Claude Fable 5 vs GPT-5.6 Luna
Claude Fable 5 was superseded by Claude Fable 5.1. GPT-5.6 Luna was superseded by GPT-6 Luna.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 leads overall, C to D. GPT-5.6 Luna is the cheaper side per task (at most $0.012). By suite, Claude Fable 5 takes real-world issues (C- to D) and spec planning (C to C-). Take GPT-5.6 Luna anyway when you want the side that is cheaper per task.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
Claude Fable 5 leads overall, C to D. By suite: Claude Fable 5 takes real-world issues, C- to D (60% vs 45% capability); Claude Fable 5 takes spec planning, C to C- (63% vs 57% capability); Claude Fable 5 takes vibe coding, C+ to F (74% vs 25% 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-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's $0.169, so GPT-5.6 Luna is the cheaper side. Claude Fable 5 is faster on real-world issues: a median task takes 2m 37s against 3m 7s. Both finished every task on it. Reasons to pick GPT-5.6 Luna anyway: it is cheaper per task (at most $0.012 vs $0.169).
Changed since the previous graded week: Claude Fable 5: real-world issues C+ to C-, vibe coding C to C+; GPT-5.6 Luna: spec planning D+ to C-, vibe coding C- to F.
| Dimension | Claude Fable 5 | GPT-5.6 Luna | Advantage |
|---|---|---|---|
| Capability | 60% | 45% | +15% (Claude Fable 5 leads) |
| Median task time | 2m 37s | 3m 7s | 30s faster (Claude Fable 5 leads) |
| Approx. task cost | $0.153 / task | <$0.015 / task | $0.138 / task lower (GPT-5.6 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-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 | GPT-5.6 Luna | Advantage |
|---|---|---|---|
| Capability | 63% | 57% | +6% (Claude Fable 5 leads) |
| Median task time | 5m 32s | 3m 25s | 2m 6s faster (GPT-5.6 Luna leads) |
| Approx. task cost | $0.230 / task | <$0.057 / task | $0.172 / task lower (GPT-5.6 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-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 | GPT-5.6 Luna | Advantage |
|---|---|---|---|
| Capability | 74% | 25% | +49% (Claude Fable 5 leads) |
| Median task time | 5m 28s | 3m 40s | 1m 48s faster (GPT-5.6 Luna leads) |
| Approx. task cost | $0.184 / task | <$0.230 / task | $0.046 / task lower (Claude Fable 5 leads) |
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-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 or GPT-5.6 Luna?
Claude Fable 5 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 2026-08-30.
Which is cheaper per task, Claude Fable 5 or GPT-5.6 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-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's $0.169, so GPT-5.6 Luna is the cheaper side.
What do Claude Fable 5 and GPT-5.6 Luna cost a month?
Claude Fable 5 runs on Claude Max 20x at $200/month (public price as of 2026-08-05). GPT-5.6 Luna runs on ChatGPT Pro 5x at $100/month (public price as of 2026-09-05).