Claude Fable 5.1 vs GPT-5.6 Terra
Claude Fable 5.1 leads overall, B- to D. GPT-5.6 Terra is about 5.0x cheaper per task. By suite, Claude Fable 5.1 takes real-world issues (B+ to D+) and spec planning (C+ to D+). Take GPT-5.6 Terra 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-5.6 Terra 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 D+ (87% vs 50% capability); Claude Fable 5.1 takes spec planning, C+ to D+ (71% vs 53% capability); Claude Fable 5.1 takes vibe coding, C+ to F (74% vs 34% 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-5.6 Terra used 8% of ChatGPT Plus's weekly limit per pass, $0.019 per task at the listed price; at the ~$100 tier the board defaults to that is an estimated ~1.6% of ChatGPT Pro 5x's weekly limit (est, about $0.019 per task). Per task, GPT-5.6 Terra is about 5.0x cheaper ($0.019 vs $0.097). GPT-5.6 Terra is faster on real-world issues: a median task takes 2m 18s against 3m 38s. Both finished every task on it. Reasons to pick GPT-5.6 Terra anyway: it is cheaper per task ($0.019 vs $0.097) and it is faster on real-world issues (2m 18s vs 3m 38s median).
Changed since the previous graded week: 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-5.6 Terra: real-world issues D to D+, real-world issues usage 9.2% to 7% of the weekly limit, spec planning C- to D+, spec planning usage 2.0% to 1% of the weekly limit, vibe coding F to D, vibe coding usage 0.5% to 0.6% of the weekly limit.
| Dimension | Claude Fable 5.1 | GPT-5.6 Terra | Advantage |
|---|---|---|---|
| Capability | 87% | 50% | +37% (Claude Fable 5.1 leads) |
| Median task time | 3m 38s | 2m 18s | 1m 20s faster (GPT-5.6 Terra leads) |
| Approx. task cost | $0.077 / task | $0.021 / task | $0.055 / task lower (GPT-5.6 Terra leads) |
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-5.6 Terra: 7.0% of ChatGPT Plus (46.0% / 5h, 7.0% / 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 Terra | Advantage |
|---|---|---|---|
| Capability | 71% | 53% | +17% (Claude Fable 5.1 leads) |
| Median task time | 8m 55s | 2m 37s | 6m 18s faster (GPT-5.6 Terra leads) |
| Approx. task cost | $0.172 / task | $0.011 / task | $0.161 / task lower (GPT-5.6 Terra 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-5.6 Terra: 1.0% of ChatGPT Plus (10.0% / 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 Terra | Advantage |
|---|---|---|---|
| Capability | 74% | 34% | +40% (Claude Fable 5.1 leads) |
| Median task time | 4m 14s | 2m 47s | 1m 26s faster (GPT-5.6 Terra leads) |
| Approx. task cost | $0.092 / task | $0.028 / task | $0.064 / task lower (GPT-5.6 Terra 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-5.6 Terra: 0.6% of ChatGPT Plus (3.0% / 5h, 0.6% / 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 Terra?
Claude Fable 5.1 clears higher: tier 4 of 5 versus tier 2 for GPT-5.6 Terra 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-5.6 Terra?
Claude Fable 5.1 used 8% of Claude Max 5x's weekly limit per pass, $0.097 per task at the listed price. GPT-5.6 Terra used 8% of ChatGPT Plus's weekly limit per pass, $0.019 per task at the listed price; at the ~$100 tier the board defaults to that is an estimated ~1.6% of ChatGPT Pro 5x's weekly limit (est, about $0.019 per task). Per task, GPT-5.6 Terra is about 5.0x cheaper ($0.019 vs $0.097).
What do Claude Fable 5.1 and GPT-5.6 Terra cost a month?
Claude Fable 5.1 runs on Claude Max 5x at $100/month (public price as of 2026-07-27). GPT-5.6 Terra runs on ChatGPT Plus at $20/month (public price as of 2026-07-20).