Some of it is also what a "branch" means to different people can mean different things: https://jvns.ca/blog/2023/11/23/branches-intuition-reality/
But yeah, as others have said, not really possible in a general way, sadly.
- 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) ~
> 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) ~
Also note this bakes in a lot of assumptions and is brittle. As many others have said, it’s not generalizable.