Open benchmarking challenge says Opus 5’s ARC-AGI-3 leap doesn’t carry over
rbhar90 · x · 2026-07-25
The post argues for more open benchmarking, saying it no longer trusts strong reasoning claims from labs because too much money is now tied to the narrative.
It quotes a comparison around Opus 5 on ARC-AGI-3 versus the authors’ held-out Witness suite:
- Opus 5 reportedly scores 30% on ARC-AGI-3, far ahead of prior models.
- On Witness, the jump does not transfer: Opus 5 lands at 43.4 ± 3.2, essentially tied with kimi-k3 (42.8 ± 1.9) and Fable-5 (43.8 ± 9.7).
- It is better than Opus 4.8 (34.8), but the authors say this is not a generational leap.
- The traces suggest Opus 5 already “knows” this class of puzzle: it states hidden rules before acting and then plays a byte-identical optimal solution in all 5 seeds at temperature 1.0, with zero exploration.
The broader point is that benchmark scores can overstate reasoning gains if the evaluation set and the real task distribution diverge.
Related event: Opus 5's High ARC-AGI-3 Score Sparks Cheating and Overfitting Controversy(11 posts)→
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