Claude Fable 5.1
Joint first on the re-based Artificial Analysis Intelligence Index v4.3 (53, level with GPT-6 Astra), first on the official Terminal-Bench v4.0 board at 57.9%, and first on Vellum's quality/speed/cost composite at 65%. On Arena text it sits at 1504 inside a 1-6 tie band with the rest of the Anthropic top group. Costs more per Index task than GPT-6 Astra ($7.63 against $3.26), so price the alternative if volume matters.
Challenger: Gemini 4 Carbon (High) — Tops the live Artificial Analysis Intelligence Index v5 on 59, ahead of Claude Opus 5.5 (max) on 58 and GPT-6 Astra (max) on 57, with the incumbent down at 50 and outside the top eight. It is no longer a single-board lead: Arena's published 28 October data has Carbon first on text and second behind Claude Opus 5.5 on the overall agent board, though Vellum has not refreshed since 28 September. Proprietary and hosted, as the incumbent is, so no licence or availability constraint changes; note it costs $7.76 per Index task.
Artificial Analysis has re-based again: the live board is Intelligence Index v5, where Gemini 4 Carbon (high) leads on 59, Claude Opus 5.5 (max) is on 58 and GPT-6 Astra (max) on 57, while Claude Fable 5.1 reads 50 (xhigh, $10.85 per task) and 49 (max, $12.99) — so the v4.3 score of 53 and the $7.63 against $3.26 comparison no longer apply. On Terminal-Bench 4.0 the 57.9% figure is a Claude Code max-effort row on the official board, which runs each vendor in its own agent and so cannot rank vendors against each other; Vals AI's neutral mini-SWE-agent run puts Claude Opus 5.5 first at 65.15%, with Anthropic quoting 66.4% itself. Arena has moved on too: its published 28 October board has Gemini 4 Carbon and Claude Opus 5.5 at the top with Fable 5.1 fourth overall, so ignore the 2 September 1507/1504 snapshot and read the live board.