Every app that accepts photo uploads meets HEIC sooner or later. iPhones save in it by default. Neither Java nor Rust can read it out of the box, and the usual answer, libheif, is a C library: native packages in every container image, a C toolchain in the build, and a parser written in C reading files that strangers upload.I had the problem twice, in a Java product and in a Rust one. So I went the other way: decoders written in the language itself, built and improved by AI coding agents, with me steering. Here is how it went.