A decoder nobody had documented
Figma’s .fig files are a Kiwi-encoded binary format wrapped in zstd, with no published schema. I wrote the byte-buffer reader and varint decoding by hand, then re-projected the decoded structures into a normalised document tree — around 5,000 lines just for that projection layer. The lesson that transfers: when the vendor API cannot give you what you need, the format itself is usually still tractable.