[ikawe]

Custom dictionaries are a feature of vanilla (non-seekable) zstd. As I understand it, all seekable-zstd are valid zstd, so it should be possible?

https://github.com/facebook/zstd?tab=readme-ov-file#the-case...

[rorosen]

Yes, dictionaries should be totally possible. However, I've never tried them to be honest because I usually only compress big files. They can be set on the (de)compression contexts the same way as with regular zstd.