Current as of March 2026. Grok 4 Fast is xAI’s budget-friendly powerhouse, designed for high-throughput tasks that require a massive 2M token context window. It targets developers who need to process large datasets without the premium pricing of flagship models.
Specs
| Provider | xAI |
| Input cost | $0.20 / M tokens |
| Output cost | $0.50 / M tokens |
| Context window | 2M tokens |
| Max output | 30K tokens |
| Parameters | N/A |
| Features | function_calling, vision, reasoning, web_search |
What it’s good at
Aggressive Pricing
At $0.2 per million input tokens and $0.5 per million output tokens, it is one of the most cost-effective models in its performance class.
Massive 2M Context
The 2 million token context window allows for ingesting entire codebases or hundreds of PDF documents in a single request.
Large Output Buffer
A 30K max output token limit is significantly higher than the 4K-8K limits found in many competing fast models.
Where it falls short
Reasoning Depth
It lacks the deep logical reasoning capabilities of Claude 3.5 Sonnet or the full Grok 4 model, occasionally failing on complex multi-step instructions.
Proprietary Constraints
The model is closed-source and proprietary, meaning you are fully dependent on xAI’s API availability and privacy policies.
Best use cases with OpenClaw
- High-Volume Data Summarization — The low cost and 2M context window make it perfect for summarizing thousands of pages of text at once.
- Web-Search Agents — Integrated web_search features combined with low latency allow OpenClaw agents to perform real-time research efficiently.
Not ideal for
- Complex Software Architecture — It can miss subtle logical dependencies in large codebases that a more reasoning-heavy model would catch.
- Privacy-Critical Local Apps — Since it requires an external API call to xAI, it is not suitable for air-gapped or strictly local environments.
OpenClaw setup
Use the OpenAI provider configuration in OpenClaw, but set the base URL to https://api.x.ai/v1 and provide your xAI API key. Ensure the model ID is set exactly to xai/grok-4-fast.
{
"models": {
"mode": "merge",
"providers": {
"xai": {
"baseUrl": "https://api.x.ai/v1",
"apiKey": "YOUR-XAI-API-KEY",
"api": "openai-completions",
"models": [
{
"id": "grok-4-fast",
"name": "Grok 4 Fast",
"cost": {
"input": 0.2,
"output": 0.5
},
"contextWindow": 2000000,
"maxTokens": 30000
}
]
}
}
}
}
How it compares
- vs GPT-4o mini — Grok 4 Fast offers a much larger 2M context window compared to mini’s 128K, though mini is slightly cheaper on input at $0.15/1M.
- vs Gemini 1.5 Flash — Both offer massive context windows, but Grok’s $0.5/1M output price is more competitive for long-form generation tasks.
Bottom line
If your priority is processing enormous amounts of data for the lowest possible price, Grok 4 Fast is currently the model to beat.
For a full comparison of all Grok models, see our Grok models roundup. For setup instructions, see our API key guide. For all available models, see the complete models guide.