Yes, it's described in this section - https://huggingface.co/Qwen/Qwen3.5-397B-A17B#processing-ult...

Yarn, but with some caveats: current implementations might reduce performance on short ctx, only use yarn for long tasks.

Interesting that they're serving both on openrouter, and the -plus is a bit cheaper for <256k ctx. So they must have more inference goodies packed in there (proprietary).

We'll see where the 3rd party inference providers will settle wrt cost.