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Grok-1.5 vs Grok-4 Fast Reasoning

Comparing Grok-1.5 and Grok-4 Fast Reasoning across benchmarks, pricing, and capabilities.

xAI · xAI · Updated for 2026

Which is better?

Grok-1.5 and Grok-4 Fast Reasoning trade strengths across price, capabilities, and technical limits. The better choice depends on the workload.

Based on current LLM Stats indexes, shared benchmarks, pricing, and model metadata for 2026.

Choose Grok-1.5

  • you are already invested in the xAI ecosystem

Choose Grok-4 Fast Reasoning

  • you want the most recent training data — it shipped Aug 2025

At a glance

The differences that matter most.

Grok-1.5
xAI
Grok-4 Fast Reasoning
xAI
Benchmark wins
—
—
Input price
— / M
$0.20 / M
Output price
— / M
$0.50 / M
Context window
—
2,000,000

Individual benchmarks

9 reported for Grok-1.5 · 0 for Grok-4 Fast Reasoning

No common benchmarks found

Grok-1.5 and Grok-4 Fast Reasoningdon't have any common benchmark datasets to compare. They may have been evaluated on different testing suites.

Human preference

Blind head-to-head votes and playground preference scores

Context Window

Maximum input and output token capacity

Only Grok-4 Fast Reasoning specifies input context (2,000,000 tokens). Only Grok-4 Fast Reasoning specifies output context (30,000 tokens).

xAI
Grok-1.5
Input- tokens
Output- tokens
xAI
Grok-4 Fast Reasoning
Input2,000,000 tokens
Output30,000 tokens
Sun Sep 20 2026 • llm-stats.com

Input capabilities

Documented input modalities across available providers

Grok-4 Fast Reasoning supports multimodal inputs, whereas Grok-1.5 does not.

Grok-4 Fast Reasoning can handle both text and other forms of data like images, making it suitable for multimodal applications.

Grok-1.5

Text
Images
Audio
Video

Grok-4 Fast Reasoning

Text
Images
Audio
Video

License

Usage and distribution terms

Both models are licensed under proprietary licenses.

Both models have usage restrictions defined by their respective organizations.

Grok-1.5

Proprietary

Closed source

Grok-4 Fast Reasoning

Proprietary

Closed source

Release Timeline

When each model was launched

Grok-1.5 was released on 2024-03-28, while Grok-4 Fast Reasoning was released on 2025-08-28.

Grok-4 Fast Reasoning is 17 months newer than Grok-1.5.

Grok-1.5

Mar 28, 2024

2.5 years ago

Grok-4 Fast Reasoning

Aug 28, 2025

1.1 years ago

1.4yr newer

Knowledge Cutoff

When training data ends

Neither model specifies a knowledge cutoff date.

Unable to compare the recency of their training data.

No cutoff dates available

Outputs Comparison

Notice missing or incorrect data?Start an Issue discussion→

Judge for yourself.

Run your own prompts against Grok-1.5 and Grok-4 Fast Reasoning side-by-side, then vote on the output you prefer.

Grok-1.5
✓ Preferred
Grok-4 Fast Reasoning
Open in Playground

FAQ

Common questions about Grok-1.5 vs Grok-4 Fast Reasoning.

Which is better, Grok-1.5 or Grok-4 Fast Reasoning?

Grok-1.5 (xAI) and Grok-4 Fast Reasoning (xAI) each have strengths in different areas. Compare their benchmark scores, pricing, context windows, and capabilities above to determine which fits your needs.

How does Grok-1.5 compare to Grok-4 Fast Reasoning in benchmarks?

Grok-1.5 scores GSM8k: 90.0%, DocVQA: 85.6%, MMLU: 81.3%, HumanEval: 74.1%, MMMU: 53.6%.

What are the context window sizes for Grok-1.5 and Grok-4 Fast Reasoning?

Grok-1.5 supports an unknown number of tokens and Grok-4 Fast Reasoning supports 2.0M tokens. A larger context window lets you process longer documents, conversations, or codebases in a single request.

What are the main differences between Grok-1.5 and Grok-4 Fast Reasoning?

Key differences include multimodal support (no vs yes). See the full comparison above for benchmark-by-benchmark results.

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