GPT-5.6 Luna vs DeepSeek V4.1 Flash: API Cost Compared
The verdict, in numbers: On our reference 200-step agent workload, DeepSeek V4.1 Flash bills $0.09 against $0.29 for GPT-5.6 Luna — 3.2x apart. List rates alone would not have told you that.
This page compares list rates and workload cost only. It does not tell you which model is better — capability belongs to your own evaluation. What it does give you is the half of the decision that can be verified: what each provider will actually charge, read from their own documentation on September 18, 2026.
Rate cards side by side
| Item | GPT-5.6 Luna | DeepSeek V4.1 Flash |
|---|---|---|
| Provider | OpenAI | DeepSeek |
| Input / 1M | $0.20 | $0.15 |
| Output / 1M | $1.20 | $0.60 |
| Cache read / 1M | $0.020 | $0.0030 |
| Context window | 1.05M | 1M |
Sources: OpenAI pricing ↗ and DeepSeek pricing ↗, both checked September 18, 2026.
The same 200-step agent task, priced twice
One coding-agent task: 200 steps, a stable 40,000-token prefix written to cache on step 1 and re-read on the remaining 199, 500 output tokens per step. Cache-write billing follows each provider’s own rules (cache writes bill at 1.25x the uncached input rate; cache writes bill at the standard input rate).
| Component | GPT-5.6 Luna | DeepSeek V4.1 Flash |
|---|---|---|
| Cache write (once) | $0.01 | $0.01 |
| Cache reads (199 × 40K) | $0.16 | $0.02 |
| Output (200 × 500) | $0.12 | $0.06 |
| Total per task | $0.29 | $0.09 |
A heavy month on each
50M fresh input, 150M cached input, 10M output tokens in a month:
| Component | GPT-5.6 Luna | DeepSeek V4.1 Flash |
|---|---|---|
| Fresh input (50M) | $10.00 | $7.50 |
| Cached input (150M) | $3.00 | $0.45 |
| Output (10M) | $12.00 | $6.00 |
| Monthly total | $25.00 | $13.95 |
DeepSeek figures use off-peak rates; weekday peak hours (01:00–04:00 and 06:00–10:00 UTC) double input and output.
What this comparison does and does not say
GPT-5.6 Luna: Lowest-cost GPT-5.6 tier; cheapest current-generation frontier rate published by OpenAI.
DeepSeek V4.1 Flash: Low-cost current DeepSeek API model with thinking mode, vision, and Responses API support.
Cost is the verifiable half of the decision; quality is the half only your workload can answer. Run both candidates on a representative evaluation set and measure cost per accepted task — the cost planning guide walks through that method, and the calculator lets you re-price this exact comparison with your own token mix.
Frequently asked questions
Is GPT-5.6 Luna cheaper than DeepSeek V4.1 Flash?
On list rates, GPT-5.6 Luna costs $0.20/$1.20 per 1M input/output tokens versus $0.15/$0.60 for DeepSeek V4.1 Flash. On a 200-step agent workload (40K cached prefix, 500 output tokens per step), DeepSeek V4.1 Flash totals $0.09 versus $0.29 for GPT-5.6 Luna. Prices verified September 18, 2026.
GPT-5.6 Luna vs DeepSeek V4.1 Flash: which has the longer context window?
GPT-5.6 Luna offers 1.05M tokens; DeepSeek V4.1 Flash offers 1M tokens, per provider documentation checked September 18, 2026.
Where do these GPT-5.6 Luna and DeepSeek V4.1 Flash prices come from?
Both rate cards were read from the providers' own pricing pages on September 18, 2026: https://developers.openai.com/api/docs/models/gpt-5.6-luna and https://api-docs.deepseek.com/quick_start/pricing/.