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Head-to-head comparison

GPT-5.6 Terra vs GPT-6 Luna

GPT-5.6 Terra (OpenAI) and GPT-6 Luna (OpenAI) compared on the ModelCap Index, API price, context window, provider availability, weight access and every public benchmark board they share. Figures are the same ones shown on the live rankings; nothing here is a hidden score.

Snapshot as of 22 September 2026

As of 22 September 2026, GPT-5.6 Terra holds the stronger ModelCap Index position (#21 vs #25); and GPT-6 Luna is 24× cheaper per output token ($0.50/1M vs $12.00/1M).

Which should you choose?

Choose GPT-5.6 Terra if…

  • you want the stronger overall ModelCap Index position — #21 against #25 (79.7 vs 78.6 points).
  • you want provider choice — 3 listed API providers against 2.
  • ARC-AGI-2 is your yardstick — 83.9% against 59.3%.
  • ARC-AGI-3 is your yardstick — 0.8% against 0.2%.
  • AA Intelligence Index is your yardstick — 42.1 against 37.3.
  • AA Humanity's Last Exam is your yardstick — 42.9% against 38.5%.

Choose GPT-6 Luna if…

  • API cost matters — $0.50/1M output tokens against $12.00/1M, about 24× cheaper.
  • your workload is prompt-heavy — input tokens cost $0.10/1M against $2.00/1M.
  • you want the more recently listed model — GPT-6 Luna was listed 22 September 2026, GPT-5.6 Terra 9 July 2026.

GPT-5.6 Terra vs GPT-6 Luna: specs, pricing and context

Specification comparison of GPT-5.6 Terra and GPT-6 Luna
FieldGPT-5.6 TerraOpenAIGPT-6 LunaOpenAI
ModelCap Index position#21#25
Index score79.778.6
EvidenceMeasuredMeasured
Input price / 1M tokens$2.00$0.10
Output price / 1M tokens$12.00$0.50
Context window1M tokens1M tokens
Max output tokens128K128K
API providers listed32
Weight accessAPI onlyAPI only
Input modalitiesFile, Image, TextFile, Image, Text
First listed9 July 202622 September 2026
PublisherOpenAIOpenAI

Evidence: 6 public benchmark observations across 6 boards · 3 public benchmark observations across 3 boards. Prices are the lowest listed API offer per million tokens observed on the OpenRouter catalogue.

Benchmark scores: GPT-5.6 Terra vs GPT-6 Luna

Public benchmark boards where GPT-5.6 Terra or GPT-6 Luna has a published result
BoardGPT-5.6 TerraGPT-6 LunaLeads
Arena codingArena (LMArena)15181510–1525X-High—Only one result
LMArena AgentLMArena1.4%0.3%–2.6%X-High—Only one result
ARC-AGI-2ARC Prize Foundation83.9%Max59.3%MaxGPT-5.6 Terra
ARC-AGI-3ARC Prize Foundation0.8%Max0.2%MediumGPT-5.6 Terra
AA Intelligence IndexArtificial Analysis42.1intelligence-index:v4.3:max37.3intelligence-index:v4.3:maxGPT-5.6 Terra
AA τ²-Bench TelecomArtificial Analysis86.3%tau2:telecom:dual-control:pass-at-1:3-repeats:source-model="GPT-5.6 Terra (max)":reasoning=true—Only one result
AA Humanity's Last ExamArtificial Analysis42.9%hle:may-2025:text-only-2158:no-tools:pass-at-1:source-model="GPT-5.6 Terra (max)":reasoning=true38.5%hle:may-2025:text-only-2158:no-tools:pass-at-1:source-model="GPT-6 Luna (max)":reasoning=trueGPT-5.6 Terra
Terminal-Bench 4.0Terminal-Bench (Laude Institute)21.5%18.3%–24.8%Codex · Max—Only one result

Scores are the sources' own published figures for each model's best evaluated configuration; ModelCap never re-runs a benchmark.

Want a different pairing? Open the interactive comparison tool to swap either model for any current ranked language model.

GPT-5.6 Terra vs GPT-6 Luna: common questions

Is GPT-5.6 Terra better than GPT-6 Luna?

GPT-5.6 Terra ranks higher on the ModelCap Index as of 22 September 2026: #21 against #25. That is a capability ranking built from public benchmark evidence with published uncertainty; whether it is "better" for you also depends on price, context and where you can run it. Their published uncertainty intervals overlap, so the rank difference alone does not establish a reliable capability advantage for your workload.

Is GPT-5.6 Terra cheaper than GPT-6 Luna?

GPT-6 Luna is cheaper on output tokens: $0.50/1M against $12.00/1M. Input tokens are $2.00/1M for GPT-5.6 Terra and $0.10/1M for GPT-6 Luna. Prices are the lowest listed API offer ModelCap observed, in USD per million tokens. For 1,000 requests with 2,000 input and 500 output tokens each (2M input + 0.5M output), the listed-rate estimate is $10.00 for GPT-5.6 Terra versus $0.45 for GPT-6 Luna. GPT-6 Luna costs 95.5% less in this scenario. This excludes caching, batch discounts, prompt-length tiers, tool charges and retries; verify the selected endpoint before budgeting.

Which has the bigger context window, GPT-5.6 Terra or GPT-6 Luna?

Both publish a 1M-token context window.

Which is better for coding, GPT-5.6 Terra or GPT-6 Luna?

The two models do not share a coding benchmark board on ModelCap yet, so no head-to-head coding score is published; the ModelCap Index position is the closest overall signal.

Are GPT-5.6 Terra and GPT-6 Luna open-weight models?

GPT-5.6 Terra: API only. GPT-6 Luna: API only. Open weights mean the checkpoint can be downloaded and self-hosted under its licence; API-only models are available solely through hosted endpoints.

Where can I run GPT-5.6 Terra and GPT-6 Luna?

ModelCap currently lists 3 API providers for GPT-5.6 Terra and 2 for GPT-6 Luna, from the OpenRouter catalogue snapshot the site serves; each model page lists the providers and their prices.

How current is this GPT-5.6 Terra vs GPT-6 Luna comparison?

Every figure comes from the sealed ModelCap dataset published 22 September 2026; the page re-renders within a minute of each data refresh, and the ModelCap Index positions are the same ones shown on the live rankings.

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