Claude Fable 5.1 vs GPT-6 Luna
Claude Fable 5.1 leads overall, C- to D. GPT-6 Luna is about 55x cheaper per task. By suite, real-world issues is even at D+ and Claude Fable 5.1 takes spec planning (C+ to C-). Take GPT-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-6 Luna vs Claude Fable 5.1
Claude Fable 5.1 leads overall, C- to D. By suite: real-world issues is even at D+ (53% vs 50% capability); Claude Fable 5.1 takes spec planning, C+ to C- (70% vs 61% capability); Claude Fable 5.1 takes vibe coding, C- to F (59% vs 33% capability). Claude Fable 5.1 used 11% of Claude Max 5x's weekly limit per pass, $0.133 per task at the listed price. GPT-6 Luna used 1% of ChatGPT Plus's weekly limit per pass, $0.002 per task at the listed price; at the ~$100 tier the board defaults to that is an estimated ~0.2% of ChatGPT Pro 5x's weekly limit (est, about $0.002 per task). Per task, GPT-6 Luna is about 55x cheaper ($0.002 vs $0.133). GPT-6 Luna is faster on real-world issues: a median task takes 1m 0s against 2m 20s. Both finished every task on it. Reasons to pick GPT-6 Luna anyway: it is cheaper per task ($0.002 vs $0.133) and it is faster on real-world issues (1m 0s vs 2m 20s median).
| Dimension | Claude Fable 5.1 | GPT-6 Luna | Advantage |
|---|---|---|---|
| Capability | 53% | 50% | +3% (Claude Fable 5.1 leads) |
| Median task time | 2m 20s | 1m 0s | 1m 20s faster (GPT-6 Luna leads) |
| Approx. task cost | $0.107 / task | $0.003 / task | $0.104 / task lower (GPT-6 Luna leads) |
Claude Fable 5.1: 100.0% of Claude Max 5x (100.0% / 5h, 7.0% / 1w) per run. Its weekly limit holds about 14.6 of the week's 33.6 5h windows. GPT-6 Luna: 1.0% of ChatGPT Plus (1.0% / 5h, 1.0% / 1w) 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 | Claude Fable 5.1 | GPT-6 Luna | Advantage |
|---|---|---|---|
| Capability | 70% | 61% | +9% (Claude Fable 5.1 leads) |
| Median task time | 6m 1s | 1m 8s | 4m 53s faster (GPT-6 Luna leads) |
| Approx. task cost | $0.230 / task | <$0.011 / task | $0.218 / task lower (GPT-6 Luna leads) |
Claude Fable 5.1: 49.0% of Claude Max 5x (49.0% / 5h, 4.0% / 1w) per run. Its weekly limit holds about 14.6 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 | Claude Fable 5.1 | GPT-6 Luna | Advantage |
|---|---|---|---|
| Capability | 59% | 33% | +26% (Claude Fable 5.1 leads) |
| Median task time | 3m 58s | 1m 37s | 2m 21s faster (GPT-6 Luna leads) |
| Approx. task cost | $0.138 / task | not measured | - |
Claude Fable 5.1: 6.2% of Claude Max 5x (6.2% / 5h, 0.6% / 1w) per good solve. Its weekly limit holds about 14.6 of the week's 33.6 5h windows. GPT-6 Luna: under 1% of ChatGPT Plus (0.4% / 5h) 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, Claude Fable 5.1 or GPT-6 Luna?
Claude Fable 5.1 clears higher: tier 3 of 5 versus tier 2 for GPT-6 Luna on real-world@v3, our private suite of real-world coding problems. Latest measurement 2026-09-20.
Which is cheaper per task, Claude Fable 5.1 or GPT-6 Luna?
Claude Fable 5.1 used 11% of Claude Max 5x's weekly limit per pass, $0.133 per task at the listed price. GPT-6 Luna used 1% of ChatGPT Plus's weekly limit per pass, $0.002 per task at the listed price; at the ~$100 tier the board defaults to that is an estimated ~0.2% of ChatGPT Pro 5x's weekly limit (est, about $0.002 per task). Per task, GPT-6 Luna is about 55x cheaper ($0.002 vs $0.133).
What do Claude Fable 5.1 and GPT-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-6 Luna runs on ChatGPT Plus at $20/month (public price as of 2026-07-20).