Claude Fable 5.1 vs GPT-6 Luna
Claude Fable 5.1 leads overall, B- to D. By suite, Claude Fable 5.1 takes real-world issues (B+ to C-) and spec planning (C+ to D+). Take GPT-6 Luna anyway when you want the side that is 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, B- to D. By suite: Claude Fable 5.1 takes real-world issues, B+ to C- (87% vs 57% capability); Claude Fable 5.1 takes spec planning, C+ to D+ (71% vs 55% capability); Claude Fable 5.1 takes vibe coding, C+ to F (74% vs 31% 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-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 3m 38s. Both finished every task on it. Reasons to pick GPT-6 Luna anyway: it is faster on real-world issues (1m 4s vs 3m 38s median).
Changed since W39: 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-6 Luna: real-world issues D+ to C-, spec planning C- to D+.
| Dimension | Claude Fable 5.1 | GPT-6 Luna | Advantage |
|---|---|---|---|
| Capability | 87% | 57% | +30% (Claude Fable 5.1 leads) |
| Median task time | 3m 38s | 1m 4s | 2m 33s faster (GPT-6 Luna leads) |
| Approx. task cost | $0.077 / task | not measured | - |
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-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 | Claude Fable 5.1 | GPT-6 Luna | Advantage |
|---|---|---|---|
| Capability | 71% | 55% | +16% (Claude Fable 5.1 leads) |
| Median task time | 8m 55s | 1m 20s | 7m 35s faster (GPT-6 Luna leads) |
| Approx. task cost | $0.172 / task | <$0.011 / task | $0.161 / task lower (GPT-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-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 | 74% | 31% | +43% (Claude Fable 5.1 leads) |
| Median task time | 4m 14s | 1m 24s | 2m 50s faster (GPT-6 Luna leads) |
| Approx. task cost | $0.092 / task | <$0.046 / task | $0.046 / task lower (GPT-6 Luna 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-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, Claude Fable 5.1 or GPT-6 Luna?
Claude Fable 5.1 clears higher: tier 4 of 5 versus tier 2 for GPT-6 Luna on real-world@v3, our private suite of real-world coding problems. Latest measurement 2026-09-27.
Which is cheaper per task, Claude Fable 5.1 or GPT-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-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 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).