Where do I get the SHA256 of a specific commit point from?

[aaomidi]

I'm not sure where to get the SHA256 value

[avdv]

I assume you're using http_archive?!

You could just leave it out. Bazel will print something like this:

DEBUG: Rule 'io_tweag_rules_nixpkgs' indicated that a canonical reproducible form can be obtained by modifying arguments sha256 = "6f763feca1a7526f30177a7a685b9c24beee5ca6df9c47358e885b02e50ffe61"

You could also manually download the file given in the url attribute to the http_archive and compute the hash using sha256sum.