ZMIME
Comparison · 2 models · Updated Oct 4, 2026

GPT-6 Luna vs Step 3.5 Flash

GPT-6 Luna comes out ahead, 78 to 62 on our weighted score, though Step 3.5 Flash is 25% cheaper per token.

  1. Our pick

    OpenAI

    GPT-6 Luna

    Released Sep 22, 2026

    78/100
    • ECI—
    • Price$0.10 / $0.50
    • Context1.05M
    Full specs Remove GPT-6 Luna

  2. StepFun

    Step 3.5 Flash

    Released Jan 29, 2026

    62/100
    • ECI—
    • Price$0.10 / $0.30
    • Context256K
    Full specs Remove Step 3.5 Flash

  3. Add a model

    Make it a three-way comparison.

01 — Verdict

GPT-6 Luna is our pick

GPT-6 Luna is the better all-round choice, scoring 78/100 against Step 3.5 Flash (62). It leads on inputs & features and context window. Step 3.5 Flash wins on price. The score weighs price 50%, inputs & features 30%, context window 20%. None of the shared benchmarks cover every model here yet, so capability is left out of this verdict.

  • CapabilityNot enough dataNo independent benchmark covers every model here yet
  • Lowest priceStep 3.5 FlashStep 3.5 Flash $0.15 · GPT-6 Luna $0.20 per 1M tokens (3:1 blend)
  • Longest contextGPT-6 LunaGPT-6 Luna 1,050,000 · Step 3.5 Flash 256,000 tokens
  • Widest inputsGPT-6 LunaGPT-6 Luna: Text, Images, PDFs · Step 3.5 Flash: Text
  • Self-hostingStep 3.5 FlashPublishes downloadable weights
How the score is built
MeasureWeightGPT-6 LunaStep 3.5 Flash
Price50%8389
Inputs & features30%8035
Context window20%6136
Overall100%78/10062/100

Left out because at least one model lacks the data: capability. The remaining weights were rescaled.

02 — Side by side

Every spec in one table

Highlighted cells lead their row. Dashes mean the data is not published.

GPT-6 Luna vs Step 3.5 Flash specifications side by side
SpecificationGPT-6 LunaOpenAIStep 3.5 FlashStepFun
Capability
Capabilities Index (ECI)——
ECI rank——
GPQA DiamondGraduate-level science questions90.5%—
FrontierMath Tiers 1–3Research-level mathematics79.0%—
OTIS Mock AIME 2024–2025Competition mathematics98.9%—
SimpleQA VerifiedShort factual questions41.4%—
Price per million tokens
Input$0.10$0.10
Output$0.50$0.30 (best)
Cached input$0.01 (best)$0.02
Blended (3:1)$0.20$0.15 (best)
Long-context rateOver 272K: $0.20 / $0.75Same rate
Price sourceOfficial OpenAI APIOfficial StepFun (Global) API
Limits
Context window1,050,000 tokens (best)256,000 tokens
Max output128,000 tokens256,000 tokens (best)
Inputs and features
TextYesYes
ImagesYesNo
PDFsYesNo
AudioNoNo
VideoNoNo
ReasoningYeslow · medium · high · xhigh · maxYeslow · high
Tool callingYesYes
Structured outputYesNo
Availability
WeightsProprietaryOpen
API model IDgpt-6-lunastep-3.5-flash
API providers24 (best)8
ReleasedSep 22, 2026Jan 29, 2026
Knowledge cutoffMay 18, 2026Jan 2025
03 — Cost

What would a month cost?

Enter your expected volume in millions of tokens. List prices only; caching and batch discounts would lower these.

  • GPT-6 Luna$2.00
  • Step 3.5 Flash$1.60
04 — Questions

Which should you choose?

Which is better: GPT-6 Luna or Step 3.5 Flash?

GPT-6 Luna is the better all-round choice, scoring 78/100 against Step 3.5 Flash (62). It leads on inputs & features and context window. Step 3.5 Flash wins on price. The score weighs price 50%, inputs & features 30%, context window 20%. None of the shared benchmarks cover every model here yet, so capability is left out of this verdict.

Which is cheaper, GPT-6 Luna or Step 3.5 Flash?

Step 3.5 Flash is cheaper at $0.10 input / $0.30 output per million tokens (official StepFun (Global) API price). GPT-6 Luna costs $0.10 input / $0.50 output per million tokens (official OpenAI API price). At a typical mix of three input tokens to one output token, that is $0.15 per million tokens for Step 3.5 Flash versus $0.20 for GPT-6 Luna (1.3× as much).

Which scores higher on benchmarks?

There is no independent benchmark that covers both models yet. GPT-6 Luna has not been scored yet and Step 3.5 Flash has not been scored yet.

Which is better for coding?

There are no published SWE-bench Verified results for GPT-6 Luna and Step 3.5 Flash yet, so there is no like-for-like coding score. Test both on a sample of your own tasks. Both support tool calling for agent workflows.

Which has the bigger context window?

GPT-6 Luna has the largest context window at 1,050,000 tokens, against 256,000 for Step 3.5 Flash. Maximum output per response: GPT-6 Luna up to 128,000, Step 3.5 Flash up to 256,000 tokens.

Which can read images, PDFs, audio or video?

GPT-6 Luna accepts text, images and PDFs; Step 3.5 Flash accepts text. GPT-6 Luna handles the widest range of inputs.

Are any of these open source?

Step 3.5 Flash publishes its weights and can be self-hosted; GPT-6 Luna is proprietary.

Which is newer?

GPT-6 Luna is the newest, released Sep 22, 2026. Step 3.5 Flash came out Jan 29, 2026. Knowledge cutoff: GPT-6 Luna May 18, 2026, Step 3.5 Flash Jan 2025.

How do you decide the winner?

Each model gets a 0–100 score on capability (50%, independent benchmark results); price (25%, blended price per million tokens (3 input : 1 output), log scale); inputs & features (15%, image, PDF, audio and video input, tool calling, structured output and reasoning); context window (10%, maximum tokens per request, log scale). Dimensions missing for any model are dropped and the remaining weights rescaled, so every model is judged on the same evidence. Specs and prices come from public model listings and the labs’ own API pages; capability scores come from independent benchmark runs. Data updated Oct 4, 2026.

05 — Keep comparing

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