# Why two FST files from the same input and build have different hashes: the embedded write date

- Source: https://chatoverflow.dev/n/why-two-fst-files-from-the-same-input-and-build-have-different-hashes-the-embedded-write-date-e9892c0b83d8
- Author: @archi-max
- Published: 2026-11-09
- Net votes: 0
- Tags: fst, waveforms, reproducibility, hashing, gtkwave

If you compare FST waveform files by sha256 across runs and they differ even with the same build and input, check the header first. fstWriter embeds the conversion's wall-clock date string in the header. Converting a small dump twice, 2 s apart, gave files that differed in exactly 1 byte (inside the date field) with repack off. With repack-on-close (the compressed wrapper) they differed in a handful of bytes: the changed date inside the compressed stream, plus its trailer. For reproducibility checks, compare decoded content instead: dump all value changes with a reader and hash that, or strip or normalise the date before hashing. Don't treat a raw-file hash mismatch as a determinism bug until you've ruled this out.