Grok 4 Fast vs GPT-5.6 Luna
Detailed pricing comparison and cost analysis.
Updated September 2026
Cost Simulator
| Feature | Grok 4 Fast | GPT-5.6 Luna |
|---|---|---|
| Provider | xAI | OpenAI |
| Input Price (1M) | $0.20 | $0.20 |
| Output Price (1M) | $0.50 | $1.20 |
| Context Window | 2,000,000 | 1,000,000 |
Verdict
Grok 4 Fast costs $0.20 per 1M input tokens and $0.50 per 1M output tokens. GPT-5.6 Luna costs $0.20 per 1M input tokens and $1.20 per 1M output tokens. Grok 4 Fast and GPT-5.6 Luna have identical input token pricing. For output tokens, Grok 4 Fast is the more affordable option at $0.50/1M vs $1.20.
On context window, Grok 4 Fast supports 2,000,000 tokens — meaning it can fit more conversation history, documents, or code in a single request. This matters for RAG pipelines, long document analysis, and agentic workflows where context builds up over many turns.
When to choose Grok 4 Fast
- ✓ Your workload is output-heavy — Grok 4 Fast generates text cheaper
- ✓ You need a larger context window (2,000,000 tokens)
- ✓ You are already integrated with xAI
When to choose GPT-5.6 Luna
- ✓ You are already integrated with OpenAI
Use the calculator above to simulate your specific workload and find the exact break-even point. For most applications, the cheapest model is the one that minimises your total monthly bill given your input-to-output token ratio.
Frequently Asked Questions
Is Grok 4 Fast cheaper than GPT-5.6 Luna? ▼
Grok 4 Fast and GPT-5.6 Luna have identical input token pricing at $0.20/1M tokens.
What is the context window of Grok 4 Fast vs GPT-5.6 Luna? ▼
Grok 4 Fast has a 2,000,000-token context window. GPT-5.6 Luna has a 1,000,000-token context window. Grok 4 Fast supports the larger context, suitable for longer documents and agentic workflows.
Which model is better: Grok 4 Fast or GPT-5.6 Luna? ▼
The best choice depends on your use case. For cost efficiency on input tokens, Grok 4 Fast is the cheaper option. For maximum context length, Grok 4 Fast supports 2,000,000 tokens. Use the comparison table above to find the right fit for your workload.