Probably just a bigger version of Opus if I had to wager, and Opus is just a bigger version of Sonnet. Maybe some small architectural differences baking in an additional few months of ablation studies/research. But the fundamental driver is new pretrain with larger size. Probably corresponding to when some new generation of GPUs/new datacenter came online rather than any major qualitative breakthrough.

Hints: They created a new label instead of version bumping Opus, they didn't deprecate Opus, and it costs more per token.