GPT-5.6 Terra vs GPT-6 Luna
Both grade D overall. By suite, GPT-6 Luna takes real-world issues (C- to D+) and spec planning is even at D+. Both agents run the same private suite cut headless through their own tools, each on its own subscription. The grades are re-measured every week, and each cost figure is read off the provider's own usage meter.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-6 Luna vs GPT-5.6 Terra
Both grade D overall. By suite: GPT-6 Luna takes real-world issues, C- to D+ (57% vs 50% capability); spec planning is even at D+ (53% vs 55% capability); vibe coding is even at F (34% vs 31% capability). GPT-5.6 Terra used 8% of ChatGPT Plus's weekly limit per pass, $0.019 per task at the listed price; at the ~$100 tier the board defaults to that is an estimated ~1.6% of ChatGPT Pro 5x's weekly limit (est, about $0.019 per task). GPT-6 Luna used <1% of ChatGPT Plus's weekly limit per pass; at the ~$100 tier the board defaults to that is an estimated ~<0.2% of ChatGPT Pro 5x's weekly limit (est). GPT-6 Luna is faster on real-world issues: a median task takes 1m 4s against 2m 18s. Both finished every task on it.
Changed since the previous graded week: GPT-5.6 Terra: real-world issues D to D+, real-world issues usage 9.2% to 7% of the weekly limit, spec planning C- to D+, spec planning usage 2.0% to 1% of the weekly limit, vibe coding F to D, vibe coding usage 0.5% to 0.6% of the weekly limit; GPT-6 Luna: real-world issues D+ to C-, spec planning C- to D+.
| Dimension | GPT-5.6 Terra | GPT-6 Luna | Advantage |
|---|---|---|---|
| Capability | 50% | 57% | +7% (GPT-6 Luna leads) |
| Median task time | 2m 18s | 1m 4s | 1m 13s faster (GPT-6 Luna leads) |
| Approx. task cost | $0.021 / task | not measured | - |
GPT-5.6 Terra: 7.0% of ChatGPT Plus (46.0% / 5h, 7.0% / 1w) per run. Its weekly limit holds about 6 of the week's 33.6 5h windows. GPT-6 Luna: under 1% of ChatGPT Plus (2.0% / 5h) per run. Its weekly limit holds about 6 of the week's 33.6 5h windows. Each figure is a share of that agent's own plan, so there is no delta to draw.
| Dimension | GPT-5.6 Terra | GPT-6 Luna | Advantage |
|---|---|---|---|
| Capability | 53% | 55% | +1% (GPT-6 Luna leads) |
| Median task time | 2m 37s | 1m 20s | 1m 17s faster (GPT-6 Luna leads) |
| Approx. task cost | $0.011 / task | <$0.011 / task | Even |
GPT-5.6 Terra: 1.0% of ChatGPT Plus (10.0% / 5h, 1.0% / 1w) per run. Its weekly limit holds about 6 of the week's 33.6 5h windows. GPT-6 Luna: under 1% of ChatGPT Plus per run. Its weekly limit holds about 6 of the week's 33.6 5h windows. Each figure is a share of that agent's own plan, so there is no delta to draw.
| Dimension | GPT-5.6 Terra | GPT-6 Luna | Advantage |
|---|---|---|---|
| Capability | 34% | 31% | +3% (GPT-5.6 Terra leads) |
| Median task time | 2m 47s | 1m 24s | 1m 24s faster (GPT-6 Luna leads) |
| Approx. task cost | $0.028 / task | <$0.046 / task | $0.018 / task lower (GPT-5.6 Terra leads) |
GPT-5.6 Terra: 0.6% of ChatGPT Plus (3.0% / 5h, 0.6% / 1w) per good solve. Its weekly limit holds about 6 of the week's 33.6 5h windows. GPT-6 Luna: under 1% of ChatGPT Plus per good solve. Its weekly limit holds about 6 of the week's 33.6 5h windows. 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, GPT-5.6 Terra or GPT-6 Luna?
Both clear tier 2 of 5, but GPT-6 Luna scores higher on capability: 57% versus 50% on real-world@v3, our private suite of real-world coding problems. Latest measurement 2026-09-28.
Which is cheaper per task, GPT-5.6 Terra or GPT-6 Luna?
GPT-5.6 Terra used 8% of ChatGPT Plus's weekly limit per pass, $0.019 per task at the listed price; at the ~$100 tier the board defaults to that is an estimated ~1.6% of ChatGPT Pro 5x's weekly limit (est, about $0.019 per task). GPT-6 Luna used <1% of ChatGPT Plus's weekly limit per pass; at the ~$100 tier the board defaults to that is an estimated ~<0.2% of ChatGPT Pro 5x's weekly limit (est).
What do GPT-5.6 Terra and GPT-6 Luna cost a month?
GPT-5.6 Terra runs on ChatGPT Plus at $20/month (public price as of 2026-07-20). GPT-6 Luna runs on ChatGPT Plus at $20/month (public price as of 2026-07-20).