Reply only to top-level comments on files
Replying to a reply on a file download page is not allowed. Only top-level comments can have replies.
Why this happens
Comment threading is intentionally one level deep. parentCommentId must point at a root comment (parentCommentId: null). Nested reply attempts fail parent lookup / validation.
Diagnose and fix
1. Visitor: reply to the original comment, not to someone else’s reply.
2. Admin: no depth setting — product design; use moderation if abuse continues.
3. Confirm Enable file comments is on.
4. Folder pages have the folder-specific sibling message.