Claude Sonnet 5.5 vs Grok-4 Fast Reasoning
Comparing Claude Sonnet 5.5 and Grok-4 Fast Reasoning across benchmarks, pricing, and capabilities.
Anthropic · xAI · Updated for 2026
Which is better?
Claude Sonnet 5.5 and Grok-4 Fast Reasoning trade strengths across price, capabilities, and technical limits. The better choice depends on the workload.
On price, Grok-4 Fast Reasoning is roughly 14.5x cheaper per token on a blended 3:1 input/output basis, which adds up quickly at production volume.
Grok-4 Fast Reasoning also accepts a larger context window (2,000,000 input tokens), making it the stronger choice for long documents and large codebases.
Based on current LLM Stats indexes, shared benchmarks, pricing, and model metadata for 2026.
Choose Claude Sonnet 5.5
- you want the most recent training data — it shipped Sep 2026
Choose Grok-4 Fast Reasoning
- cost matters — it's about 14.5x cheaper per token
- you process long inputs — it offers a 2,000,000 token context window
At a glance
The differences that matter most.
Individual benchmarks
47 reported for Claude Sonnet 5.5 · 0 for Grok-4 Fast Reasoning
Claude Sonnet 5.5 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
Pricing Analysis
Price comparison per million tokens
For input processing, Claude Sonnet 5.5 ($2.00/1M tokens) is 10.0x more expensive than Grok-4 Fast Reasoning ($0.20/1M tokens).
For output processing, Claude Sonnet 5.5 ($10.00/1M tokens) is 20.0x more expensive than Grok-4 Fast Reasoning ($0.50/1M tokens).
In conclusion, Claude Sonnet 5.5 is more expensive than Grok-4 Fast Reasoning.*
* Using a 3:1 ratio of input to output tokens
Context Window
Maximum input and output token capacity
Grok-4 Fast Reasoning accepts 2,000,000 input tokens compared to Claude Sonnet 5.5's 1,000,000 tokens. Claude Sonnet 5.5 can generate longer responses up to 128,000 tokens, while Grok-4 Fast Reasoning is limited to 30,000 tokens.
Input capabilities
Documented input modalities across available providers
Both Claude Sonnet 5.5 and Grok-4 Fast Reasoning support multimodal inputs.
They are both capable of processing various types of data, offering versatility in application.
Claude Sonnet 5.5
Grok-4 Fast Reasoning
License
Usage and distribution terms
Both models are licensed under proprietary licenses.
Both models have usage restrictions defined by their respective organizations.
Proprietary
Closed source
Proprietary
Closed source
Release Timeline
When each model was launched
Claude Sonnet 5.5 was released on 2026-09-28, while Grok-4 Fast Reasoning was released on 2025-08-28.
Claude Sonnet 5.5 is 13 months newer than Grok-4 Fast Reasoning.
Sep 28, 2026
3 weeks ago
1.1yr newerAug 28, 2025
1.1 years ago
Knowledge Cutoff
When training data ends
Claude Sonnet 5.5 has a documented knowledge cutoff of 2026-06-01, while Grok-4 Fast Reasoning's cutoff date is not specified.
We can confirm Claude Sonnet 5.5's training data extends to 2026-06-01, but cannot make a direct comparison without Grok-4 Fast Reasoning's cutoff date.
Jun 2026
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Provider Availability
Claude Sonnet 5.5 is available from Anthropic. Grok-4 Fast Reasoning is available from xAI.
Claude Sonnet 5.5
Grok-4 Fast Reasoning
Outputs Comparison
Judge for yourself.
Run your own prompts against Claude Sonnet 5.5 and Grok-4 Fast Reasoning side-by-side, then vote on the output you prefer.
FAQ
Common questions about Claude Sonnet 5.5 vs Grok-4 Fast Reasoning.