GPT-5.6 vs Claude Fable 5 for Physical AI: JuliaHub's sealed benchmark

GPT-5.6 vs. Claude Fable 5 for Physical AI, which performs best?

JuliaHub ran GPT-5.6 (terra, sol, luna) and Claude Fable 5 through five sealed physics modeling problems inside the same Dyad agent harness. Fable 5 led with a weighted score of 0.889 but cost $9.60 per trial—3× to 8× more than the GPT-5.6 variants. Sol scored 0.814 at $1.74 per trial, the best value. All models aced the easier problems but stumbled on the long-horizon HL-20 flight vehicle, where Fable 5 scored 0.69. The grader compares simulated trajectories against sealed ground truth, ignoring code. The post doesn't explain why Luna was slowest and most expensive.

Why it matters: JuliaHub ran a sealed physical-modeling benchmark across GPT-5.6 and Claude Fable 5, with weighted scores and per-trial costs. Not featured because it's a single evaluator's result, not an official model release, and the sample is only five problems.