Unclear What content or feature actually sets attached_by, and how it is used once present, is not stated.
What
The schema Claude Code uses for chat/session messages (which already includes fields like id, in_reply_to, and author) now accepts an optional attached_by field, whose only currently allowed value is "server".
Why
This appears to let a message record note that something in it was attached by the server rather than by the user or the model, which could support features that add server-side content to a message after the fact, though the finding does not say what specifically uses this field.
Something disagreesSomething we can check disagrees with this entry, or the writer said they could not settle it.
The writer flagged doubt
What content or feature actually sets attached_by, and how it is used once present, is not stated.