Model Comparison
GPT-5.5 Instant vs Grok 4 FastWhich is better in 2026?
Grok 4 Fast significantly outperforms across most benchmarks. Grok 4 Fast is 40.9x cheaper per token.
Verdict: GPT-5.5 Instant vs Grok 4 Fast — which is better?
GPT-5.5 Instant (by OpenAI) and Grok 4 Fast (by xAI) are two of the AI models people compare most. Here is how they stack up on benchmarks, price and capabilities, and which one to pick in 2026.
GPT-5.5 Instant outperforms in 0 benchmarks, while Grok 4 Fast is better at 2 benchmarks (AIME 2025, GPQA). Grok 4 Fast significantly outperforms across most benchmarks.
On price, Grok 4 Fast is roughly 40.9x cheaper per token on a blended 3:1 input/output basis, which adds up quickly at production volume.
Grok 4 Fast also accepts a larger context window (2,000,000 input tokens), making it the stronger choice for long documents and large codebases.
Choose GPT-5.5 Instant if…
- you want the most recent training data — it shipped May 2026
Choose Grok 4 Fast if…
- you want the strongest raw capability — it leads on 2 of 2 shared benchmarks
- cost matters — it's about 40.9x cheaper per token
- you process long inputs — it offers a 2,000,000 token context window
Performance Benchmarks
Comparative analysis across standard metrics
GPT-5.5 Instant outperforms in 0 benchmarks, while Grok 4 Fast is better at 2 benchmarks (AIME 2025, GPQA).
Grok 4 Fast significantly outperforms across most benchmarks.
Arena Performance
Human preference votes
Pricing Analysis
Price comparison per million tokens
For input processing, GPT-5.5 Instant ($5.00/1M tokens) is 25.0x more expensive than Grok 4 Fast ($0.20/1M tokens).
For output processing, GPT-5.5 Instant ($30.00/1M tokens) is 60.0x more expensive than Grok 4 Fast ($0.50/1M tokens).
In conclusion, GPT-5.5 Instant is more expensive than Grok 4 Fast.*
* Using a 3:1 ratio of input to output tokens
Context Window
Maximum input and output token capacity
Grok 4 Fast accepts 2,000,000 input tokens compared to GPT-5.5 Instant's 400,000 tokens. GPT-5.5 Instant can generate longer responses up to 128,000 tokens, while Grok 4 Fast is limited to 30,000 tokens.
Input Capabilities
Supported data types and modalities
Both GPT-5.5 Instant and Grok 4 Fast support multimodal inputs.
They are both capable of processing various types of data, offering versatility in application.
GPT-5.5 Instant
Grok 4 Fast
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
GPT-5.5 Instant was released on 2026-05-05, while Grok 4 Fast was released on 2025-08-28.
GPT-5.5 Instant is 8 months newer than Grok 4 Fast.
May 5, 2026
3 months ago
8mo newerAug 28, 2025
11 months ago
Knowledge Cutoff
When training data ends
GPT-5.5 Instant has a documented knowledge cutoff of 2025-08-31, while Grok 4 Fast's cutoff date is not specified.
We can confirm GPT-5.5 Instant's training data extends to 2025-08-31, but cannot make a direct comparison without Grok 4 Fast's cutoff date.
Aug 2025
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Provider Availability
GPT-5.5 Instant is available from OpenAI. Grok 4 Fast is available from xAI.
GPT-5.5 Instant
Grok 4 Fast
Outputs Comparison
Key Takeaways
No standout differentiators in the data we have for this pair.
Detailed Comparison
Interactive Arena
Judge for yourself.
Run your own prompts against GPT-5.5 Instant and Grok 4 Fast side-by-side, then vote on the output you prefer.
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FAQ
Common questions about GPT-5.5 Instant vs Grok 4 Fast.