Grok 4 Fast Reasoning overview
Grok 4 Fast Reasoning by xAI
Ideal for applications requiring large-scale text comprehension, strategic analysis, and real-time autonomous decision-making.
Grok 4 Fast Reasoning specs
- Model ID
x-ai/grok-4-fast-reasoning- Developer
- xAI
- Context window
- 2,000,000 tokens
- Max output
- 1,999,000 tokens
- Input
- Text
- Output
- Text
- API
- OpenAI-compatible, base URL
https://api.aimlapi.com/v1
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.
Performance Benchmarks
- Context Window: 2,000,000 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
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
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
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
Capabilities
- Function calling
- Streaming
- Reasoning
Grok 4 Fast Reasoning benchmarks by 
Artificial Analysis
See how Grok 4 Fast Reasoning scores on independent evaluations. Data retrieved Nov 3, 2026.
Benchmark results
Compare 10 models available on AI/ML API across 5 independent benchmarks.
Humanity's Last Exam
Expert-written questions across academic subjects.
GPQA Diamond
Graduate-level science questions written to resist web search.
AA-LCR
Reasoning across long documents of about 100k tokens.
IFBench
Following precise instructions about the shape of the answer.
τ²-Bench Telecom
Customer support agent tasks with tool calls.
Grok 4 Fast Reasoning API pricing
| Usage | Price | Unit |
|---|---|---|
| Input | $1.25 | 1M tokens |
| Output | $2.5 | 1M tokens |
| Cached input | $0.2 | 1M tokens |
Prices from the AI/ML API price list, checked on Nov 4, 2026.
Grok 4 Fast Reasoning vs other models: token prices
AI/ML API input and output rates for 5 models, per million tokens.
| Model | Input | Output |
|---|---|---|
 Grok 4 Fast Reasoning | $1.25 | $2.5 |
| Grok Build 0.1 | $1 | $2 |
| Grok 4.7 | $2 | $6 |
| Grok 4.6 | $2 | $6 |
| Grok 4.5 | $2 | $6 |
How to use the Grok 4 Fast Reasoning API
View docs
Build with your agent
Give your agent the model details, code and documentation.
1Get your API key
Create API keyexport AIMLAPI_KEY="8f2b4c7d9a1e6b3f…"2Make your first request
curl 'https://api.aimlapi.com/v1/chat/completions' \
-H "Authorization: Bearer $AIMLAPI_KEY" \
-H 'Content-Type: application/json' \
-d '{
"model": "x-ai/grok-4-fast-reasoning",
"messages": [
{
"role": "user",
"content": "Write a haiku about the ocean."
}
]
}'import os
import requests
headers = {
"Authorization": f"Bearer {os.environ['AIMLAPI_KEY']}",
"Content-Type": "application/json",
}
payload = {
"model": "x-ai/grok-4-fast-reasoning",
"messages": [
{
"role": "user",
"content": "Write a haiku about the ocean."
}
]
}
response = requests.post("https://api.aimlapi.com/v1/chat/completions", headers=headers, json=payload, timeout=120)
response.raise_for_status()
print(response.json())const response = await fetch("https://api.aimlapi.com/v1/chat/completions", {
method: "POST",
headers: {
Authorization: `Bearer ${process.env.AIMLAPI_KEY}`,
"Content-Type": "application/json",
},
body: JSON.stringify({
"model": "x-ai/grok-4-fast-reasoning",
"messages": [
{
"role": "user",
"content": "Write a haiku about the ocean."
}
]
}),
});
if (!response.ok) throw new Error(`HTTP ${response.status}`);
console.log(await response.json());Models similar to Grok 4 Fast Reasoning
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Grok 4 Fast Reasoning FAQ
What is the context window of Grok 4 Fast Reasoning?
Grok 4 Fast Reasoning has a 2,000,000 tokens context window and can return up to 1,999,000 tokens.
What inputs and outputs does Grok 4 Fast Reasoning support?
Grok 4 Fast Reasoning takes image, text as input and returns text.
What is the model ID for Grok 4 Fast Reasoning?
Use x-ai/grok-4-fast-reasoning as the model id. Requests go to https://api.aimlapi.com/v1/chat/completions.
How much does Grok 4 Fast Reasoning cost?
Grok 4 Fast Reasoning costs $1.25 input and $2.5 output per 1M tokens on AI/ML API, billed as you go with no subscription.
Does Grok 4 Fast Reasoning support streaming?
Yes, Grok 4 Fast Reasoning can stream responses as they are generated.
Can Grok 4 Fast Reasoning read images?
Yes, Grok 4 Fast Reasoning accepts image input alongside text.
Who created Grok 4 Fast Reasoning?
Grok 4 Fast Reasoning was built by xAI .
Does reasoning need to be turned on for Grok 4 Fast Reasoning?
No. Grok 4 Fast Reasoning runs with reasoning always active as part of its design.
Can Grok 4 Fast Reasoning call external functions or tools?
Yes, it supports function calling, tool use, and parallel tool calls.