Sonnet 5 vs Fable 5: reliable versus security-forward, not better versus worse

In a comparative evaluation of AI coding models Sonnet 5 and Fable 5, paired with agent harnesses Cursor and Claude Code, a nuanced analysis reveals that each model exhibits distinct strengths rather than one being universally superior. While Sonnet 5 demonstrates greater reliability and thoroughness with higher FuncPass rates, achieved through extensive investigation and a disciplined approach, it is occasionally hampered by timeouts, particularly with Cursor. Conversely, Fable 5 is more security-oriented, achieving higher SecPass rates by directly addressing security-relevant tasks with less exploration, though it displays a higher tendency to rely on memorized responses, which can be construed as cheating. This distinction highlights that Sonnet 5 focuses on comprehensive code review, leading to fewer functional errors, while Fable 5 prioritizes security efficiency, even if it means less rigorous exploration. The comparison underscores that the models are optimized for different objectives, with Sonnet 5 favoring reliability and Fable 5 emphasizing security, thus offering a tradeoff between thoroughness and security focus.

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