gpt-5 with medium reasoning, no search: "Short answer: use the system zstd C library that ships with iOS; Apple’s Compression framework still doesn’t expose zstd. Link libzstd and call its streaming API from Swift." (and then like 50 lines of code plus some explanation)

gpt-5 with medium reasoning + search: "Short answer: you can’t. Apple’s built‑in Compression framework on iOS exposes LZFSE, LZ4, ZLIB/DEFLATE, and LZMA—not Zstandard—so there is no system API to do zstd without bundling an external library. (https://www.hackingwithswift.com/example-code/system/how-to-...)

If you can switch to a built‑in algorithm, here’s a minimal streaming compressor using Compression (LZFSE shown):" (etc etc)