Claude Fable 5.1 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GPT-5.6 Luna was superseded by GPT-6 Luna.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 Fable 5.1 leads overall, B- to D. GPT-5.6 Luna is the cheaper side per task (at most $0.012). By suite, Claude Fable 5.1 takes real-world issues (B+ to D) and spec planning (C+ to C-). Take GPT-5.6 Luna anyway when you want the side that is cheaper per task and faster on real-world issues.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 Fable 5.1
Claude Fable 5.1 leads overall, B- to D. By suite: Claude Fable 5.1 takes real-world issues, B+ to D (87% vs 45% capability); Claude Fable 5.1 takes spec planning, C+ to C- (71% vs 57% capability); Claude Fable 5.1 takes vibe coding, C+ to F (74% vs 25% capability). Claude Fable 5.1 used 8% of Claude Max 5x's weekly limit per pass, $0.097 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 Claude Fable 5.1's $0.097, 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 3m 38s. Both finished every task on it. Reasons to pick GPT-5.6 Luna anyway: it is cheaper per task (at most $0.012 vs $0.097) and it is faster on real-world issues (3m 7s vs 3m 38s median).
Changed since the previous graded week: Claude Fable 5.1: real-world issues D+ to B+, real-world issues usage 7% to 5% of the weekly limit, spec planning usage 4% to 3% of the weekly limit, vibe coding C- to C+, vibe coding usage 0.6% to 0.4% of the weekly limit; GPT-5.6 Luna: spec planning D+ to C-, vibe coding C- to F.
| Dimension | Claude Fable 5.1 | GPT-5.6 Luna | Advantage |
|---|---|---|---|
| Capability | 87% | 45% | +42% (Claude Fable 5.1 leads) |
| Median task time | 3m 38s | 3m 7s | 31s faster (GPT-5.6 Luna leads) |
| Approx. task cost | $0.077 / task | <$0.015 / task | $0.061 / task lower (GPT-5.6 Luna leads) |
Claude Fable 5.1: 64.0% of Claude Max 5x (64.0% / 5h, 5.0% / 1w) 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 Fable 5.1 | GPT-5.6 Luna | Advantage |
|---|---|---|---|
| Capability | 71% | 57% | +14% (Claude Fable 5.1 leads) |
| Median task time | 8m 55s | 3m 25s | 5m 29s faster (GPT-5.6 Luna leads) |
| Approx. task cost | $0.172 / task | <$0.057 / task | $0.115 / task lower (GPT-5.6 Luna leads) |
Claude Fable 5.1: 33.0% of Claude Max 5x (33.0% / 5h, 3.0% / 1w) 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 Fable 5.1 | GPT-5.6 Luna | Advantage |
|---|---|---|---|
| Capability | 74% | 25% | +49% (Claude Fable 5.1 leads) |
| Median task time | 4m 14s | 3m 40s | 33s faster (GPT-5.6 Luna leads) |
| Approx. task cost | $0.092 / task | <$0.230 / task | $0.138 / task lower (Claude Fable 5.1 leads) |
Claude Fable 5.1: 4.2% of Claude Max 5x (4.2% / 5h, 0.4% / 1w) 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 Fable 5.1 or GPT-5.6 Luna?
Claude Fable 5.1 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, Claude Fable 5.1 or GPT-5.6 Luna?
Claude Fable 5.1 used 8% of Claude Max 5x's weekly limit per pass, $0.097 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 Claude Fable 5.1's $0.097, so GPT-5.6 Luna is the cheaper side.
What do Claude Fable 5.1 and GPT-5.6 Luna cost a month?
Claude Fable 5.1 runs on Claude Max 5x at $100/month (public price as of 2026-07-27). GPT-5.6 Luna runs on ChatGPT Pro 5x at $100/month (public price as of 2026-09-05).