Claude Fable 5 vs GPT-6 Luna
Claude Fable 5 was superseded by Claude Fable 5.1. 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 leads overall, C to D. GPT-6 Luna is about 70x cheaper per task. By suite, Claude Fable 5 takes real-world issues (C- to D+) and 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
Claude Fable 5 leads overall, C to D. By suite: Claude Fable 5 takes real-world issues, C- to D+ (60% vs 50% capability); Claude Fable 5 takes spec planning, C to C- (63% vs 61% capability); Claude Fable 5 takes vibe coding, C+ to F (74% vs 33% capability). Claude Fable 5 used 7% of Claude Max 20x's weekly limit per pass, $0.169 per task at the listed price; at the ~$100 tier the board defaults to that is an estimated ~14% of Claude Max 5x's weekly limit (est, about $0.169 per task). 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 70x cheaper ($0.002 vs $0.169). GPT-6 Luna is faster on real-world issues: a median task takes 1m 0s against 2m 37s. Both finished every task on it. Reasons to pick GPT-6 Luna anyway: it is cheaper per task ($0.002 vs $0.169) and it is faster on real-world issues (1m 0s vs 2m 37s median).
Changed since W35: Claude Fable 5: real-world issues C+ to C-, vibe coding C to C+.
| Dimension | Claude Fable 5 | GPT-6 Luna | Advantage |
|---|---|---|---|
| Capability | 60% | 50% | +10% (Claude Fable 5 leads) |
| Median task time | 2m 37s | 1m 0s | 1m 37s faster (GPT-6 Luna leads) |
| Approx. task cost | $0.153 / task | $0.003 / task | $0.150 / task lower (GPT-6 Luna leads) |
Claude Fable 5: 33.0% of Claude Max 20x (33.0% / 5h, 5.0% / 1w) per run. Its weekly limit holds about 7 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 | GPT-6 Luna | Advantage |
|---|---|---|---|
| Capability | 63% | 61% | +2% (Claude Fable 5 leads) |
| Median task time | 5m 32s | 1m 8s | 4m 24s 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: 10.0% of Claude Max 20x (10.0% / 5h, 2.0% / 1w) per run. Its weekly limit holds about 7 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 | GPT-6 Luna | Advantage |
|---|---|---|---|
| Capability | 74% | 33% | +41% (Claude Fable 5 leads) |
| Median task time | 5m 28s | 1m 37s | 3m 52s faster (GPT-6 Luna leads) |
| Approx. task cost | $0.184 / task | not measured | - |
Claude Fable 5: 2.8% of Claude Max 20x (2.8% / 5h, 0.4% / 1w) per good solve. Its weekly limit holds about 7 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 or GPT-6 Luna?
Claude Fable 5 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 2026-08-30.
Which is cheaper per task, Claude Fable 5 or GPT-6 Luna?
Claude Fable 5 used 7% of Claude Max 20x's weekly limit per pass, $0.169 per task at the listed price; at the ~$100 tier the board defaults to that is an estimated ~14% of Claude Max 5x's weekly limit (est, about $0.169 per task). 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 70x cheaper ($0.002 vs $0.169).
What do Claude Fable 5 and GPT-6 Luna cost a month?
Claude Fable 5 runs on Claude Max 20x at $200/month (public price as of 2026-08-05). GPT-6 Luna runs on ChatGPT Plus at $20/month (public price as of 2026-07-20).