# Grok 4.1 Fast Reasoning

Grok 4.1 Fast Reasoning combines structured chain-of-thought reasoning with fast inference and a context window of 1M tokens for high-accuracy agentic operations.

- **Model ID:** `spacexai/grok-4.1-fast-reasoning`
- **Type:** chat
- **Providers:** vertex
- **Context window:** 1,000,000
- **Maximum output tokens:** 1,000,000
- **Pricing:** $0.2/1M input tokens, $0.5/1M output tokens
- **Canonical page:** https://vercel.com/ai-gateway/models/grok-4.1-fast-reasoning

## Supported parameters

Detailed capability metadata has not been reported for this model.

## Example

```ts
import { streamText } from 'ai'

const result = streamText({
  model: 'spacexai/grok-4.1-fast-reasoning',
  prompt: 'Why is the sky blue?'
})
```

## About

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.

## What to consider

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.

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.

## When to use

### Best For

- **High-stakes agentic operations** where incorrect tool calls or decisions have significant consequences
- **Complex planning tasks** where 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** where 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** where reasoning overhead on every call makes the workload uneconomical. Use selective reasoning instead
- **Non-agentic reasoning tasks** where Grok 4 or Grok 3 may be better general-purpose choices

## 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

- **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

## Frequently 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-reasoning` as 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.

## Links

- [Model page](https://vercel.com/ai-gateway/models/grok-4.1-fast-reasoning)
- [AI Gateway documentation](https://vercel.com/docs/ai-gateway)
- [Provider model documentation](https://docs.cloud.google.com/vertex-ai/generative-ai/docs/partner-models/grok/grok-4-1-fast#reasoning)


If you encounter incorrect information, can’t find what you need, need a capability Vercel doesn’t currently offer, or have any other feedback to report, please do it by following the instructions at https://vercel.com/feedback.md.