Grok 4 Fast is an AI model developed by xAI and released on September 19, 2025. It is designed to provide multimodal reasoning while prioritizing efficiency and cost reduction. This post offers an overview of the model’s capabilities and compares it to Grok 4, Grok 3, and the upcoming Grok 5.
What Is Grok 4 Fast?
Introducing Grok 4 Fast, a multimodal reasoning model with a 2M context window that sets a new standard for cost-efficient intelligence.
Available for free on https://t.co/AnXpIEOhOD, https://t.co/53pltypvkw, iOS and Android apps, and OpenRouter.https://t.co/3YZ1yVwueV
— xAI (@xai) September 19, 2025
Grok 4 Fast is a multimodal language model capable of processing text, images, and other data types. It is optimized to deliver strong performance while using fewer resources.
- Efficiency: Uses ~40% fewer processing tokens than Grok 4, resulting in a 98% decrease in cost per unit of output.
- Context window: Up to 2 million tokens, enabling it to handle very large inputs.
- Modes:
- Fast Mode for quick responses.
- Auto Mode for tool use, such as web search and code execution.
- Benchmarks: Ranked #1 in search and #8 in general text reasoning on the LMSYS leaderboard.
- Multimodality: Supports analysis across mixed media (text + images).
Grok is #1 https://t.co/Of5pCrWWEy
— Elon Musk (@elonmusk) October 2, 2025
Access and Pricing:
- Available free on grok.com, x.com, and Grok mobile apps.
- API pricing: $0.20 per million input tokens, $0.50 per million output tokens.
Kilocode + Grok 4 Fast is peak vibe coding.
The 2M token context window is insane. pic.twitter.com/O9E7cYJmq0
— George Andrew (@TrecentTrill) September 27, 2025
not sure folks are grok'ing (pun intended) how fast token cost/performance is improving
Grok 4-Fast, released just two months after Grok 4, is a 98% cheaper model on equivalent tasks
RL is driving massive performance savings pic.twitter.com/w1YqfsUnkf
— Spencer Peterson (@spencermpeter) September 22, 2025
Comparison to Grok 4
Grok 4, released on July 9, 2025, is xAI’s flagship model. It is described as highly intelligent, with native tool use and real-time search. Access is limited to SuperGrok and Premium+ subscribers.
In benchmarks:
- Grok 4 Fast offers comparable performance overall, with notable gains in efficiency.
- It outperforms Grok 4 in areas such as web search and writing, while Grok 4 remains stronger on GPQA Diamond and Humanity’s Last Exam (HLE) without tools.
- Grok 4 Fast’s reduced token usage makes it more cost-effective for many tasks.
| Aspect | Grok 4 Fast | Grok 4 |
|---|---|---|
| Token Reduction | 40% fewer on average | Baseline |
| Cost Efficiency | 98% reduction per output | Higher relative cost |
| Benchmarks | #1 Search, #8 Text (LMSYS) | Strong in GPQA, HLE |
| Context Window | 2 million tokens | Not specified (larger focus) |
| Availability | Free for general use | Subscription required |
Comparison to Grok 3
Grok 3 is an earlier free model with usage limits, emphasizing reasoning and pretraining knowledge.
Both Grok 4 and Grok 4 Fast improve significantly over Grok 3 in reasoning and handling complex tasks.
| Aspect | Grok 4 Fast | Grok 3 |
|---|---|---|
| Reasoning | Comparable to Grok 4 | Lower than Grok 4 |
| Context Window | 2 million tokens | 128,000 tokens |
| Efficiency | High (40% fewer tokens) | Standard |
| Benchmarks | Superior in search/text | Outperformed by later models |
| Availability | Free | Free with limits |
Upcoming Grok 5
Grok 5 is in development, with training expected to begin in October 2025. Elon Musk has suggested it could be released before the end of 2025 and described it as potentially “crushingly good.”
- Musk estimates a 10% chance of reaching AGI (artificial general intelligence), a milestone not expected from earlier models.
- No confirmed features or benchmarks are available yet.