Usernames are limited to 1024 bytes, so no account can exist that is unable to log in. The username rides in the session cookie, which browsers and securecookie refuse past about 4 KB, leaving room for roughly 2000 bytes of username; the limit is about half that.
User.BeforeSave returns ErrUsernameTooLong when a whole User is created or saved. A byte-counting check constraint on users.username catches every other write, including a column update. The limit appears in the constant and in the struct tag; a test fails if they disagree.
Model: opus-5-5
clawbot