Claude Fable 5.1 vs GPT-5.6 Luna
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.
| Dimension | Claude Fable 5.1 | GPT-5.6 Luna | Advantage |
|---|---|---|---|
| Capability | 73% | 42% | +32% (Claude Fable 5.1 leads) |
| Median task time | 1m 46s | 1m 53s | 7s faster (Claude Fable 5.1 leads) |
| Approx. task cost | $0.026 / task | $0.003 / task | $0.022 / task lower (GPT-5.6 Luna leads) |
Claude Fable 5.1: 28.0% of Claude Max 20x (28.0% / 5h, 5.0% / 1w) per run. Its weekly limit holds about 7.2 of the week's 33.6 5h windows. GPT-5.6 Luna: 1.0% of ChatGPT Plus (5.0% / 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-5.6 Luna | Advantage |
|---|---|---|---|
| Capability | 74% | 58% | +17% (Claude Fable 5.1 leads) |
| Median task time | 6m 51s | 2m 19s | 4m 32s faster (GPT-5.6 Luna leads) |
| Approx. task cost | $0.051 / task | not measured | - |
Claude Fable 5.1: 15.0% of Claude Max 20x (15.0% / 5h, 2.0% / 1w) per run. Its weekly limit holds about 7.2 of the week's 33.6 5h windows. GPT-5.6 Luna: 0.0% of ChatGPT Plus (1.0% / 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 | Claude Fable 5.1 | GPT-5.6 Luna | Advantage |
|---|---|---|---|
| Capability | 85% | 42% | +43% (Claude Fable 5.1 leads) |
| Median task time | 3m 48s | 3m 5s | 43s faster (GPT-5.6 Luna leads) |
| Approx. task cost | $0.025 / task | $0.003 / task | $0.022 / task lower (GPT-5.6 Luna leads) |
Claude Fable 5.1: 1.8% of Claude Max 20x (1.8% / 5h, 0.2% / 1w) per good solve. Its weekly limit holds about 7.2 of the week's 33.6 5h windows. GPT-5.6 Luna: 0.1% of ChatGPT Plus (0.4% / 5h, 0.1% / 1w) 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-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 2026-09-01.
How much do Claude Fable 5.1 and GPT-5.6 Luna cost?
Claude Fable 5.1 runs on Claude Max 20x at $200/month (public price as of 2026-08-05). GPT-5.6 Luna runs on ChatGPT Plus at $20/month (public price as of 2026-07-20).