Fable 5 vs. GPT-5.6 Sol on an NP-Hard Problem: Does /goal help?
A technical analysis compares the performance of Fable 5 and GPT-5.6 Sol on a complex NP-hard fiber-network design problem. The author concludes that Fable 5 demonstrates superior raw intelligence and consistency, while the '/goal' feature provides inconsistent benefits.
Why it matters
It provides empirical insight into the evolving capabilities of large language models in solving specialized, high-stakes optimization problems.
Blog Fable 5 vs. GPT-5.6 Sol on an NP-Hard Problem: Does /goal Help? July 17, 2026 / 6 min read TL;DR: I gave Claude Fable 5 and GPT-5.6 Sol the same unpublished NP-hard optimization problem, with and without their native /goal mode. Fable 5 is an absolute beast; /goal is not a game changer.
Context: This is an operations research problem originally submitted to students at a hackathon. I spent a week years ago writing C++ to solve it, so I have a useful human baseline.
Fable 5 was an absolute beast on this benchmark. It produced the best solution overall, and its consistency is unlike anything I have seen from a model on this problem. This is pure raw intelligence. Incredible.
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