Side-by-side comparison of pricing, 12 benchmarks, and generation speed.
| Metric | Step 3.7 Flash | GPT-5.6 Luna (low) |
|---|---|---|
| Input ($/M tokens) | $0.2 | $0.2 |
| Output ($/M tokens) | $1.15 | $1.2 |
Verdict. GPT-5.6 Luna (low) takes the aggregate benchmark matchup 2–0 across 2 categories. Real workloads usually care about a handful of specific tasks — see the per-benchmark table above.
Pricing. Both models sit in the budget bracket for output-token pricing. At 1.0× the per-million-token cost, Step 3.7 Flash is meaningfully cheaper if your traffic is output-heavy (long completions, document generation, agent loops). Step 3.7 Flash makes more sense when output volume is low and absolute reasoning quality justifies the premium.
Strengths. Step 3.7 Flash is strongest on Coding Index (39.6), Intelligence Index (30.9). GPT-5.6 Luna (low) leads on Coding Index (44.2), Intelligence Index (33.9).
Speed. Throughput is comparable — 106 tok/s vs 109 tok/s — so generation speed shouldn't drive your choice here. Look at the per-benchmark wins instead.
Provider. StepFun and OpenAI sell to overlapping but distinct developer audiences: StepFun tends to ship frontier reasoning models with premium positioning, while OpenAI often prices more aggressively. Your existing vendor relationships, billing, and SLA preferences may matter as much as the raw numbers above.
Workload cost. Workload scenarios (per million requests at 30M input + 15M output tokens): Step 3.7 Flash costs $23.25 ($279/year); GPT-5.6 Luna (low) costs $24.00 ($288/year). At a smaller 5M-input/2M-output scale (single-developer tool or prototype): Step 3.7 Flash ≈ $3.30/run, GPT-5.6 Luna (low) ≈ $3.40/run. At agent/realtime scale (200M input / 100M output per million requests): Step 3.7 Flash ≈ $155/run, GPT-5.6 Luna (low) ≈ $160/run. Step 3.7 Flash becomes more attractive at higher volume — the absolute per-token pricing difference compounds when you ship at scale.
Recommendation. Both models have legitimate use cases — the right answer depends on whether you are optimizing for benchmark ceiling, latency, or unit cost. Start with the cheaper / faster model, evaluate against your specific task, and only switch if the upgrade shows a meaningful lift.
Head-to-head deltas
Data from Artificial Analysis API — 12 benchmarks
Step 3.7 Flash is cheaper overall. Its blended price (3:1 input/output ratio) is $0.44/M tokens vs $0.45/M for GPT-5.6 Luna (low).
GPT-5.6 Luna (low) wins 2 out of 12 benchmarks compared to 0 for Step 3.7 Flash. See the detailed benchmark chart above for per-category results.
GPT-5.6 Luna (low) generates tokens faster at 109 tok/s vs 106 tok/s. However, GPT-5.6 Luna (low) has lower time-to-first-token (1.73s vs 2.73s).
Choose based on your priorities: Step 3.7 Flash for lower cost, GPT-5.6 Luna (low) for stronger benchmark performance, and GPT-5.6 Luna (low) for faster generation. For latency-sensitive apps, check the TTFT comparison above.