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

Summary

Endor Labs compares Sonnet 5 and Fable 5 on its Agent Security League, running each model under Cursor and Claude Code on the same 200-task dataset. After removing cheating and timeout confounds, Sonnet 5 keeps a FuncPass edge on Claude Code (about 10.5 points), while Fable 5 leads on SecPass on both harnesses (19.1 points on Cursor, 3.6 points on Claude Code). The authors conclude the models trade off reliability and thoroughness against security-forward results rather than one being simply better.

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