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Head-to-head comparison

GPT-5.6 Luna vs GPT-6 Astra

GPT-5.6 Luna (OpenAI) and GPT-6 Astra (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 9 September 2026

As of 9 September 2026, GPT-6 Astra holds the stronger ModelCap Index position (#1 vs #25); and GPT-5.6 Luna is 41.7× cheaper per output token ($1.20/1M vs $50.00/1M).

Which should you choose?

Choose GPT-5.6 Luna if…

  • API cost matters — $1.20/1M output tokens against $50.00/1M, about 41.7× cheaper.
  • your workload is prompt-heavy — input tokens cost $0.20/1M against $10.00/1M.
  • you want provider choice — 3 listed API providers against 2.

Choose GPT-6 Astra if…

  • you want the stronger overall ModelCap Index position — #1 against #25 (93.7 vs 70.5 points).
  • LMArena Agent is your yardstick — 12.5% against 1.1%.
  • ARC-AGI-2 is your yardstick — 95.0% against 59.5%.
  • ARC-AGI-3 is your yardstick — 62.7% against 0.2%.
  • AA Intelligence Index is your yardstick — 52.8 against 37.5.
  • AA Terminal-Bench 2.1 is your yardstick — 88.4% against 80.9%.
  • AA Humanity's Last Exam is your yardstick — 54.7% against 39.5%.
  • Terminal-Bench 4.0 is your yardstick — 58.2% against 17.3%.
  • you want the more recently listed model — GPT-6 Astra was listed 4 September 2026, GPT-5.6 Luna 9 July 2026.

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

Specification comparison of GPT-5.6 Luna and GPT-6 Astra
FieldGPT-5.6 LunaOpenAIGPT-6 AstraOpenAI
ModelCap Index position#25#1
Index score70.593.7
EvidenceMeasuredMeasured
Input price / 1M tokens$0.20$10.00
Output price / 1M tokens$1.20$50.00
Context window1M tokens1M tokens
Max output tokens128K128K
API providers listed32
Weight accessAPI onlyAPI only
Input modalitiesFile, Image, TextFile, Image, Text
First listed9 July 20264 September 2026
PublisherOpenAIOpenAI

Evidence: 6 public benchmark observations across 6 boards · 4 public benchmark observations across 4 boards. Prices are the lowest listed API offer per million tokens observed on the OpenRouter catalogue.

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

Public benchmark boards where GPT-5.6 Luna or GPT-6 Astra has a published result
BoardGPT-5.6 LunaGPT-6 AstraLeads
Arena codingArena (LMArena)14971489–1505X-High—Only one result
LMArena AgentLMArena1.1%0.0%–2.2%X-High12.5%10.4%–14.7%MaxGPT-6 Astra
ARC-AGI-2ARC Prize Foundation59.5%Max95.0%MaxGPT-6 Astra
ARC-AGI-3ARC Prize Foundation0.2%Max62.7%MaxGPT-6 Astra
AA Intelligence IndexArtificial Analysis37.5intelligence-index:v4.3:max52.8intelligence-index:v4.3:maxGPT-6 Astra
AA Terminal-Bench 2.1Artificial Analysis80.9%terminal-bench:2.1:terminus-2:e2b:pass-at-1:3-repeats:source-model="GPT-5.6 Luna (max)":reasoning=true88.4%terminal-bench:2.1:terminus-2:e2b:pass-at-1:3-repeats:source-model="GPT-6 Astra (max)":reasoning=trueGPT-6 Astra
AA Humanity's Last ExamArtificial Analysis39.5%hle:may-2025:text-only-2158:no-tools:pass-at-1:source-model="GPT-5.6 Luna (max)":reasoning=true54.7%hle:may-2025:text-only-2158:no-tools:pass-at-1:source-model="GPT-6 Astra (max)":reasoning=trueGPT-6 Astra
Terminal-Bench 4.0Terminal-Bench (Laude Institute)17.3%14.4%–20.1%Codex · Max58.2%55.4%–61.0%Codex · MaxGPT-6 Astra

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 Luna vs GPT-6 Astra: common questions

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

GPT-6 Astra ranks higher on the ModelCap Index as of 9 September 2026: #1 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 Luna cheaper than GPT-6 Astra?

GPT-5.6 Luna is cheaper on output tokens: $1.20/1M against $50.00/1M. Input tokens are $0.20/1M for GPT-5.6 Luna and $10.00/1M for GPT-6 Astra. 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 $1.00 for GPT-5.6 Luna versus $45.00 for GPT-6 Astra. GPT-5.6 Luna costs 97.8% 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 Luna or GPT-6 Astra?

Both publish a 1M-token context window.

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

AA Terminal-Bench 2.1: GPT-5.6 Luna 80.9%, GPT-6 Astra 88.4% — GPT-6 Astra leads. Terminal-Bench 4.0: GPT-5.6 Luna 17.3%, GPT-6 Astra 58.2% — GPT-6 Astra leads.

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

GPT-5.6 Luna: API only. GPT-6 Astra: 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 Luna and GPT-6 Astra?

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

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

Every figure comes from the sealed ModelCap dataset published 9 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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