If you're looking to model git branches as much as possible with jj bookmarks and assume that:

- the working copy has a bookmark pointing to it

- there's some ancestor with a bookmark

- there's a single linear path between the two with no other bookmarks in between

Here's an example that represents a branch containing 3 commits named "bookmark-05ff" branched off of "bookmark-6825".

    > jj log -r y:: -T builtin_log_redacted
    @  urplyywu user-482a 2025-10-23 13:24:52 bookmark-05ff a334e2e1
    │  (empty) (redacted)
    ○  lxxtnlxw user-482a 2025-10-23 13:24:46 git_head() 7e32fa6b
    │  (empty) (redacted)
    ○  omktyuos user-482a 2025-10-23 13:24:13 7668b0bb
    │  (empty) (redacted)
    ○  ykzktoux user-7b7f 2025-10-23 13:23:47 bookmark-6825 98bfcbde
    │  (empty) (redacted)
    ~

In this case, the following log would get you the commit pointed to by "bookmark-6825":

    > jj log -r 'latest(ancestors(@) & bookmarks(), 2) ~ @' -T builtin_log_redacted
    ○  ykzktoux user-7b7f 2025-10-23 13:23:47 bookmark-6825 98bfcbde
    │  (empty) (redacted)
    ~

I'm using the builtin_log_redacted output template. Normally you'd have actual bookmark names, descriptions, user, etc.

Also note this bakes in a lot of assumptions and is brittle. As many others have said, it’s not generalizable.