What is Grok 4.1 Fast?

Grok 4.1 Fast is a fast, low-cost, tool-calling large language model released by xAI on November 19, 2025, alongside the Agent Tools API. xAI calls it "our best tool-calling model with a 2M context window" and positions it as a model that "combines frontier tool-calling performance with blazing-fast inference and cost effectiveness" for enterprise agents such as customer support, finance, and deep research.[1] At launch it scored a perfect 100 percent on the tau2-bench Telecom agentic benchmark and 72 percent on the Berkeley Function Calling Leaderboard v4, while pricing started at just $0.20 per million input tokens and $0.50 per million output tokens.[1][7]

The model ships with a 2 million-token context window and is offered in two variants: a reasoning version (grok-4-1-fast-reasoning) that pauses to think before acting, and a low-latency non-reasoning version (grok-4-1-fast-non-reasoning) for instant replies. Both are accessible through the xAI API and through OpenAI-compatible surfaces such as the OpenAI Responses API. The cheap blended price (about $0.28 per million tokens at a 3:1 input-to-output mix) makes Grok 4.1 Fast one of the cheapest frontier-class agentic models available at launch.[1][7]

Grok 4.1 Fast was trained with reinforcement learning inside simulated tool environments covering dozens of domains. xAI says the goal was to make tool use a core competency rather than an add-on, so the model can plan, invoke multiple tools in parallel, and continue across many turns until it has enough evidence to answer.[1]

Overview

Grok 4.1 Fast is a tool-calling specialist in the Grok 4.x family from xAI, the AI company founded by Elon Musk. It was announced on November 19, 2025, and xAI describes it as the company's best model for agentic tool use, built for enterprise scenarios such as customer support, finance, and deep research.[1] The launch paired the model with the Agent Tools API, a hosted suite of server-side tools that xAI describes as tools "that allow Grok 4.1 Fast to operate as a fully autonomous agent."[1]

Where does Grok 4.1 Fast sit in the Grok lineup?

Grok 4.1 Fast sits in the Grok 4 generation as a smaller, cheaper, agent-focused sibling to the flagship Grok 4 reasoning model. The full release sequence places it as the eighth public Grok model.

Model	Release date	Notes
Grok 1	November 3, 2023	First Grok model. Weights open-sourced March 2024.
Grok 1.5	March 28, 2024	Context window expanded to 128k tokens.
Grok 1.5V	April 12, 2024	First xAI model with image understanding.
Grok 2	August 14, 2024	Multimodal input, image generation via Black Forest Labs.
Grok 3	February 17, 2025	Reasoning-focused, debuted on the X platform.
Grok 4	July 9, 2025	Frontier reasoning model with native tool calling.
Grok 4 Fast	September 19, 2025	First Fast variant, optimized for cost and latency.
Grok 4.1	November 17, 2025	Conversational refresh with improved emotional intelligence and lower hallucination.
Grok 4.1 Fast	November 19, 2025	Agent-tuned variant, paired with the Agent Tools API launch.

Grok 4.1 Fast inherits the conversational and factual improvements of Grok 4.1 but is purpose-built for autonomous, tool-using workflows rather than open-ended chat.[1][6]

What are the model variants and specifications?

The model is published under two API identifiers, separating reasoning behaviour into distinct surfaces.

Specification	grok-4-1-fast-reasoning	grok-4-1-fast-non-reasoning
Release date	November 19, 2025	November 19, 2025
Context window	2,000,000 tokens	2,000,000 tokens
Maximum output	30,000 tokens	30,000 tokens
Reasoning mode	Extended chain of thought	Direct response
Input modalities	Text and image	Text and image
Output modality	Text	Text
Tool calling	Native	Native
Structured outputs	Yes	Yes
Cached input pricing	Yes	Yes
Throughput limit	4,000,000 tokens per minute, 480 requests per minute	4,000,000 tokens per minute, 480 requests per minute
Time to first token (Artificial Analysis)	8.69 seconds	0.56 seconds
Output speed (Artificial Analysis)	113.6 tokens per second	133.4 tokens per second

On the xAI API the variants are selected by model name, while on the OpenRouter and Oracle Cloud surfaces the same names appear with provider prefixes such as xai.grok-4-1-fast-reasoning on Oracle and x-ai/grok-4.1-fast on OpenRouter, with reasoning enabled through a request parameter.[2][3][7]

How was Grok 4.1 Fast trained and designed?

xAI trained Grok 4.1 Fast with long-horizon reinforcement learning inside synthetic, fully simulated tool environments. Each episode required the model to chain many tool calls together, recover from errors, and maintain state across the full 2 million-token context. The training set covered tools across dozens of domains, so that performance on any single tool surface generalises rather than overfitting to a narrow benchmark.[1]

Two design choices shape the model's behaviour. First, both variants share the same backbone, with reasoning toggled at inference rather than baked into a separate weight set, which keeps latency predictable when developers switch modes mid-conversation. Second, xAI trained the model to prefer conservative tool selection: it tends to call only the tools it believes it needs, then waits for the results before deciding what to do next. The launch post claims this reduces wasted calls in long agent sessions and helps keep cost under control.[1]

The company also reports that Grok 4.1 Fast roughly halves hallucination rates compared to Grok 4 Fast. On the FActScore factuality benchmark of 500 biography questions, the non-reasoning variant scored 2.97 percent, down from 9.89 percent for an earlier configuration, and on a stratified sample of real-world production queries the non-reasoning hallucination rate fell to 4.22 percent from 12.09 percent for Grok 4 Fast.[1][16]

How does Grok 4.1 Fast perform on benchmarks?

xAI's launch post highlights agentic tool-use evaluations rather than traditional knowledge benchmarks. The headline numbers come from the company's own measurements and from third-party evaluators such as Artificial Analysis.

Evaluation	Result	Source
tau2-bench Telecom (dual-control agent)	100 percent, top score among major closed models	xAI launch post[1]
Berkeley Function Calling Leaderboard v4	72 percent, top score among major closed models	xAI launch post[1]
FActScore (factuality, non-reasoning)	2.97 percent error, about half the rate of Grok 4 Fast	xAI launch post[1][16]
Artificial Analysis Intelligence Index (reasoning)	39 (median for the price tier was 21)	Artificial Analysis[8]
Artificial Analysis Intelligence Index (non-reasoning)	24	Artificial Analysis[7]

The tau-bench Telecom split is a dual-control evaluation in which both an agent and a simulated customer can edit shared state, which makes it a stress test for long-horizon planning under partial information. The Berkeley Function Calling Leaderboard v4 evaluates how reliably a model selects, formats, and parallelises function calls across more than 2,000 question and tool pairs that include multi-turn and parallel call cases.[1][9][10]

In Artificial Analysis's Intelligence Index v4.0, the reasoning variant's score of 39 sat well above the median of 21 for similarly priced models, a ranking the evaluator attributes to the model's mix of low price and strong agentic scores rather than to raw knowledge depth.[8]

On traditional academic benchmarks Grok 4.1 Fast is competitive but not class-leading. Independent measurements report MMLU Pro around 74.3 percent, GPQA Diamond around 63.7 percent, AIME 2025 around 34.3 percent, and LiveCodeBench around 39.9 percent, which place it below the much larger Grok 4 on pure coding and math but above many similarly priced peers.[5]

What is the Agent Tools API?

The Agent Tools API launched on the same day as Grok 4.1 Fast and is the model's natural companion. xAI describes it as "a suite of powerful server-side tools that allow Grok 4.1 Fast to operate as a fully autonomous agent," packaging tools that xAI hosts so developers do not need to manage their own search APIs, code sandboxes, or vector stores.[1][11]

Tool	Purpose
Web search	Real-time queries across the open web with citations.
X search	Real-time queries against posts on the X platform.
Code execution	Python in a secure sandbox for data analysis and simulation.
Collections search	Retrieval over user-uploaded document collections.
Remote MCP	Connections to third-party servers that speak the Model Context Protocol.

The model decides when and how to invoke each tool, often calling several in parallel during a single turn. xAI charges separately for token use and for tool invocations, metering tool calls at $5 per 1,000 successful invocations, although it offered all tool access free through December 3, 2025 (in partnership with OpenRouter), with the model itself also free during that window on OpenRouter.[1][11][13]

The Agent Tools API is also the foundation for the grok-4.20-multi-agent model, which orchestrates either four or sixteen specialist agents that share the same tool stack and synthesise findings through a designated leader agent.[12]

How much does Grok 4.1 Fast cost?

Grok 4.1 Fast is positioned as a low-cost option compared with peer agentic models from OpenAI, Anthropic, and Google.

Tier	Price
Input tokens	$0.20 per 1M
Output tokens	$0.50 per 1M
Cached input tokens	About $0.05 per 1M
Blended (3:1) rate	About $0.28 per 1M
Agent Tools API calls	$5 per 1,000 successful invocations

Pricing is identical for the reasoning and non-reasoning variants. On OpenRouter the model carries the same headline rates, and during the launch promotion the :free route allowed unlimited use at no charge through December 3, 2025. Cached input tokens, which are billed at roughly a quarter of the fresh input rate, make Grok 4.1 Fast especially attractive for repetitive agent loops where the same system prompt and tool schemas are sent on every call.[1][7][13]

How do you access Grok 4.1 Fast?

Developers can call the model through several surfaces.

Surface	Notes
xAI API (native)	Direct access via `grok-4-1-fast-reasoning` and `grok-4-1-fast-non-reasoning`.
OpenAI-compatible Responses API	Same endpoints accept Grok 4.1 Fast as a drop-in.
OpenRouter	Routes to xAI with optional fallback providers.
Oracle Cloud Generative AI	Hosted as `xai.grok-4-1-fast-reasoning` and `xai.grok-4-1-fast-non-reasoning`.
AI/ML API and other aggregators	Available with provider-prefixed model names.

The OpenAI-compatible surface is important because it lets teams that already use the OpenAI Responses API point at Grok 4.1 Fast with little more than a base URL change. The xAI native SDK adds first-class support for the Agent Tools stack and for streaming, structured output, and asynchronous batch jobs.[1][2][3]

What is Grok 4.1 Fast used for?

The launch post and early third-party reviews focus on a few concrete patterns.

Customer support automation is the canonical example. The dual-control nature of the tau2-bench Telecom benchmark mirrors a real call centre, where the agent and the customer both edit shared account state, and Grok 4.1 Fast was tuned to handle exactly that pattern of long, conversational, tool-mediated work.[1]

Finance and agentic search make up the second main category. Because the model can fan out across web search, X search, and collections search at the same time, it can pull together a stock dossier or a competitive analysis in a single turn, then keep refining as the user pushes back. xAI cites finance specifically as a target domain, alongside customer support.[1]

Long-running multi-turn tasks benefit from the 2 million-token context. The model can keep an entire incident transcript, a long codebase, or a multi-day chat history in working memory without forced summarisation. The Artificial Analysis evaluations measured performance across the full window, which xAI presents as a contrast to other agentic models that degrade past a few hundred thousand tokens.[1][7]

Browser automation and data analysis round out the use cases. The hosted code execution tool gives the model a Python sandbox for spreadsheet work, plotting, and quick calculations, while remote MCP connections let it drive third-party browsers, ticket systems, or databases without custom glue code.[1][11]

How does Grok 4.1 Fast compare with peer agentic models?

Grok 4.1 Fast competes with a small cluster of agent-focused offerings from the major frontier labs.

Model	Context window	Headline tool stack	Input or output price (per 1M)
Grok 4.1 Fast (xAI)	2,000,000	Web, X, code, collections, remote MCP via Agent Tools API	$0.20 in, $0.50 out
OpenAI o4-mini with Responses API	200,000	Web search, file search, code interpreter, computer use	Mid-tier o-series pricing
Anthropic Claude Sonnet 4.5 with agent tools	200,000	Computer Use, web search, code execution, MCP	Mid-tier Claude pricing
Google Gemini 2.5 Flash with function calling	1,000,000	Function calling, Google Search grounding, code execution	Low Flash pricing
Mistral Le Chat with function calling	Up to 256,000	Function calling, web search, code interpreter	Mid-tier Mistral pricing

The headline differentiators for Grok 4.1 Fast are the size of the context window, the bundling of an MCP-compatible tool stack at the API level, and the unusually low blended price for a model that posts top scores on agent benchmarks. Its main weaknesses are also visible from the table: it is newer than its peers, the xAI ecosystem has fewer pre-built integrations than the OpenAI Responses API or the Anthropic Claude Computer Use stack, and it is less battle-tested in production agent deployments.[1][7][8]

How was Grok 4.1 Fast received?

The launch drew steady coverage in the developer-focused press. Artificial Analysis ranked the reasoning variant 17th overall and the non-reasoning variant 18th in its non-reasoning class on the Intelligence Index at release, while singling out the model for its unusually long context and aggressive pricing.[7][8] OpenRouter promoted the free window through December 3, 2025 heavily and saw the model climb its trending charts during that promotion period.[13] Independent reviewers on Medium and developer blogs echoed the headline claims about agent benchmark dominance but noted that on raw coding and math the model still trails the larger Grok 4 and the top tier of OpenAI and Anthropic models.[5]

Social reaction was driven in part by xAI's own posts on X, which framed Grok 4.1 Fast as the first frontier model designed specifically for autonomous tool use rather than chat. Posts comparing its tau2-bench Telecom scores to GPT-5.1, Claude Opus 4.5, and Gemini 3 Pro circulated widely in the agent-development community, although direct apples-to-apples comparisons depend heavily on the exact benchmark version and prompt template.[6][13]

What are the limitations of Grok 4.1 Fast?

The model is real but young, and its limitations matter for anyone planning a production deployment.

First, it is xAI ecosystem specific. The Agent Tools API, the multi-agent orchestrator, and the deepest integrations with the X platform all live inside xAI's hosted infrastructure. Teams that want to run their own search or code sandboxes get less benefit from the model's tool-calling training, which is biased towards the xAI tool surface shapes.

Second, it is newer than its peer agent models. The OpenAI Responses API, Anthropic's Computer Use stack, and Google's function-calling tooling all have a longer track record in production, more SDK support, and broader third-party tooling. Grok 4.1 Fast launched with strong benchmarks but with a thinner integration ecosystem.

Third, the model is conservative outside xAI's hosted tools. Independent reviewers report that when developers wire up custom function schemas and run the model outside the Agent Tools API, the gap between Grok 4.1 Fast and the top OpenAI and Anthropic models narrows substantially. The reinforcement learning curriculum was tuned on simulated environments that closely resemble the hosted tool stack.[5]

Finally, on raw academic and coding benchmarks Grok 4.1 Fast trails the larger flagship models, including the original Grok 4. For workloads that depend on hard math, deep code generation, or exotic knowledge tasks rather than tool-mediated workflows, the larger reasoning models from xAI, OpenAI, Anthropic, and Google still post higher scores at the cost of much higher prices.[5][8]

ELI5: Grok 4.1 Fast in plain terms

Imagine a very fast assistant who is great at using tools (a web browser, a calculator, a notebook) instead of just talking. Grok 4.1 Fast is that assistant: it is built to pick the right tool, use several at once, and keep going for a long time until it finishes a job, like answering a tricky customer-service question. It is also cheap to run and can remember a huge amount of text at once (about 2 million words' worth of tokens), so it does not forget the start of a long conversation.

See also

References

xAI. "Grok 4.1 Fast and Agent Tools API." November 19, 2025. https://x.ai/news/grok-4-1-fast
xAI Docs. "Models and Pricing." https://docs.x.ai/developers/models
Oracle Cloud Infrastructure. "xAI Grok 4.1 Fast." https://docs.oracle.com/en-us/iaas/Content/generative-ai/xai-grok-4-1-fast.htm
xAI. "Grok 4 Fast Model Card." September 19, 2025. https://data.x.ai/2025-09-19-grok-4-fast-model-card.pdf
Barnacle Goose. "Grok 4.1 Fast: Independent Reviews and Benchmarks." Medium, November 2025. https://medium.com/@leucopsis/grok-4-1-fast-independent-reviews-and-benchmarks-3aa61849858a
xAI. "Grok 4.1." November 17, 2025. https://x.ai/news/grok-4-1
Artificial Analysis. "Grok 4.1 Fast: Intelligence, Performance and Price Analysis." https://artificialanalysis.ai/models/grok-4-1-fast
Artificial Analysis. "Grok 4.1 Fast (Reasoning): Intelligence, Performance and Price Analysis." https://artificialanalysis.ai/models/grok-4-1-fast-reasoning
Artificial Analysis. "tau2-Bench Telecom Benchmark Leaderboard." https://artificialanalysis.ai/evaluations/tau2-bench
Patil et al. "The Berkeley Function Calling Leaderboard (BFCL): From Tool Use to Agentic Evaluation of Large Language Models." ICML 2025. https://gorilla.cs.berkeley.edu/leaderboard.html
xAI Docs. "Tools Overview." https://docs.x.ai/docs/guides/tools/overview
xAI Docs. "Multi Agent." https://docs.x.ai/developers/model-capabilities/text/multi-agent
OpenRouter. "Grok 4.1 Fast: API, Pricing and Benchmarks." https://openrouter.ai/x-ai/grok-4.1-fast
Wikipedia. "Grok (chatbot)." https://en.wikipedia.org/wiki/Grok_(chatbot)
BleepingComputer. "xAI's Grok 4.1 rolls out with improved quality and speed for free." November 17, 2025. https://www.bleepingcomputer.com/news/artificial-intelligence/xais-grok-41-rolls-out-with-improved-quality-and-speed-for-free/
LangCopilot. "Grok 4.1 Released: xAI's 2M Context AI with Lower Hallucination and Aggressive Pricing." November 21, 2025. https://langcopilot.com/posts/2025-11-21-grok-4-1-xai-release-2m-context

Editing: Grok 4.1 Fast