Quick answer
- What it is
- Budget/fast tier of the GPT-5.6 family (Sol › Terra › Luna)
- Price
- $1 in / $6 out per MTok
- Headline score
- 84.7% on Terminal-Bench 2.1 (Terra: 87.4%, Sol: 88.8%)
- Not included
- Max reasoning effort and ultra mode, Sol only
What Is GPT-5.6 Luna?
GPT-5.6 Luna went generally available on July 9, 2026, alongside Sol and Terra. In OpenAI's naming scheme the number is the generation and the name is the tier; Luna is the bottom of the three, the budget/fast option. The API identifier is gpt-5.6-luna, and it is selectable in Codex like the rest of the current generation.
The pitch is not capability, it is arithmetic: $1 in / $6 out per MTok makes Luna the cheapest way to run the current OpenAI generation, and for a specific class of work, mechanical, well-specified, high-volume, that price is the feature. The rest of this page is about drawing that class precisely, because the same price becomes a trap one step outside it. For the full lineup context, see the models hub.
Specs: The Verified Data
| Spec | GPT-5.6 Luna |
|---|---|
| Vendor | OpenAI |
| API identifier | gpt-5.6-luna |
| Generally available | July 9, 2026 |
| Price | $1 in / $6 out per MTok |
| Tier | Budget / fast (bottom of the GPT-5.6 family) |
| Terminal-Bench 2.1 | 84.7% (OpenAI launch materials) |
| Codex | Selectable in Codex (current generation) |
Data verified July 18, 2026 against OpenAI's published pricing and launch materials. Family context: Luna trails Terra by 2.7 points on Terminal-Bench 2.1 (84.7% vs 87.4%) and, like Terra, has neither max reasoning effort nor ultra mode, those are Sol exclusives.
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The Cost Math That Justifies Luna
Take a realistic mechanical task, a lint sweep, a rename, a formulaic edit, consuming 100k input and 20k output tokens. Per task the absolute numbers look trivial; multiplied by a pipeline they stop being trivial:
| 100k in / 20k out | GPT-5.6 Luna | GPT-5.6 Terra | GPT-5.6 Sol |
|---|---|---|---|
| Price per MTok | $1 in / $6 out per MTok | $2.5 in / $15 out per MTok | $5 in / $30 out per MTok |
| Cost per task | $0.22 | $0.55 | $1.10 |
| 1,000 tasks | $220 | $550 | $1,100 |
That is the whole argument: at 2.5x below Terra and 5x below Sol on both sides of the meter, a 1,000-task pipeline costs $220 on Luna versus $550 on Terra and $1,100 on Sol. When the tasks are easy enough that all three tiers would succeed, paying the Terra or Sol rate buys nothing. Budgeting across a whole Codex plan is covered in the best Codex model guide.
What Luna Is Actually Good For
Mechanical edits
High-volume small tasks
Cost-capped pipelines
The common thread: the task must not need the 2.7 points Luna gives up to Terra. If success depends on multi-step reasoning, escalate before you start, to Terra for normal work, or to Sol for the genuinely hard stories. The three-way decision rule is laid out in Sol vs Terra vs Luna.
When Luna Is a False Economy
The practical fix is routing, not loyalty to a tier. Decide per task, and let outcomes tune the rule: mechanical cards to Luna, default work to Terra, hard stories to Sol. That is the model AIDEN operationalizes, your Codex CLI on a kanban board where each story runs its own model on its own branch, so a wrong routing costs you one card, not a pipeline.