Claude Sonnet 5.5 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GPT-5.6 Luna was superseded by GPT-6 Luna.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.
Claude Sonnet 5.5 leads overall, C- to D. By suite, Claude Sonnet 5.5 takes real-world issues (C- to D) and spec planning (C+ to C-). The data gives no reason to pick GPT-5.6 Luna over Claude Sonnet 5.5 this week.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 Claude Sonnet 5.5
Claude Sonnet 5.5 leads overall, C- to D. By suite: Claude Sonnet 5.5 takes real-world issues, C- to D (60% vs 45% capability); Claude Sonnet 5.5 takes spec planning, C+ to C- (74% vs 57% capability); Claude Sonnet 5.5 takes vibe coding, D+ to F (48% vs 25% capability). Claude Sonnet 5.5 used <1% of Claude Max 5x's weekly limit per pass, $0.003 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. Claude Sonnet 5.5 is faster on real-world issues: a median task takes 37s against 3m 7s. Both finished every task on it. The data gives no reason to pick GPT-5.6 Luna over Claude Sonnet 5.5 this week.
Changed since W38: GPT-5.6 Luna: spec planning D+ to C-, vibe coding C- to F.
| Dimension | Claude Sonnet 5.5 | GPT-5.6 Luna | Advantage |
|---|---|---|---|
| Capability | 60% | 45% | +15% (Claude Sonnet 5.5 leads) |
| Median task time | 37s | 3m 7s | 2m 30s faster (Claude Sonnet 5.5 leads) |
| Approx. task cost | $0.002 / task | <$0.015 / task | $0.014 / task lower (Claude Sonnet 5.5 leads) |
Claude Sonnet 5.5: 4.0% of Claude Max 5x (4.0% / 5h) per run. Its weekly limit holds about 13.8 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 | Claude Sonnet 5.5 | GPT-5.6 Luna | Advantage |
|---|---|---|---|
| Capability | 74% | 57% | +18% (Claude Sonnet 5.5 leads) |
| Median task time | 3m 25s | 3m 25s | 1s faster (Claude Sonnet 5.5 leads) |
| Approx. task cost | $0.007 / task | <$0.057 / task | $0.051 / task lower (Claude Sonnet 5.5 leads) |
Claude Sonnet 5.5: 4.0% of Claude Max 5x (4.0% / 5h) per run. Its weekly limit holds about 13.8 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 | Claude Sonnet 5.5 | GPT-5.6 Luna | Advantage |
|---|---|---|---|
| Capability | 48% | 25% | +23% (Claude Sonnet 5.5 leads) |
| Median task time | 56s | 3m 40s | 2m 44s faster (Claude Sonnet 5.5 leads) |
| Approx. task cost | $0.001 / task | <$0.230 / task | $0.229 / task lower (Claude Sonnet 5.5 leads) |
Claude Sonnet 5.5: 0.2% of Claude Max 5x (0.2% / 5h) per good solve. Its weekly limit holds about 13.8 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, Claude Sonnet 5.5 or GPT-5.6 Luna?
Claude Sonnet 5.5 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 2026-09-28.
Which is cheaper per task, Claude Sonnet 5.5 or GPT-5.6 Luna?
Claude Sonnet 5.5 used <1% of Claude Max 5x's weekly limit per pass, $0.003 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.
What do Claude Sonnet 5.5 and GPT-5.6 Luna cost a month?
Claude Sonnet 5.5 runs on Claude Max 5x at $100/month (public price as of 2026-07-20). GPT-5.6 Luna runs on ChatGPT Pro 5x at $100/month (public price as of 2026-09-05).