xAI: Grok-4-Fast
Grok-4-Fast is a reasoning model from xAI released on Sep 20, 2025. It reads text, images, audio and video and writes text, with a 2M token context window and up to 2M tokens per reply.
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It supports tool calling.
- Modalities
- In / out price
- n/a
- Context
- 2M
- Max output
- 2M
- Released
- Sep 20, 2025
- Knowledge cutoff
- n/a
- Tool calling
- Structured output
- Reasoning
- Prompt caching
- Open weights
- Free to use
Supported request parameters
What you can set when you call Grok-4-Fast through an OpenAI compatible API.
- Tools
tools - Reasoning
reasoning - Temperature
temperature
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Grok-4-Fast questions
What is the context window of Grok-4-Fast?
2,000,000 tokens of input, and it can write up to 2,000,000 tokens in one reply. As a rough guide, a million tokens is around 750,000 English words.
Does Grok-4-Fast support tool calling?
Yes. It accepts tool definitions, which is what an agent needs to call your APIs.
Is Grok-4-Fast open source?
No. xAI serves it through an API only, so you cannot download the weights or host it yourself.
When was Grok-4-Fast released?
Sep 20, 2025.
Catalog data from models.dev (MIT licensed), refreshed daily, last on Oct 15, 2026. The summary is written from these facts.
Building on Grok-4-Fast?
I test your real tasks on it and the two closest alternatives, then ship the agent on whichever wins on cost and accuracy.
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