Fable 5 vs. GPT-5.6 Sol on an NP-Hard Problem: Does /goal Help?

Fable 5 vs. GPT-5.6 Sol on an NP-Hard Problem: Does /goal help?

The author tested Claude Fable 5 and GPT-5.6 Sol on an unpublished fiber-network optimization problem, each with three 30-minute runs, comparing plain mode against /goal. Fable 5's plain mean was 32,386—1,875 points lower than Sol's 34,261—and its three plain runs stayed within a 319-point range, showing remarkable consistency. /goal won four of six trials but made both models' means worse: Fable 5 by 759 points, Sol by 868. The feature occasionally gives a small edge but can also cause large regressions. The post also breaks down how /goal differs under the hood: Claude Code uses Haiku as a transcript-only evaluator, while Codex has persisted state and lifecycle tools. Bottom line: Fable 5 is the real story here; /goal is not a safe default.

Why it matters: First-person experiment with concrete numbers across 3 runs per model. The counterintuitive finding that /goal mode destabilizes Fable 5 is worth surfacing. Docked slightly because the problem domain is narrow and this is a personal blog, not an official release.