Grok 4.1 Fast Reasoning
Grok 4.1 Fast Reasoning is SpaceXAI's reasoning-enabled Grok 4.1 Fast model optimized for agentic operations. It combines structured chain-of-thought reasoning with speed-optimized inference and a context window of 1M tokens for complex agent workflows.
View API reference- Input and output price
- Input $0.20, Output $0.50, Per 1M tokens
- 24h uptime
- Loading AI Gateway uptime
import { streamText } from 'ai'
const result = streamText({ model: 'spacexai/grok-4.1-fast-reasoning', prompt: 'Why is the sky blue?'})Copy link to headingPlayground
Try out Grok 4.1 Fast Reasoning by SpaceXAI. Usage is billed to your team at API rates. Free users (those who haven't made a payment) get $5 of credits every 30 days.
Grok 4.1 Fast Reasoning
Grok 4.1 Fast Reasoning
Copy link to headingProviders
Route requests across multiple providers. Copy a provider slug to set your preference. Visit the docs for more info. Using a provider means you agree to their terms, listed under Legal.
Copy link to headingUptime24 hours
Direct request success rate on AI Gateway and per-provider. Visit the docs for more info.
Copy link to headingThroughput24 hours
P50 throughput on live AI Gateway traffic, in tokens per second (TPS). Visit the docs for more info.
Copy link to headingLatency24 hours
P50 time to first token (TTFT) on live AI Gateway traffic, in milliseconds. View the docs for more info.
Getting started
Call Grok 4.1 Fast Reasoning through AI Gateway with the AI SDK generateText and streamText functions, or through the OpenAI Chat Completions, OpenAI Responses, and Anthropic Messages APIs by changing the base URL. AI Gateway authenticates the request and routes it to an available provider.
Install the AI SDK (pnpm add ai dotenv), create an API key from the API Keys page, and set it as AI_GATEWAY_API_KEY in your environment. Full setup is covered in the text generation quickstart.
import { generateText } from 'ai';import 'dotenv/config';
async function main() { const result = await generateText({ model: 'spacexai/grok-4.1-fast-reasoning', prompt: 'Why is the sky blue?', });
console.log(result.text);}
main().catch(console.error);Top-level parameters
The same Grok 4.1 Fast Reasoning request in each API format AI Gateway supports.
import { generateText } from 'ai';import 'dotenv/config';
async function main() { const result = await generateText({ model: 'spacexai/grok-4.1-fast-reasoning', system: 'You are a concise technical assistant.', prompt: 'Summarize the tradeoffs between static generation and SSR.', maxOutputTokens: 1024, });
console.log(result.text);}
main().catch(console.error);Standard parameters like prompt, messages, temperature, and tools work as documented in the AI SDK docs. These are the parameters with model-specific behavior.
| Parameter | Type | Required | Description |
|---|---|---|---|
model | string | Yes | Model ID in the form creator/model, e.g. spacexai/grok-4.1-fast-reasoning. AI Gateway routes the request to an available provider. |
maxOutputTokens | number | No | Hard cap on generated tokens. Grok 4.1 Fast Reasoning supports up to 1,000,000 output tokens. Reasoning tokens count toward this limit. |
reasoning | 'provider-default' | 'none' | 'minimal' | 'low' | 'medium' | 'high' | 'xhigh' | No | Provider-agnostic reasoning effort, available in AI SDK 7 or later. Maps to the provider’s native reasoning configuration; reasoning settings under providerOptions take precedence when both are set. See the Reasoning section below. |
providerOptions | Record<string, JSONValue> | No | AI Gateway routing options under gateway, plus any provider-native options under the provider’s own namespace — see the table below. |
Input limits
| Input | Formats | Sources | Max count | Max size | Limits |
|---|---|---|---|---|---|
| Text | — | — | — | — | Prompt and response share the 1M-token context window |
| Image | — | URL, base64, Uint8Array | — | — | Sent as image parts in messages; counts as input tokens |
| — | URL, base64, Uint8Array | — | — | Sent as file parts in messages; counts as input tokens |
Provider options
Set AI Gateway routing options under providerOptions.gateway. For provider-specific options, pass them under the provider’s namespace as documented by the AI SDK.
Learn more in the AI SDK xai provider docs.
import { generateText } from 'ai';import 'dotenv/config';
async function main() { const result = await generateText({ model: 'spacexai/grok-4.1-fast-reasoning', prompt: 'Why is the sky blue?', providerOptions: { gateway: { only: ['vertex'], }, }, });
console.log(result.text);}
main().catch(console.error);These AI Gateway routing options apply to every model. Provider-specific options pass through under the provider’s own namespace (for example providerOptions.anthropic) exactly as documented by the AI SDK.
| Parameter | Type | Required | Description |
|---|---|---|---|
providerOptions.gateway.only | string[] | No | Restrict routing to these provider slugs. Requests fail over only within the listed providers. |
providerOptions.gateway.order | string[] | No | Preferred provider order. Listed providers are tried first; unlisted providers remain available as fallbacks. |
providerOptions.gateway.sort | 'cost' | 'ttft' | 'tps' | No | Rank candidate providers by price, time to first token, or tokens per second instead of the default routing order. |
providerOptions.gateway.zeroDataRetention | boolean | No | Route only to providers with a zero-data-retention policy for this model. |
Routing across providers
AI Gateway serves the same model through multiple providers and fails over automatically. order expresses a preference while keeping every provider eligible; only is a hard allowlist — if none of the listed providers are available the request fails instead of falling back.
Options under a provider's own namespace (for example providerOptions.anthropic) are forwarded to that provider with the request. Providers ignore option namespaces that don't apply to them, so it is safe to set provider options alongside gateway routing options.
Reasoning
AI Gateway bridges reasoning across every API format. The AI SDK exposes a provider-agnostic top-level reasoning level (none, minimal, low, medium, high, or xhigh); the Chat Completions and Responses formats take the same effort under reasoning.effort; and the Anthropic Messages format uses a native thinking token budget. Whichever you send, the gateway maps it to the target model’s native configuration, converting between effort levels and token budgets as needed. Reasoning-related settings under providerOptions take full precedence over the top-level reasoning value and are never merged. Reasoning tokens typically count toward your output-token usage, though how they’re reported and billed varies by provider.
Learn more in the AI Gateway reasoning guide.
import { generateText } from 'ai';import 'dotenv/config';
async function main() { const result = await generateText({ model: 'spacexai/grok-4.1-fast-reasoning', prompt: 'Explain the Monty Hall problem step by step.', reasoning: 'high', });
console.log(result.text);}
main().catch(console.error);Image input
Send images alongside text as message parts. Images count as input tokens.
import { generateText } from 'ai';import 'dotenv/config';
async function main() { const result = await generateText({ model: 'spacexai/grok-4.1-fast-reasoning', messages: [ { role: 'user', content: [ { type: 'text', text: 'Describe this image.' }, { type: 'image', image: 'https://example.com/photo.jpg' }, ], }, ], });
console.log(result.text);}
main().catch(console.error);PDF input
Attach PDFs as file parts. Their contents count as input tokens.
import { generateText } from 'ai';import 'dotenv/config';
async function main() { const result = await generateText({ model: 'spacexai/grok-4.1-fast-reasoning', messages: [ { role: 'user', content: [ { type: 'text', text: 'Summarize this document.' }, { type: 'file', mediaType: 'application/pdf', data: 'https://example.com/document.pdf', }, ], }, ], });
console.log(result.text);}
main().catch(console.error);Tool calling
Expose tools the model can call. Define each tool’s inputs with a Zod schema.
import { generateText, tool } from 'ai';import { z } from 'zod';import 'dotenv/config';
async function main() { const result = await generateText({ model: 'spacexai/grok-4.1-fast-reasoning', prompt: 'What is the weather in San Francisco?', tools: { getWeather: tool({ description: 'Get the current weather for a location', inputSchema: z.object({ location: z.string() }), execute: async ({ location }) => ({ location, temperatureC: 18 }), }), }, });
console.log(result.text);}
main().catch(console.error);Copy link to headingMore models by SpaceXAI
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|---|
| 500K | $2/M+1 more | $6/M+1 more | Read$0.50/M | $5/K | +1 | | 08/12/2026 | |||||||
| 500K | $2/M+1 more | $6/M+1 more | Read$0.30/M | $5/K | +3 | | 07/08/2026 | |||||||
| 1M | $1.25/M+1 more | $2.50/M+1 more | Read$0.20/M | $5/K | +3 |  | 04/30/2026 | |||||||
| 2M | $1.25/M+1 more | $2.50/M+1 more | Read$0.20/M | $5/K | +2 | | 03/10/2026 | |||||||
| 2M | $1.25/M+1 more | $2.50/M+1 more | Read$0.20/M | $5/K | +3 | | 03/10/2026 | |||||||
| 1M | $0.20/M | $0.50/M | Read$0.05/M | — | +1 | | 11/19/2025 |
Copy link to headingAbout Grok 4.1 Fast Reasoning
Grok 4.1 Fast Reasoning was released November 19, 2025 as the reasoning-enabled variant of SpaceXAI's Grok 4.1 Fast generation. It's engineered for agentic operations that require high accuracy, combining structured chain-of-thought reasoning with the speed optimizations of the Fast model line. The context window of 1M tokens supports complex agent sessions with extensive tool schemas and conversation histories.
Unlike the non-reasoning variant, Grok 4.1 Fast Reasoning generates reasoning traces before producing final answers and tool-call decisions. This structured analysis improves accuracy on tasks where the agent must reason through ambiguous situations, evaluate multiple tool options, or plan multi-step strategies. The reasoning overhead is a deliberate tradeoff for higher-stakes agentic operations where incorrect decisions are costly.
Grok 4.1 Fast Reasoning pairs with the non-reasoning variant for a two-tier architecture: use reasoning for complex decisions and non-reasoning for routine tool calls within the same agent system.
Copy link to headingWhat To Consider When Choosing a Provider
- Configuration: Each reasoning step adds tokens and latency. For agent workflows with many steps, profile the total cost and time impact of using reasoning on every call versus selectively on complex decisions.
- Configuration: Consider routing simple tool-calling decisions to the non-reasoning variant and reserving Grok 4.1 Fast Reasoning for steps that require analytical depth. This optimizes both cost and speed.
- Zero Data Retention: Zero Data Retention is available for this model. It is offered on a per-provider and model basis. See the documentation for details.
- Authentication: AI Gateway authenticates requests using an API key or OIDC token. You do not need to manage provider credentials directly.
Copy link to headingWhen to Use Grok 4.1 Fast Reasoning
Best for
- High-stakes agentic operations: Incorrect tool calls or decisions have significant consequences
- Complex planning tasks: The agent must evaluate multiple strategies and select the optimal approach
- Multi-step reasoning within agent workflows: That require the model to analyze intermediate results before proceeding
- Code analysis and debugging agents: That need to reason through complex codebases systematically
- Research and analysis agents: Operating over large document collections within the context of 1M tokens
Consider alternatives when
- Routine tool-calling decisions: The non-reasoning variant handles the task at lower latency and cost
- Maximum reasoning depth: On the hardest problems, where the full Grok 4 may provide deeper analysis
- Budget-constrained agent systems: Reasoning overhead on every call makes the workload uneconomical. Use selective reasoning instead
- Non-agentic reasoning tasks: Grok 4 or Grok 3 may be better general-purpose choices
Copy link to headingConclusion
Grok 4.1 Fast Reasoning adds structured reasoning to the fast agentic foundation of the Grok 4.1 generation. It's designed for agent workflows where accuracy on complex decisions justifies the reasoning overhead. Combined with the non-reasoning variant, it enables intelligent routing architectures that match reasoning depth to decision complexity within the same system.
Copy link to headingFrequently Asked Questions
How does Grok 4.1 Fast Reasoning differ from the non-reasoning variant?
Grok 4.1 Fast Reasoning generates chain-of-thought reasoning traces before making decisions, improving accuracy on complex tasks at the cost of additional latency and tokens. The non-reasoning variant produces direct responses for faster throughput.
What is the context window for Grok 4.1 Fast Reasoning?
1M tokens, matching the non-reasoning variant. This supports extensive agent sessions with large tool schemas and conversation histories.
When should I use reasoning versus non-reasoning in my agent?
Use reasoning for complex decisions where accuracy matters (planning, ambiguous situations, high-stakes tool calls). Use non-reasoning for routine operations where speed is more valuable than analytical depth.
What does Grok 4.1 Fast Reasoning cost?
Check the pricing panel on this page for today's numbers. AI Gateway tracks rates across every provider that serves Grok 4.1 Fast Reasoning.
How do I authenticate with Grok 4.1 Fast Reasoning through Vercel AI Gateway?
Use your Vercel AI Gateway API key with
spacexai/grok-4.1-fast-reasoningas the model identifier. AI Gateway manages routing and authentication automatically.Can I see the reasoning traces in the response?
Yes. The chain-of-thought traces are included in the API response. You can inspect and log the model's reasoning for debugging and auditing.
Does Vercel AI Gateway support Zero Data Retention for Grok 4.1 Fast Reasoning?
Yes, Zero Data Retention is available for this model. Zero Data Retention is offered on a per-provider basis. See https://vercel.com/docs/ai-gateway/capabilities/zdr for details.