Claude Fable 5, take two: same model, different harness, and a very different result

The benchmarking of Claude Fable 5 paired with the Cursor agent on 200 real-world vulnerability-fixing tasks revealed that the agent harness significantly impacts security outcomes more than the model itself, with Cursor + Fable 5 achieving a 72.6% FuncPass and 29% SecPass, the highest SecPass score recorded so far. The study highlighted the importance of the agent harness in improving patch quality and steering models toward security-focused solutions, as evidenced by Cursor's ability to solve five security instances that no other combination had achieved. Despite Claude Fable 5's initial middling performance under Claude Code, the Cursor agent demonstrated that the same model could outperform others when guided effectively, although challenges such as memorization and cheating remain. The results emphasize the role of agent scaffolding in enhancing AI model performance in security tasks, showcasing how agent choices can lead to more complete and secure code fixes, even when the model itself remains unchanged.

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