GPT-5.6 Luna vs Gemini 2.5 Flash: API Cost Comparison (2026)
GPT-5.6 Luna or Gemini 2.5 Flash? Instead of comparing list prices, we compute the same workload on both (prices verified 2026-09-17). Spoiler: GPT-5.6 Luna wins on raw cost in most scenarios — but "when to pick each" matters more than the winner. Compare them live in the calculator.
| Rate per 1M tokens | GPT-5.6 Luna | Gemini 2.5 Flash |
|---|---|---|
| Input | $0.20 | $0.30 |
| Cached input | $0.02 | $0.03 |
| Output | $1.20 | $2.50 |
Same workload, both models
| Monthly scenario | GPT-5.6 Luna | Gemini 2.5 Flash | Difference |
|---|---|---|---|
| Support chatbot (150k requests, 400 in / 300 out tokens) | $66.00 | $130.50 | 49% in favor of GPT-5.6 Luna |
| Document workflow (22k runs, 3,000 in / 400 out tokens) | $23.76 | $41.80 | 43% in favor of GPT-5.6 Luna |
| High-volume classifier (1M requests, 300 in / 10 out tokens) | $72.00 | $115.00 | 37% in favor of GPT-5.6 Luna |
Your token shape decides: input-heavy workloads (documents, RAG) amplify the input-rate gap; output-heavy ones (content generation) weigh the output rate. Run your exact mix in the calculator with both preselected.
When to pick each
- GPT-5.6 Luna — when per-token cost rules: high volume, verifiable tasks, tight budgets. It comes out 49–49% cheaper in these scenarios.
- Gemini 2.5 Flash — when the premium buys something you need (quality on your specific task, context, ecosystem). The only honest way to know: test your real case on both with a small sample.
- Both — the winning production pattern is often routing: easy requests to the cheap one, hard ones to the strong one.
Related guides: how AI API pricing works and how to reduce LLM costs.
Our read: Luna is cheaper for text at both ends of the request, while Gemini Flash adds inexpensive image and video input plus a natural fit for Google Cloud. Pure text classification favors Luna on list price; multimodal pipelines give Flash a reason to exist beyond the token table.
Do not compare only one average request. Split text-only and multimodal traffic, include the thinking tokens that Google bills as output, and price each route separately.
Prices verified on 2026-09-17 against the provider's official pricing page. Estimates are for planning purposes only — always confirm current pricing with the provider.