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GPT-5.6 Luna vs Gemini 3.1 Pro (Preview)
A straight cost comparison of GPT-5.6 Luna (OpenAI) and Gemini 3.1 Pro (Preview) (Google Gemini), per 1,000,000 tokens, verified October 2026.
Rate-by-rate comparison
| Metric | GPT-5.6 Luna | Gemini 3.1 Pro (Preview) | Winner |
|---|---|---|---|
| Input / 1M | $0.20 | $2 | GPT-5.6 Luna |
| Cached input / 1M | $0.02 | $0.20 | GPT-5.6 Luna |
| Cache write / 1M | $0.25 | — | Gemini 3.1 Pro (Preview) |
| Output / 1M | $1.20 | $12 | GPT-5.6 Luna |
| Context window | 1,050,000 | 1,000,000 | GPT-5.6 Luna |
| Batch API | Yes | Yes | Tie |
| Long-context tier | > 272,000 | > 200,000 | — |
For per-token metrics, the lower number wins. For context window and batch support, the higher or broader option wins. A dash means the provider does not publish that rate: a missing cache-write fee means writes are not billed per token, while a missing cached-input rate means cache hits bill at the full input rate.
Blended cost: 1M input + 250K output
| Model | Blended cost | Difference |
|---|---|---|
| GPT-5.6 Luna | $0.50 | Cheaper |
| Gemini 3.1 Pro (Preview) | $5 | +$4.50 |
Ten workloads, two bills
The same two models, priced across ten workloads that differ only in their input:output ratio. Nothing else changes — no retries, no system prompt, no middleware.
| Workload | GPT-5.6 Luna | Gemini 3.1 Pro (Preview) | Winner |
|---|---|---|---|
| Classification 100K reqs · 500 in / 10 out | $11.20 | $112 | GPT-5.6 Luna |
| Data extraction 10K reqs · 2K in / 200 out | $6.40 | $64 | GPT-5.6 Luna |
| Support chat 1K reqs · 4K in / 600 out | $1.52 | $15.20 | GPT-5.6 Luna |
| Code review 2K reqs · 8K in / 1.5K out | $6.80 | $68 | GPT-5.6 Luna |
| Summarisation 1K reqs · 20K in / 1K out | $5.20 | $52 | GPT-5.6 Luna |
| Agent loop 5K reqs · 12K in / 2K out | $24 | $240 | GPT-5.6 Luna |
| Translation 500 reqs · 15K in / 15K out | $10.50 | $105 | GPT-5.6 Luna |
| Long-document QA 200 reqs · 120K in / 800 out | $4.99 | $49.92 | GPT-5.6 Luna |
| Cached RAG 1K reqs · 50K in (45K cached) / 1K out | $3.11 | $31.08 | GPT-5.6 Luna |
| Nightly batch 10K reqs · 10K in / 2K out | $22 | $220 | GPT-5.6 Luna |
GPT-5.6 Luna wins 10 of 10, Gemini 3.1 Pro (Preview) wins 0 of 10. The winner is set by the token mix, not by the headline rate.
Where the two bills cross over
GPT-5.6 Luna is cheaper on both sides of the mix: $0.20 vs $2 per million input tokens, and $1.20 vs $12 per million output. No input:output ratio reverses that, so on price alone the other model never becomes the better choice — it has to win on context, latency or output quality.
Cost at four scales
The same support-chat workload (4K input, 600 output per request) at four monthly volumes.
| Requests / month | GPT-5.6 Luna | Gemini 3.1 Pro (Preview) | Difference |
|---|---|---|---|
| 100 | $0.152 | $1.52 | GPT-5.6 Luna cheaper by $1.37 |
| 1,000 | $1.52 | $15.20 | GPT-5.6 Luna cheaper by $13.68 |
| 10,000 | $15.20 | $152 | GPT-5.6 Luna cheaper by $137 |
| 100,000 | $152 | $1,520 | GPT-5.6 Luna cheaper by $1,368 |
What caching does to each bill
Caching only helps the part of the prompt that repeats. This is the support-chat workload (1,000 requests, 4,000 input tokens each) with an increasing share of that prompt served from a warm cache, written once on the first request.
| Cached share of the prompt | GPT-5.6 Luna | Gemini 3.1 Pro (Preview) | Winner |
|---|---|---|---|
| 0% | $1.52 | $15.20 | GPT-5.6 Luna |
| 50% | $1.16 | $11.60 | GPT-5.6 Luna |
| 80% | $0.945 | $9.45 | GPT-5.6 Luna |
| 95% | $0.837 | $8.37 | GPT-5.6 Luna |
Both models publish a cached-read rate, so the discount is comparable: GPT-5.6 Luna reads cache at $0.02 per million, Gemini 3.1 Pro (Preview) at $0.20.
Which one should you pick?
GPT-5.6 Luna wins on classification, data extraction, support chat, code review, summarisation, agent loop, translation, long-document qa, cached rag, nightly batch, while Gemini 3.1 Pro (Preview) wins on none of the ten workloads above.
Beyond price, the two differ on input rate ($0.20 vs $2); output rate ($1.20 vs $12); cached-input rate ($0.02 vs $0.20); cache-write rate ($0.25 vs —); context window (1,050,000 vs 1,000,000).
Batch mode is available on GPT-5.6 Luna and Gemini 3.1 Pro (Preview), which halves the nightly job. Both publish cached-input discounts, so a stable system prompt is worth caching on either.
FAQ
Is GPT-5.6 Luna cheaper than Gemini 3.1 Pro (Preview)?
On a blended 1M-input-plus-250K-output workload, GPT-5.6 Luna is cheaper by 90% ($0.50 vs $5). The gap changes for cache-heavy prompts and for batch jobs, as the workload table shows.
Which handles longer prompts, GPT-5.6 Luna or Gemini 3.1 Pro (Preview)?
GPT-5.6 Luna has the larger context window (1,050,000 vs 1,000,000 tokens). Watch the long-context threshold on GPT-5.6 Luna and Gemini 3.1 Pro (Preview): rates rise once input passes it.
Do both models support prompt caching?
GPT-5.6 Luna bills cached reads at $0.02 per 1M; Gemini 3.1 Pro (Preview) at $0.20.
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