses over very large input sizes

API Pricing

• Input: 0–128k: $0.21; 128k+: $0.42 per 1M tokens
• Output: 0–128k: $0.525; 128k+: $1.05 per 1M tokens
• Cached input: $0.05 per 1M tokens

Optimal Use Cases

• Large-scale document analysis and synthesis where extended context is crucial
• Real-time autonomous agents requiring fast, reliable multi-step reasoning
• Complex strategic planning involving API orchestration and extended logic chains
• Advanced research evaluation for datasets with vast textual dependencies
• Text-to-text transformations including summarization, Q&A, and content generation across extensive inputs

Code Sample

Comparison with Other Models

• vs. GPT-4o: Grok 4 Fast Reasoning supports a vastly larger context window of 2 million tokens compared to GPT-4o, enabling deeper long-form understanding. While GPT-4o excels in multimodal inputs and web browsing, Grok 4 Fast offers faster inference and superior reasoning over extended texts.
• vs. Claude 4 Opus: Claude 4 Opus is known for exceptional language safety and alignment features. Grok 4 Fast outperforms Claude 4 in handling ultra-long context tasks and delivers higher throughput in multi-step reasoning scenarios.
• vs. Gemini 2.5 Pro: Gemini 2.5 Pro provides strong instruction following and speed for typical text tasks. Grok 4 Fast surpasses Gemini in zero-shot reasoning with very long inputs, leveraging its 2 million token context for complex planning and inference.
• vs. Grok 4: Grok 4 Fast Reasoning builds on Grok 4 by dramatically increasing the context window from 256K to 2 million tokens, supporting larger and more complex documents. It also features optimized compute pathways for faster execution while maintaining advanced tool integration and reasoning capabilities.

Limitations

• Text-only model without vision or audio modalities
• Tool use remains sequential, limited compositionality
• Closed-weight approach with no offline or local inference support
• Stream determinism may vary under certain high-throughput conditions