Gemini 3.8 Flash vs GPT-5.6 Luna
GPT-5.6 Luna was superseded by GPT-6 Luna. The grades below are the last ones measured, so read them as history rather than as this week's board. See the current roster.
Gemini 3.8 Flash leads overall, C- to D. GPT-5.6 Luna is the cheaper side per task (at most $0.012). By suite, Gemini 3.8 Flash takes real-world issues (C- to D) and GPT-5.6 Luna takes spec planning (C- to D+). Both agents run the same private suite cut headless through their own tools, each on its own subscription.Measured
Each agent graded on the same private suites, running headless through its own tools. Advantages are oriented so a win is a win, whichever way the metric runs.
Verdict: GPT-5.6 Luna vs Gemini 3.8 Flash
Gemini 3.8 Flash leads overall, C- to D. By suite: Gemini 3.8 Flash takes real-world issues, C- to D (60% vs 45% capability); GPT-5.6 Luna takes spec planning, C- to D+ (57% vs 50% capability); Gemini 3.8 Flash takes vibe coding, C- to F (56% vs 25% capability). Gemini 3.8 Flash used 23.7% of Google AI Pro's weekly limit per pass, $0.057 per task at the listed price. GPT-5.6 Luna used <1% of ChatGPT Pro 5x's weekly limit per pass, at most $0.012 per task at the listed price. Per task, GPT-5.6 Luna costs at most $0.012 against Gemini 3.8 Flash's $0.057, so GPT-5.6 Luna is the cheaper side. GPT-5.6 Luna is faster on real-world issues: a median task takes 3m 7s against 5m 21s. Both finished every task on it. Reasons to pick GPT-5.6 Luna anyway: it is cheaper per task (at most $0.012 vs $0.057), it is faster on real-world issues (3m 7s vs 5m 21s median) and it leads on spec planning (C- to D+).
Changed since the previous graded week: Gemini 3.8 Flash: real-world issues F to C-, real-world issues usage 18.0% to 21.9% of the weekly limit, spec planning usage 2.0% to 1.8% of the weekly limit, vibe coding D+ to C-, vibe coding usage 1.2% to 1.1% of the weekly limit; GPT-5.6 Luna: spec planning D+ to C-, vibe coding C- to F.
| Dimension | Gemini 3.8 Flash | GPT-5.6 Luna | Advantage |
|---|---|---|---|
| Capability | 60% | 45% | +15% (Gemini 3.8 Flash leads) |
| Median task time | 5m 21s | 3m 7s | 2m 14s faster (GPT-5.6 Luna leads) |
| Approx. task cost | $0.067 / task | <$0.015 / task | $0.052 / task lower (GPT-5.6 Luna leads) |
Gemini 3.8 Flash: 21.9% of Google AI Pro (131.3% / 5h, 21.9% / 1w) per run. Its weekly limit holds 6 of the week's 33.6 5h windows. GPT-5.6 Luna: under 1% of ChatGPT Pro 5x per run. Each figure is a share of that agent's own plan, so there is no delta to draw.
| Dimension | Gemini 3.8 Flash | GPT-5.6 Luna | Advantage |
|---|---|---|---|
| Capability | 50% | 57% | +6% (GPT-5.6 Luna leads) |
| Median task time | 3m 0s | 3m 25s | 25s faster (Gemini 3.8 Flash leads) |
| Approx. task cost | $0.021 / task | <$0.057 / task | $0.036 / task lower (Gemini 3.8 Flash leads) |
Gemini 3.8 Flash: 1.8% of Google AI Pro (11.0% / 5h, 1.8% / 1w) per run. Its weekly limit holds 6 of the week's 33.6 5h windows. GPT-5.6 Luna: under 1% of ChatGPT Pro 5x per run. Each figure is a share of that agent's own plan, so there is no delta to draw.
| Dimension | Gemini 3.8 Flash | GPT-5.6 Luna | Advantage |
|---|---|---|---|
| Capability | 56% | 25% | +31% (Gemini 3.8 Flash leads) |
| Median task time | 7m 51s | 3m 40s | 4m 11s faster (GPT-5.6 Luna leads) |
| Approx. task cost | $0.048 / task | <$0.230 / task | $0.182 / task lower (Gemini 3.8 Flash leads) |
Gemini 3.8 Flash: 1.1% of Google AI Pro (6.3% / 5h, 1.1% / 1w) per good solve. Its weekly limit holds 6 of the week's 33.6 5h windows. GPT-5.6 Luna: under 1% of ChatGPT Pro 5x per good solve. Each figure is a share of that agent's own plan, so there is no delta to draw.
Grades and deltas are comparable only within a suite version. See methodology.
Frequently asked questions
Which is better for coding, Gemini 3.8 Flash or GPT-5.6 Luna?
Gemini 3.8 Flash clears higher: tier 4 of 5 versus tier 1 for GPT-5.6 Luna on real-world@v3, our private suite of real-world coding problems. Latest measurement 2026-09-27.
Which is cheaper per task, Gemini 3.8 Flash or GPT-5.6 Luna?
Gemini 3.8 Flash used 23.7% of Google AI Pro's weekly limit per pass, $0.057 per task at the listed price. GPT-5.6 Luna used <1% of ChatGPT Pro 5x's weekly limit per pass, at most $0.012 per task at the listed price. Per task, GPT-5.6 Luna costs at most $0.012 against Gemini 3.8 Flash's $0.057, so GPT-5.6 Luna is the cheaper side.