Opus 5.5 does worse and costs more at Max thinking — Medium beats Max on hard benchmark

davidyin44 · x · 2026-09-26

morganlinton spent four days benchmarking Opus 5.5 on his own VulcanBench Frontier v4 (hard tasks built to stump frontier models) and found counterintuitive results:

Opus 5.5 performed better at Medium and High thinking than at Extra High and Max
Cost spikes at Max, but accuracy doesn't improve — extra thinking is a net loss here
Fable 5.1 was most accurate overall, but only at Max — and surprisingly cheaper there than at Extra High, which ties Opus 5.5

The author cautions these are deliberately hard tasks, not representative of everyday workloads.

Original post →

Opus 5.5 does worse and costs more at Max thinking — Medium beats Max on hard benchmark

More from Models