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It's hard to tell since any given dev can just use jj locally, if they want to, while everyone else uses git.

Technically, you can still tell actually - jj writes a "change-id xyz..." in the git commit object header, which remains there as it's pushed around. It's just typically not made visible by regular things. (I wonder what other random garbage has been hidden in git commit headers that noone has seen)

Huh, fortunate for it that GitHub (especially) preserves it then!

I wonder how guaranteed that is, can imagine GH deciding to 'clean' it (how much data can you put there theoretically? Or to close a range of potential security issues) and suddenly relatively niche but genuine uses like jj's change-id are unintentionally not preserved.

Cleaning it would change the commit ID, so a forge cannot clean it even if it wanted to (not without rewriting all descendant commits too and breaking signed commits, at least).

The change-id is exactly as much part of the commit object as the author/committer name/timestamp, description, parent commit ID, tree, and participates in the commit hashing as those do.

Various git commands can drop the metadata, so it’s not guaranteed to stick around.

All my prs are named phil-<hash> and have been for years now. I keep waiting for someone to care/notice, but no one has.

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